Biology Department Research Group Terrestrial Ecology ______

Academic year: 2019 – 2020

EARLY TERRITORY SELECTION BY THE THREATENED YELLOWHAMMER EMBERIZA CITRINELLA IN WEST FLANDERS

Bram Catfolis

Supervisor: Prof. Dr. Luc Lens, Ghent University Co-supervisor: Prof. Dr. Luc De Bruyn, Research Institute for Nature and Forest (INBO) Scientific tutor: Olivier Dochy, Province of West Flanders

Master’s dissertation submitted to obtain the degree of Master of Science in Biology

Photo on front page: © Rini Lamboo

© Faculty of Sciences – research group Terrestrial Ecology All rights reserved. This thesis contains confidential information and confidential research results that are property to the UGent. The contents of this master thesis may under no circumstances be made public, nor complete or partial, without the explicit and preceding permission of the UGent representative, i.e. the supervisor. The thesis may under no circumstances be copied or duplicated in any form, unless permission granted in written form. Any violation of the confidential nature of this thesis may impose irreparable damage to the UGent. In case of a dispute that may arise within the context of this declaration, the Judicial Court of Gent only is competent to be notified.

Table of contents

1 INTRODUCTION 3

1.1 HISTORY OF THE EUROPEAN LOWLAND FARMLAND 3 1.1.1 COLLAPSE OF EUROPE’S FARMLAND BIRD POPULATIONS 3 1.1.2 EUROPEAN POLICIES AND LEGISLATION CONCERNING AGRICULTURAL INTENSIFICATION 4 1.2 CHANGES IN MANAGEMENT AND IMPLICATIONS FOR FARMLAND BIRDS 4 1.2.1 LOSS OF MIXED FARMING 4 1.2.2 MECHANIZATION AND CHANGE OF CROPS 5 1.2.3 LOSS OF FIELD BOUNDARIES AND MARGINS 5 1.2.4 THE USE OF PESTICIDES 6 1.3 THE STRUGGLE OF OVERWINTER SURVIVAL 7 1.3.1 ‘WINTER BIRD CROPS’ TO THE RESCUE 7 1.4 YELLOWHAMMER PROJECT & THESIS FRAMEWORK 8 1.4.1 THE SPECIES ACTION PLAN 8 1.5 ENTERING THE BREEDING PERIOD 9 1.5.1 NEST SITE SELECTION 9 1.5.2 SURROUNDING HABITAT SPECIFICITY 10 1.5.3 CONSPECIFIC ATTRACTION 11 1.5.4 BEARING IN MIND: COUNTRY SPECIFIC ASPECTS 12 1.6 THE YELLOWHAMMER AS A STUDY SPECIES 12 1.6.1 CHARACTERISTICS 12 1.6.2 HABITAT 13 1.6.3 DIET 13 1.6.4 BEHAVIOUR 13 1.6.5 POPULATION TREND AND DISTRIBUTION 14

2 OBJECTIVES 15

2.1 OBJECTIVE 1: WHICH HABITAT FEATURES DETERMINE A PREFERRED YELLOWHAMMER TERRITORY? 15 2.2 OBJECTIVE 2: DO WINTER FOOD PLOTS HAVE AN INFLUENCE ON EARLY TERRITORY SETTLEMENT BY THE YELLOWHAMMER? 15 2.3 OBJECTIVE 3: ARE YELLOWHAMMER TERRITORIES SPATIALLY CLUSTERED? 16

3 MATERIAL & METHODS 17

3.1 STUDY SITE 17 3.2 DATA COLLECTION 17 3.2.1 COLOUR-RINGING 17 3.2.2 BIRD SURVEYS AND TERRITORY MAPPING 19 3.2.3 SURROUNDING HABITAT FEATURES 20

1 3.3 STATISTICAL ANALYSIS 22 3.3.1 TERRITORY SETTLEMENT AND THE RELATION WITH SURROUNDING HABITAT FEATURES 22 3.3.2 INFLUENCE OF WINTER FOOD PLOTS ON EARLY TERRITORY SETTLEMENT 23 3.3.3 PATTERNS OF TERRITORY DISTRIBUTION 23

4 RESULTS 25

4.1 YELLOWHAMMER TERRITORIES 25 4.2 DATA EXPLORATION 25 4.3 TERRITORY SETTLEMENT IN RELATION TO HABITAT VARIABLES 27 4.4 INFLUENCE OF WINTER FOOD PLOTS ON EARLY TERRITORY SETTLEMENT 29 4.4.1 NEAREST WINTER FOOD PLOT 30 4.4.2 THREE NEAREST WINTER FOOD PLOTS 30 4.5 TERRITORY DISTRIBUTION 30

5 DISCUSSION 32

5.1 GENERAL 32 5.2 HABITAT FEATURES DETERMINING TERRITORY SELECTION 32 5.2.1 HEDGES 33 5.2.2 FIELD MARGINS 34 5.2.3 DITCHES 35 5.2.4 METHODOLOGICAL REMARKS ON DETERMINING HABITAT PREFERENCES 35 5.3 INFLUENCE OF WINTER FOOD PLOTS ON EARLY TERRITORY SETTLEMENT 36 5.4 CLUSTERED TERRITORY DISTRIBUTION 37 5.4.1 METHODOLOGICAL REMARKS ON CLUSTER ANALYSIS 39 5.5 IMPLICATIONS FOR MANAGEMENT 39 5.6 GENERAL PROPOSAL FOR FUTURE RESEARCH 40

6 CONCLUSION 41

7 SUMMARY 42

7.1 ENGLISH SUMMARY 42 7.2 NEDERLANDSTALIGE SAMENVATTING 44

8 ACKNOWLEDGEMENTS 46

9 REFERENCES 47

10 APPENDIX 57

2 1 Introduction 1.1 History of the European lowland farmland 1.1.1 Collapse of Europe’s farmland bird populations

Traditionally, land use throughout much of western Europe is dominated by agriculture. Until the 60s of the twentieth century, farmland birds had no problems getting food all year round (Shrubb 2003). Countless fields with overwintered stubbles and spilled grain used to provide a seed source over the early and mid-winter periods (Potts 2003, Evans et al. 2004). Once the field was ploughed, new seeds from crops and weeds come to the surface. In the late winter, farmland birds were presumably dependent on these ploughing activities (Shrubb 2003). Sowing during spring, periods of fallow and varied crop rotation made it possible for every species to find suitable habitat at any time.

But due to post-war changes in European agriculture, the availability of all of these has fallen dramatically (Shrubb 2003). Governmental policies and technological advances rapidly changed this sustainable and balanced land use (Robinson & Sutherland 2002). The changes caused by this agricultural intensification were of that nature that most farmland species, especially farmland birds, could not adapt (Chamberlain et al. 2000, Anderson et al. 2001, Donald et al. 2001).

In the mid-1970s, many farmland species started to decline markedly (Siriwardena et al. 1998a). The suite of changes in the agricultural environment around that time adversely affected many species’ populations. Species, such as Wren (Troglodytes troglodytes), Pied wagtail (Motacilla alba yarrellii) and Treecreeper (Certhia familiaris) showed a similar trend in abundance decline, despite their different ecological requirements (Siriwardena et al. 1998a). Such coincidences in population trend are more likely to indicate that a large number of components of agricultural intensification changed at the same time, but the actual individual factors affecting population change differed from species to species (Chamberlain et al. 2000). However, some species like Stock dove (Columba oenas), Jackdaw (Corvus monedula) and Chaffinch (Fringella coelebs) showed increased abundances, indicating that certain agricultural changes can be beneficial for some species (Chamberlain et al. 2000). Usually, the species that benefit or experience little effect of this intensification are generalists which have more catholic habitat preferences in comparison with farmland specialists (Robinson & Sutherland 2002). These specialist species can thrive only in a narrow range of environmental conditions or have a limited diet, which makes them vulnerable to rapid and drastic habitat changes. If we want to avoid a biological impoverishment of our lowland farmland, we must counteract the negative consequences of intensified farming.

For nearly 120 bird Species of European Conservation Concern, lowland farmland provides a breeding or wintering habitat. This is the largest number of such species supported by any habitat (Donald et al. 2001). In Flanders, 10% of the birds listed in the Red List of Flemish Breeding Birds are farmland birds (Devos et al. 2016). Granivorous species, those with a substantial seed component in the diet, experience the strongest decline compared to other farmland bird species (Wilson et al. 1999). Endangered species from agricultural landscape and farmland birds in particular hardly benefit from the traditional, reserve-oriented nature policy in Flanders (Devos et al. 2004).

Even though the rate of change in agriculture has slowed, the majority of farmland specialists have continued to decline since the late 1980s (Fuller et al. 2000). A population recovery on a larger scale requires a thorough reform of the Flemish and European agricultural policy. Because farmland birds depend on a wide variety of landscape and nature factors for their survival, they are good indicators of landscape quality in general.

3 1.1.2 European policies and legislation concerning agricultural intensification

In 1962, the Common Agricultural Policy (CAP) came into force and serves as the main agricultural policy tool of the European Union (Butler et al. 2010). It implements a system of agricultural subsidies and other programs to support farmers and improve agricultural productivity. The most important measure taken by CAP in 1992 are the so-called agri-environment schemes (AES). They are designed to encourage farmers to protect and enhance the environment on their farmland by paying them for the provision of environmental services. Farmers commit themselves, for a minimum period of at least five years, to adopt environmentally-friendly farming techniques that go beyond legal obligations and that try to support biodiversity, enhance the landscape, mitigate climate change, and improve the quality of water, air and soil. AES are a financially attractive form of environmental protection because at least 50% of the costs of approved agri-environment schemes flows from the European Union Common Agricultural Policy. The rest of the money is funded by the Member States themselves (Batáry et al. 2015). However, the effectiveness and efficiency of the AES are still heavily debated. While several studies have found positive effects on the biodiversity in response to changed agricultural practices under AES programs (MacDonald et al. 2012, Evans et al. 2019), others have shown mixed or limited benefits (Kleijn et al. 2001, Feehan et al. 2005, Princé et al. 2012), and even negative biodiversity outcomes (Besnard & Secondi 2014, Fuentes-Montemayor et al. 2011). Despite their mixed success, AES now represent the dominant policy instrument for conserving biodiversity in agricultural landscapes (Ansell et al. 2016). To improve the effectiveness of the AES, a continuous multidisciplinary evaluation of conservation programs is needed.

1.2 Changes in management and implications for farmland birds

For more than 50 years, agricultural intensification leads to a population decline of bird species associated with agriculture on a European scale. This applies both to farmland birds and meadow birds. But intensification is not an unambiguous process. It consists of several components, which often occur simultaneously and are interlinked. The following components of intensification with a negative effect on farmland birds have been described in the literature.

1.2.1 Loss of mixed farming

Before the intensification, livestock and arable production often occurred on the same farm. This offers habitat diversity which is the key to a large diversity of species (Whispear & Davies 2005). The arable component serves as a rich source of seeds for granivorous birds, while the grass component is home to a large number of invertebrates, providing food to insectivorous birds.

Since the new agricultural developments, farms increasingly tend to specialize in either livestock or arable crops (Robinson & Sutherland 2002). With farm amalgamation, farm size has increased and the surface area of peripheral and transition zones of interest for birds has decreased (Stoate et al. 2001). These loss of mosaics in space and time has reduced the number of different habitats on smaller spatial scales. To guarantee a sufficiently high reproduction, a varied range of habitats within a short flight distance is of great importance during the breeding season (Batáry et al. 2010). The province of West Flanders kept his high number of small-scale farms, so that the effect of the loss of mixed farming is less noticeable here.

4 1.2.2 Mechanization and change of crops

New techniques and mechanization of agriculture have led to bigger farms with an increased on average arable field size. Sowing the same crop over several fields is more efficient but decreases the habitat diversity. Also, the flying distance between the habitats becomes too large during the breeding season (Batáry et al. 2010).

A change in crop choice and consequently harvesting times also had a great impact on farmland species. The less productive spring cereals were replaced by winter cereals, sown in the autumn instead of early spring, or by maize (Eggers et al. 2011, Glemnitz et al. 2015). This change led to fewer stubbles and annual weeds throughout the winter which are crucial as seed food for birds (Newton 2004, Glemnitz et al. 2015). In addition, sophisticated harvesters with high yields leave little grain on the stubbles. However, spillage of seeds was one of the most important food sources for granivorous species (Bos & Schröder 2009). Spring cereals also provide a more suitable breeding habitat for many species than winter cereals and maize. The latter crops achieve a dense and high crop structure early in the breeding season, making them unsuitable as a breeding habitat (Bos & Schröder 2009).

In addition to arable management, grassland management has also been intensified. Herb-rich meadows and half-open haylofts were replaced by grasslands that do not run to seed. Multiple cuts per year to ensile the grass occur during the breeding season and destroy the nests of ground-breeding birds (Benton et al. 2003, Tscharntke et al. 2005). Increased fertilizer application, regular ploughing, reseeding and drainage lead to dense and homogeneous swards that do not provide bare open ground for foraging or nesting sites for ground-breeding birds (Perkins et al. 2000, Batáry et al. 2010).

1.2.3 Loss of field boundaries and margins

As mentioned before, the sizes of field generally increased, especially on arable land. The loss of field boundaries and margins is the price to be paid for this. To allow a more efficient use of the new machinery, hedgerows have to be removed. Those that remain are often managed to keep them ‘tidy’ (Manson & Macdonald 2000). Another consequence of larger parcels is the relatively smaller proportion of weed-rich edges which are popular foraging areas with farmland birds.

Because of the loss of hedges, wooded banks and channels alongside the farmlands and grasslands, a valuable source of nesting habitat for many of the declining farmland birds decreased (Morris et al. 2001). But more important is the loss of a rich source of insect life (Glemnitz et al. 2015, Robinson & Sutherland 2002). The field boundaries act as foraging areas and overwintering habitats for many insects (Perkins et al. 2002, Whispear & Davies 2005). A varied, food-rich environment is crucial for farmland specialists, like Yellowhammer (Emberiza citrinella), to maintain a stable population size.

5 1.2.4 The use of pesticides

In the 1940s, the first hormonal herbicide was introduced. Thirty years later, 136 different compounds had been approved for agricultural use, and by 1997 344 pesticide compounds were available on the market (Robinson & Sutherland 2002). Nowadays, pesticide usage is widespread and closely linked to modern agriculture. Modern pesticides are more efficient and less persistent, thus require smaller amounts of active ingredient. However, the number and extent of applications has increased (Robinson & Sutherland 2002) and keeps increasing all over the world (Sharma et al. 2019). So, it is not surprising that pesticides have caused a tremendous change in the environment. The use of pesticides has both a direct and indirect impact on farmland species.

The main effects of most pesticides for farmland birds are indirectly manifested. Three mechanisms have been identified through which pesticides may affect food availability for birds. Firstly, the direct removal of arthropods by insecticides. Arthropods are exploited by adults and their dependent young during the breeding season. Morris et al. (2005) found a negative relationship between insecticide use and nestling body condition of the Yellowhammer. Insecticide use did not lead to a brood reduction from starvation, but nestlings leaving the nest with poor body condition are less likely to survive to reproductive age which has an impact at the population level. They also found that insecticide applications changed the foraging behaviour of Yellowhammers. Fields with breeding season applications of insecticide were used less by early broods, which have a wholly insectivorous diet, than fields without such applications. Later on, treated fields were visited more frequently, but birds were mostly taking semi-ripe grain rather than insects. Secondly, herbicides reduce the abundance of non-crop plants from which arthropods live. A reduced abundance of invertebrate food during the breeding season significantly reduces the avian reproductive success (Richmond et al. 2011). Thirdly, herbicides deplete or eliminate the plants, which provide food for both herbivorous and granivorous bird species, thereby reducing their chance of survival. Reduced weed growth and seed production not only leads to loss of immediate food supply, but also to long-term depletion of the seed bank in the soil (Vickery et al. 2001, Boatman et al. 2004, Newton 2004, Buckingham et al. 2006, Kleijn et al. 2009).

Besides their indirect impact, pesticides can also directly cause failed reproduction and mortality (Burn 2000, Newton 2004). Neurotoxic pesticides have the potential to cause behavioural disturbances in birds (Walker 2003, Eng et al. 2019). The fastest-growing class of neurotoxic insecticides are neonicotinoids. They kill insects by targeting their nervous system (Matsuda et al. 2001). Because neonicotinoids disrupt specific neural circuits of insects, they are considered to be less harmful to vertebrates (Tomizawa & Casida 2005). However, neonicotinoid use has been linked in a range of studies to adverse ecological effects, including the death of non-target invertebrate species (Henry et al. 2012, Van Dijk et al. 2013) and declines of terrestrial and aquatic vertebrate wildlife due its direct and indirect toxicity (Gibbons et al. 2015). A study of Hallmann et al. (2014) showed that imidacloprid, the most widely used neonicotinoid insecticide, has a negative impact on insectivorous bird populations in the Netherlands. Local population trends were significantly more negative in areas with higher surface-water concentrations of imidacloprid. Birds are affected both directly by the consumption of poisoned insects or coated seeds and indirectly by the reduction in insect populations.

6 1.3 The struggle of overwinter survival

The widespread decline in population sizes of granivorous farmland birds cannot be allocated only to the changes in several components of the annual breeding performance (e.g. clutch size, brood size, chick:egg ratio and daily nest failure rates). The winter also serves as a critical period and changes in the overwinter period might be of equal, or even higher, concern in saving farmland bird species (Siriwardena et al. 1998b). According to Siriwardena et al. (2000), the periods during which most mortality occurs, are likely to happen outside the breeding season, and, at least for resident species, in harsh late winter conditions when metabolic demands are high and food supplies have been depleted. The most important factor in these population declines is the reduction in food availability during winter (Siriwardena et al. 2000, Newton 2004, Kleijn et al. 2014). Bird species like Common Linnet (Carduelis cannabina), Tree sparrow (Passer montanus) and several bunting species, rely on seeds during winter and seem to be affected by the shortage (Robinson & Sutherland 2002). A lot of granivorous passerines are tied to stubble field as a winter foraging habitat. The highest mortality rate occurs in the late winter, between the end of February and beginning of March. During this so-called ‘hungry gap’ (Siriwardena et al. 2008), finding food becomes hard because seeds have become scarce and arthropods are hardly to be found yet (Kleijn et al. 2014). To tackle this problem, different conservation measures that enhance winter food resources are more and more implemented in agri-environment schemes. If seed food is supplied effectively throughout the winter, these measures may stop or even reverse the declining population trends (Siriwardena et al. 2007). Positive effects of supplementary winter seed food on several species of farmland birds are shown in a study from Siriwardena et al. (2007). Providing food at several feeding sites during winter resulted in less steep decreases in breeding abundance of several species such as Yellowhammer (Emberiza citrinella) Goldfinch (Carduelis carduelis) or Reed bunting (Emberiza schoeniclus), but also non-granivorous species like Robin (Erithacus rubecula) and Dunnock (Prunella modularis).

1.3.1 ‘Winter Bird Crops’ to the rescue

As mentioned before, there has been a large reduction in the area and quality of overwintered stubble fields due to the switch from spring to autumn-sown cereals coupled with improved weed control and harvesting techniques (Robinson & Sutherland 2002). Enhancing the winter food resources for farmland birds can be done either through the retention of overwinter stubbles or by planting seed- rich crops, also called winter bird crops (WBCs) (Henderson et al. 2004). These WBCs are a mixture of plants that are sown with the sole purpose to provide extra winter food and/or cover for farmland birds. Despite the fact that the perfect crop mixture does not exist because of different ecological requirements between species, researchers try to select appropriate seed mixtures that meet the needs of species of particular conservation concern as much as possible (Henderson et al. 2004).

Kleijn et al. (2014) experimented with several seed mixtures to look for preferences. It turned out that most bird species, except for Reed Bunting (Emberiza schoeniclus) and Common Linnet (Carduelis cannabina), are not very fussy when it comes to seed mixtures. Whether your field is sown with a specially designed crop mixture or it is fallow and overgrown by weeds, anything that increases the supply of seed during the winter period attracts seed-eating songbirds. Even buntings, which are known to have a strong preference for cereals (Perkins et al. 2007), also make use of a wide range of species from other plant families. A possible exception is Yellowhammer (Emberiza citrinella) which seems to be more of a picky eater selecting almost exclusively cereal grains (Kleijn et al. 2014). For most bird species, the quantity of seeds is more important than the type of seeds. Increasing seed

7 availability leads to a proportional increase in wintering farmland birds. Since WBCs provide a concentrated seed source, even small areas can be attractive as a valuable resource (Henderson et al. 2004). The importance of seed-rich winter habitats highlights the potential benefit of WBCs as an option within agri-environment schemes.

The province of West Flanders already anticipates on this matter by setting aside cereal strips and patches during winter for several years in Heuvelland and around Beveren aan de IJzer (Fig. 1). One of their target species is Yellowhammer, one of the most threatened farmland birds in Flanders. In order to bridge the hungry gap, they investigated the potential of bristle oat (Avena strigosa) as a winter food crop. Bristle oat is a type of cereal crop that is native to the Mediterranean region. It is used st as cover crop and, if sown before the 1 of August, Figure 1: Winter food plot in Heuvelland, sown with it seems to be a good seed source for farmland birds cereals up to the end of winter (Coelembier et al. 2015).

1.4 Yellowhammer project & thesis framework 1.4.1 The Species Action Plan

This thesis is part of the larger Species Action Plan, commissioned and financed by the province of West Flanders, in collaboration with the Research Institute for Nature and Forest (INBO) and the University of Ghent. The Species Action Plan is covered within the provincial biodiversity policy on species protection and aims to maintain and enhance the biodiversity in West Flanders in order to regain resilient populations (Dochy 2015).

Biodiversity is a broad concept and is used as an indicator of ecosystem health. Because farmland birds depend on a wide variety of landscape and nature factors for their survival, they are good indicators of landscape quality in general (Dochy & Hens 2005). We distinguish two types: (1) those that rely on an open landscape and (2) those that prefer a more small-scale landscape, characterized by landscape elements such as hedgerows, bushes, orchards, scrubs, field margins, … (Dochy & Hens 2005). This type of landscape is still present in West Flanders, but its existence suffers from the increasing degree of intensification. To create support, you have to reach the general public and make them enthusiastic about the protection of our farmland biodiversity. Species are often more appealing to the imagination than habitats do and are therefore widely used as a stepping stone for conservation projects (Dochy et al. 2007). The perfect mascot of the small-scale landscape is the Yellowhammer (Emberiza citrinella). This species is protected according to "het Soortenbesluit", a decision of the Flemish Government on 15 May 2009 regarding species protection and species management. It switched from "endangered" to “Least concern” according to the Red List of Flemish Breeding Birds (Devos et al. 2004, Devos et al. 2016), but this is mainly due to its increase in the east of Flanders. West Flanders, on the other hand, barely enjoys the population increase. Its high cuddibility factor and bright yellow colour make from the Yellowhammer a symbolic and recognizable bird species (Dochy et al. 2007). It is important to note that the Yellowhammer’s habitat is also home to many other rare and threatened species e.g. Northern crested newt (Triturus cristatus), Brown hairstreak (Thecla betulae), Garden dormouse (Eliomys quercinus), Grey partridge (Perdix perdix), Black poplar (Populus nigra), etc. (Dochy 2018b).

8 By improving the habitat conditions of the Yellowhammer, many other species may experience indirect positive effects.

The overall goal of the Species Action Plan is to preserve the Yellowhammer population. To achieve this, most measures are focusing on conservation and improvement of their current habitats. Only if the current local population increases, expansion to lost areas is possible (Dochy 2018b). The following five goals are explained by the Species Action Plan:

1) The improvement of the breeding habitat with a focus on the construction and management of Yellowhammer-friendly hedges. 2) Providing winter food in the form of large starchy seeds (wheat, barley or bristle oat). 3) Creating insect-rich zones. They harbour many insects and seeds and are therefore important all year round. 4) Refinement of the knowledge, which allows us to improve the efficiency of the current measures and to better evaluate them. 5) Raising awareness to create a large support base.

The first three measures for the Yellowhammer focus on providing food and protection. They are known as the “Big Three”, according to Dochy & Hens (2005). My thesis will focus on the fourth goal and will more specifically investigate the territory selection by the Yellowhammer in Heuvelland, located in the Province of West Flanders. Which type of habitat do they prefer? What influence do the winter food plots have on their territory selection? What is the distribution of the settled territories?

1.5 Entering the breeding period 1.5.1 Nest site selection

In February, male Yellowhammers leave the winter flock in the morning and evening to search for a suitable breeding territory. Territories must include a nest site, be close to song perches and be in close proximity to food resources, because most foraging trips to collect food for the young are made within 100 m of the nest site (Morris et al. 2001).

Trees or shrubs that clearly stand out above the other vegetation are valued as singing or observation post. Electrical wires and fence poles also fulfil this function. Only one tree may be required per territory, and hence territories need at least one boundary possessing a tree (Whittingham et al. 2005). Yellowhammers typically build their nests among herbaceous vegetation in ditches or in the shrubby vegetation of hedgerows (Stoate et al. 1998, Bradbury et al. 2000). The optimal hedges are rather low (1.40 m) and wide (1.20 m) and are not too neatly shaved (Lack, 1992). A closed bottom is preferred since Yellowhammers mostly breed either on or close to the ground. Pasture and silage leys are avoided. These preferences tie the Yellowhammer to a small-scale agricultural landscape in Europe, which is typically full of small landscape elements such as shrubs, trees, hedges, wood edges, groves, orchards and bramble stems, in combination with herb-rich verges or plots. Winter set-aside fields have a strong influence on Yellowhammer distribution (Whittingham et al. 2005). This is due to the temporal persistence of set-aside fields into the spring, in contrast to stubble fields which are often ploughed earlier in the year.

9 A regional study along both sides of the border between France and West Flanders discovered that both pollard willows and ponds occurred more than average in Yellowhammer territories (Dochy 2014). Ponds are often surrounded by bare trampled soil, the preferred foraging ground by Yellowhammers. Pollard willows act as suitable singing posts and are rich in insects from early spring. In addition, these old truncated trees often point to less intensively worked farmland.

In our area, the species is largely sedentary, wintering in the same general areas as occupied in the breeding season (Whittingham et al. 2005). In West Flanders, Yellowhammers rarely roam more than 2 km outside their local breeding area during winter (Dochy 2018a). It is assumed that the overwintering individuals are local or originating from neighbouring breeding areas in northern France, although hard evidence is still lacking. During the breeding season the socially monogamous couples are strongly territorial. The male establishes and defends its breeding territory by singing and fighting with other males from February (Andrew 1956). Linear features such as hedges or ditches are more strongly defended than the corresponding area in the field. The size of Yellowhammer territory decreases with increasing population density (Andrew 1956).

1.5.2 Surrounding habitat specificity

Yellowhammer territories must contain a nest site and be close to song perches and food resources (Fig. 2). The chicks are fed with invertebrates and later unripe grain is added to their diet. So, during the breeding season, field margins, un-cropped land and cereal crops are important sources of chick- food arthropods and seeds (Douglas et al. 2009, Henderson et al. 2012). Grass margins and other non- crop field boundary habitats, such as hedgerows and ditches, are selected relative to cropped areas by Yellowhammers (Perkins et al. 2002). However, in cereal fields, tractor tramlines with sparser vegetation than cropped areas are also a preferred foraging site. The accessibility and detectability of arthropod prey is likely to be higher in these tramlines compared to in the adjacent crop. The selection of sparsely vegetated tramlines suggests that vegetation structure is an important determinant of patch selection within cereal fields (Douglas et al. 2010), but also in the uncultivated field margins (Morris Figure 2: Yellowhammer territory with hedgerow and grass et al. 2001). margin adjacent to cropland. Photo: © Olivier Dochy

Yellowhammers initially select foraging microhabitats according to the vegetation structure and later according to food availability (Dunn et al. 2010). Lower vegetation cover provides a greater visibility of predators and thus a lower perceived predation risk (Whittingham et al. 2004). Invertebrate abundance is lower, but the lower vegetation density allows easier access to their prey. While increased vegetation cover leads to a higher perceived predation risk to the foraging bird, higher invertebrate abundances associated with increased cover can lead to birds selecting such sites for foraging. This happens in areas where the invertebrate abundance tends to be low and where a higher predation risk has to be taken in order to ensure a sufficiently high provisioning rate to chicks (Dunn et al. 2010). So, there is a payoff between an increased invertebrate availability and a higher predation

10 risk (Butler et al. 2005). A valuable measure to improve foraging sites is cutting patches within field margins to create a mosaic of cut patches where accessibility is improved, adjacent to uncut patches where invertebrate abundance remains high (Douglas et al. 2009, Perkins et al. 2002, Morris et al. 2001, Vickery et al. 2001). These enhanced arable margins, created either by sowing or through natural regeneration, have a strong influence on the Yellowhammer’s territory selection (Burgess et al. 2015).

1.5.3 Conspecific attraction

Conspecific attraction is the tendency of individuals of the same species to settle close to each other, which leads to a territorial clustering. This phenomenon is well described in colonial species, especially birds. Although this behaviour may occur in territorial birds, like the Yellowhammer, evidence has been scarce. If territorial birds do exhibit this behaviour, it would have major conservation implications (Ward & Schlossberg 2004). The theory of conspecific attraction seems contra-intuitive, because you would expect that an increase in conspecific density leads to an increased intraspecific competition resulting in a reduced individual fitness. If a clustered distribution is observed, there has to be a profound reason why a species chooses to breed close to its conspecifics instead of living in a randomly distributed population. Several mechanisms can declare conspecific attraction. First of all, conspecifics can act as cues to habitat quality. In many species, territory quality is determined by the distribution of habitat and vital resources, but accurate assessment of the patch quality or availability is often difficult or time-consuming (Stamps 1988, Herremans 1993, Muller et al. 1997, Ward & Schlossberg 2004). Secondly it can be a result from an increased predator and competitor protection. Territory owners may warn their neighbours about the presence of predators by producing specific alarm calls. Neighbours can cooperate to defend themselves against predators and interspecific competitors. Working together reduces the overall defence costs for individuals living within groups of territories (Stamps 1988, Muller et al. 1997). Furthermore, aggregations can be caused by social attraction. Males in territorial clusters might attract more females and thus have an increased chance to find a mate or to achieve extra pair copulations. "Social stimulation" provided by neighbours can result in synchronous reproduction and decreased juvenile predation rates (Stamps 1988, Herremans 1993, Reed & Dobson 1993, Muller et al. 1997).

No literature was found about possible conspecific attraction by the Yellowhammer. However, aggregated territories were found at a member of the Emberiza genus, more specifically the Ortolan Bunting (Emberiza hortulana) (Vepsäläinen et al. 2007, Dale & Steifetten 2011). This bunting is among the species with the most severe population declines in Europe during recent decades. It suffers from the same habitat degrading forces as the Yellowhammer. In Belgium, the Ortolan Bunting is extinct since the beginning of the 21th century. Berg (2008) surveyed the habitat selection and reproductive success of Ortolan Buntings on farmland in central Sweden. He found that Ortolan Buntings were strongly aggregated with many territories in a few areas. At the 1-km2 scale, territories occupied by pairs aggregated strongly in areas with high proportions of preferred habitats. The number of territories with single males correlated positively with the number of pairs, which suggests that conspecific attraction may influence territory distribution. Dale et al. (2006) investigated the search for breeding areas at the landscape level by the whole Norwegian population of the Ortolan Bunting. They stated that young Ortolan Bunting males searching for new breeding areas should take advantage of conspecifics to locate breeding areas and perhaps assess the quality of these.

11 1.5.4 Bearing in mind: country specific aspects

It is important to note that a lot of studies concerning the relationship between farmland bird abundances and food provisioning or habitat features is conducted in Great Britain (Kleijn et al. 2014). Agriculture in Great Britain happens on a much larger scale compared to our regions and is characterized by extensive plots and a crop rotation that is mostly dominated by winter wheat and oilseed rape. The loss of mixed farming led to a geographical separation between arable (eastern Britain) and livestock farming (western Britain) (Stoate et al. 2001, Robinson & Sutherland 2002). The Flemish landscape, on the other hand, is still characterized by a more small-scale agriculture. The larger English farm companies also allow an easier integration of conservational measures. The results and solutions from British studies cannot always be directly translated to the situation in Flanders. We have to interpret them with care. Therefore, it is important to conduct supplementary research that takes our Flemish situation into account.

1.6 The Yellowhammer as a study species 1.6.1 Characteristics

Kingdom Animalia

Phylum Chordata

Class Aves

Order Passeriformes

Family Emberizidae

Genus Emberiza

Species Emberiza citrinella

Figure 3: Yellowhammer in its different plumages. Source: Dochy, O. (2014). Actieprogramma soortbescherming Geelgors [Actieplan]: Provincie West-Vlaanderen.

The Yellowhammer is a rather large bunting with a fairly long, slightly forked tail, belonging to the bunting family Emberizidae. The males have a bright yellow head and breast, and a darkly streaked back. The females, on the other hand, are less brightly coloured and more streaked on the crown, breast, and flanks (Fig. 3). There is a lot of individual variation in the amount of yellow in both sexes. Their reddish-brown rump and white outer tail feathers are a striking and species-specific characteristic during take-off and landing. The bill is grey and the legs are flesh-brown. The juvenile is much duller and less yellow than the adults, and often has a paler rump. Both sexes are less strongly marked outside the breeding season, when the dark fringes on new feathers obscure the yellow plumage.

12 1.6.2 Habitat

The Yellowhammer occupies a wide range of vegetation types across Europe. Originally, it is an inhabitant of semi-open areas where it prefers transition zones between short herb-rich vegetation and shrubs or forest edges from which the male likes to sing. Across Europe, the Yellowhammer has been able to adapt well to a small-scale arable and mixed farmland full of small landscape elements such as shrubs, solitary trees, hedges, wood edges, groves, stubble fields and herb-rich margins or parcels (Dochy, 2014). However, this type of landscape is dramatically disappearing due to the agricultural intensification.

1.6.3 Diet

During winter, Yellowhammers feed themselves almost exclusively with starchy cereal grains (Robinson 2004). Wheat (Triticum spp.) is the most preferred grain type, due to its ease of handling and consumption, followed by oats (Avena spp.) and, if nothing else is available, barley (Hordeum vulgare) (Perkins et al. 2007). Their diet can be supplemented with some weed seeds, especially from genera like Polygonum, Centaurea, Stellaria and Poa (Robinson 2004). In contrast to other farmland birds, Yellowhammers are quite picky. Maize seeds are too big to swallow and oily seeds, such as those of brassicas, are ignored in favour of more starchy items (Dochy & Hens 2005).

During the breeding season, Yellowhammers add invertebrates to their diet, particularly as food for their growing chicks. A wide range of species is taken including grasshoppers, flies, beetles, caterpillars, earthworms and spiders. During the first few days, chicks are exclusively fed invertebrate prey, but later on, unripe grain is added to their diet (Stoate et al. 1998, Morris et al. 2005). Adult birds eat 3/4 invertebrates and 1/4 plant seeds in the breeding season. Nestlings receive 2/3 invertebrates and 1/3 seeds (Dochy 2014).

1.6.4 Behaviour

Most European Yellowhammers winter within their breeding range, only the northern populations move southwards. During late summer, the young birds start to form winter flocks. Adults join later during autumn (Clarysse 2003). These groups can greatly differ in size and often consist of other seed- eating birds. They exploit seed-rich places, such as (grain) stubble fields, set-aside areas or manure heaps. Yellowhammers prefer a nearby bramble or thorn bush to rest or hide in. In February, the males leave the flock in the morning and evening to search for a suitable breeding territory. They search a noticeable song post and show their presence by their striking singing. It sounds like the swelling tune from Beethoven's fifth symphony, with a series of short notes, gradually increasing in volume and followed by one protracted note. The omission of the long final strophe happens regularly. Yellowhammer males learn their songs from their fathers, and over time, many local and individual variations have developed (Diblíková et al. 2019).

Females leave the winter flock later. The entire process, from winter group to the occupation of a breeding habitat, takes two months. It can be interrupted by cold or rainy weather, causing the birds to return to the winter groups (Clarysse 2003).

13 During the breeding season, couples are strongly territorial. The female builds the nest low against the ground in high grass, often along shrubs and hedges, usually close to arable land. She lays 3-5 eggs around mid-April. After 12 – 14 days, the eggs are hatched. The young remain in the nest for 11-13 days and are cared for by both parents. After fledging they are fed for another 8 days before becoming fully independent. A second brood during the summer is certainly no exception (Dochy 2014).

1.6.5 Population trend and distribution

The Yellowhammer is the most common and widespread European bunting (Snow & Perrins 1998). They are mostly sedentary birds, living from Spain up to Scandinavia and from Ireland up to Central- Siberia. In the Mediterranean it occurs only as a winter visitor, while in the far north they are migratory birds. They are absent from high mountains, arctic regions and places with very intensive agriculture, like large parts of Flanders and the Netherlands. Yellowhammers have also been successfully introduced in New Zealand in the 19th century (Tietze et al. 2012). Three subspecies occur throughout Europe: E. citrinella citrinella (Northern Spain – Scandinavia), E. citrinella caliginosa (western Great- Britain) and E. citrinella erythrogenys (Eastern-Europa, Russia and the Baltic states).

The Yellowhammer used to be common and widespread as well in our Flanders. Since the eighties, their distribution area dramatically decreased. Nowadays, there are only 3400 – 4000 breeding pairs left from the 10 000 – 11 000 estimated pairs in the period of 1973 – 1977 (Devos et al. 2004). Only in Limburg and Flemish Brabant, we still find some core populations. In West Flanders we find some relict populations along the French border. According to Dochy (2014), about 100-120 couples remain between Beveren-aan-de-IJzer and Nieuwkerke.

Population numbers of Yellowhammers in Flanders are slightly rising recently (+50%, 2007-2016), especially in the east, but the high level of more than 50 years ago is still a long way off. Together with the fact that its breeding area seems to be expanding in a westerly direction (Vermeersch et al. 2004), the species has been included in the new Red List in the category ‘Least Concern' (Devos et al. 2016). However, the recent increase in the Flemish population is contrary to the European trend, which shows a lasting decline both in the long term (-45%, 1980-2015) and in the short term (-11%, 2006-2015).

Red List category of the Yellowhammer:

Europe: Least Concern (IUCN List of Threatended Species, www.iucnredlist.org, 2020) United Kingdom: Threatened (RSPB, www.rspb.org.uk, 2020) France: Vulnerable (UICN, List Rouge des espèces menaces en France, 2016) Netherlands: Least Concern (Rode Lijst, www.sovon.nl, 2017) Belgium (Wallonia): Least Concern (Liste Rouge des oiseaux nicheurs, biodiversite.wallonie.be, 2010) Belgium (Flanders): Least Concern (Red List of Flemish Breeding Birds, www.inbo.be, 2016)

14 2 Objectives

Improving our knowledge about the habitat preferences and territorial behaviour of Yellowhammers is one of the key instruments to adjust the current conservation measures. By improving the quality of their habitat, local populations get the chance to increase and to become more resistant to natural population fluctuations. Only if the current local population grows, expansion to lost areas is possible again. This study will be performed in the West Flemish Heuvelland, characterized by the increasingly rare small-scale landscape harbouring some relict populations of the Yellowhammer.

2.1 Objective 1: Which habitat features determine a preferred Yellowhammer territory?

The first part of this master thesis will focus on the relative importance of habitat and boundary features during the early settlement process. Habitat associations have been studied by several researchers (Bradbury et al. 2000, Whittingham et al. 2009, Burgess et al. 2015, McHugh et al. 2016), but their results may not be directly translated to the situation in West Flanders. Therefore, it is important to investigate our local populations and adjust conservation measures to their specific needs. We did an effort to gain extra knowledge about the preferences of surrounding small landscape elements and land-use that define territory selection.

We will develop a model in which we try to predict which habitat features have an influence on the territory choice. Yellowhammer territories will be compared to a set of random points inside the survey area. Do Yellowhammers build their nest in close proximity of AES-options? Which type of surrounding crops do they prefer? We hypothesize that nearby invertebrate food resources are the main driver of habitat choice (Dunn et al. 2010, Douglas et al. 2012). We expect the presence of a hedgerow to be important as both song perch and nesting site (Bradbury et al. 2000, Stoate et al. 2001, Batáry et al. 2010). Tree lines are also expected to be an important source of song posts (Bradbury et al. 2000).

2.2 Objective 2: Do winter food plots have an influence on early territory settlement by the Yellowhammer?

Fields sown with winter bird crops are an effective conservation measure to help Yellowhammers and other bird species survive the winter. On the first spring-like days at the end of the winter male Yellowhammers leave these winter food crops to search for a suitable breeding territory. But do Yellowhammers settle in closer proximity to the food plots than expected by chance? Are proper territories closest to the plots taken first? Is the difference in distance related to the crop type of the winter food plot? We expect male Yellowhammers to mark their territory near the winter food plots where they mainly reside during winter.

15 2.3 Objective 3: Are Yellowhammer territories spatially clustered?

We will investigate if the distribution of Yellowhammer territories deviates from spatial homogeneity. Does conspecific attraction play a significant role in the distribution of the Yellowhammer? Or does the observed distribution merely reflect a patchy distribution of suitable habitat? The hypothesis is that a Yellowhammer likes to nest at an audible distance from its neighbour (Clarysse 2003). Based on my own observations, we estimate that when the weather is calm, Yellowhammers can hear each other up to 400 meters away.

16 3 Material & methods 3.1 Study site

The study was carried out between the end of February and mid-April 2020 in a region centred around Heuvelland, situated in the Province of West Flanders (Fig. 4). The total study area is approximately 47 square kilometres and ranges from the French border in the west to Dikkebus and Kemmel in the east, and from Reningelst in the north to Dranouter in the south. The region is characterized by mixed agriculture, with fields bounded by ditches, hedges, tree lines, fences or grass margins. Land use consisted of a combination of arable crops (spring- and autumn- sown cereals, legumes, maize Zea mays, beets Beta vulgaris and linseed Linum usitatissimum), hay and silage for winter forage, and pasture grazed by cattle or horses. Spread out, we also find farmsteads, private Figure 4: Map op West Flanders with a frame houses with their gardens, small villages and small around the study area woodlands. Different AES options are present, delivering winter food plots as well as enhanced field margins and hedges. Winter food plots are under management of Natuurpunt, ANB (Agency for Nature and Forests), the Province of West Flanders, RLW (Regional Landscape Westhoek) or local farmers on contract. They were sown with seed-supplying crops like spring or winter wheat (Triticum spp.), bristle oat (Avena strigosa), oil radish (Raphanus sativus subsp. oleiferus) or a seed mixture. Unfortunately, bristle oat failed to set seed this winter and was therefore unable to bridge the hungry gap. The wheat quality was unusually good luckily, and filled that gap.

3.2 Data collection 3.2.1 Colour-ringing

As a first technique to investigate if the early settlement of Yellowhammers is related to the presence of winter food plots, we used colour-ringing. Resightings of colour-ringed individuals allow us to investigate if the Yellowhammers that make use of the food plot during winter, also breed in the nearby area.

Local bird ringers ensured the ringing of Yellowhammers and chose the right ringing sites. They were selected based on the number of Yellowhammers occurring on the site and their practicality for catching birds. Birds were captured by using mist-nets, which are commonly used to capture passerines. The effectiveness of this technique is highly dependent on weather conditions. Rain and wind increase the visibility of the nets, thus making them useless. Only when the conditions were optimal, the mist-nets could be installed. They are best placed against a dark background, e.g. dense, high hedges. Mist-nets are designed in such a way that when a bird flies against the net, it becomes entangled in a loose, baggy pocket of netting created by horizontally strung lines (Sutherland & Green

17 2004). To catch Yellowhammers, we used 9 m and 12 m wide mist nets with 4 shelves and a mesh size of 16.

At each capture, birds were (1) sexed, (2) aged, (3) measured for wing length, (4) measured for weight and (5) given a fat score (according to Svensson, 1992). Aging was done by distinguishing between “young birds” (hatched during 2019) and “adult birds” (hatched prior to 2019) (Table 1). Each individual was marked with 4 different rings: three colour rings and one metal ring (2.8mm). Two colour-rings were attached on the right tarsus and one colour-ring on top of a metal ring were attached on the left tarsus. This combination of colour-ringing was opposite to the one used in the master script of Ostyn (2016), so we could avoid conflicts. The colour rings had an inner diameter of 2.8 mm and a height of 9 mm. In this project, six different colours were used: yellow, orange, red, white, blue and dark green (Fig. 5). Colour combinations that are hard to identify in the field (e.g. white-yellow, red- orange) were excluded.

Figure 5: Scheme of possible colour combinations with "M" referring to the metal ring

The colour-ring on the left tarsus is indicative to the food plot where the bird was captured and ringed. The two colour-rings on the right tarsus allows us to identify each Yellowhammer individually (Fig. 6).

Figure 6: Colour-ringed Yellowhammer Photo: © Stefaan Beydts

18 Due to turbulent weather conditions in February and March, only two ringing events could take place (14 February 2020 and 4 March 2020). A total of 21 individuals have been caught and colour-ringed (Table 1). Volunteers of the monthly simultaneous counts of farmland birds were called on watching out for colour-ringed birds. Nevertheless, resightings of colour-ringed Yellowhammers were scarce. Only two resightings were recorded, each time of the same individual. They are therefore excluded from the statistical analysis.

Table 1: Overview of the Yellowhammers colour-ringed, 14 February 2020 & 4 March 2020

3.2.2 Bird surveys and territory mapping

Fields were surveyed between the end of February and mid-April, following Breeding Bird Monitoring Project (BMP) methodology (Vergeer et al. 2016). Bird surveys were carried out once or twice a week between 8:00 AM and 3:00 PM. Periods of heavy rain, strong wind or poor visibility were avoided, as birds are harder to locate by sight and sound in these conditions. In addition, these adverse conditions will reduce bird activity in general. Routes were reversed between surveys to minimize any effects of time of day. When behaviour indicative of territoriality, typically singing males, was spotted, the Yellowhammer’s exact location was digitally mapped on the mobile app Maps.me. Each possible territory location was at least four times visited. Repeated visits are necessary to distinguish between true and false absences and thus to reduce the likelihood of double-counting territories. To define the locations of breeding territories, observations of Yellowhammers showing territorial behaviour and being less than 150 m apart were aggregated into one territory. If a territory was made up of more than one observation, the centroid of that cluster was calculated using GIS software (QGIS 3.12.0-București) and defined as the core territory. Finally, a circular buffer had to be drawn around each territory centroid. Yellowhammers can forage up to 400 m from the nest, although trips are typically shorter in high quality habitat and take place within 100 m from the nest (Morris et al. 2001, Perkins et al. 2002). Therefore, we chose to generate a 150 m buffer around each territory centre.

With the use of GIS, random points with a minimum distance of 300 m between each other were generated inside the study area. Each random point was given a 150 m buffer zone. When the buffer of a random point overlapped with the buffer of a territory, it was excluded and a new random point was added. Using random points, we can statistically compare the characteristics of occupied territories with characteristics expected under random territory selection (null model).

19 3.2.3 Surrounding habitat features

3.2.3.1 Habitat mapping and data extraction Habitats within 150 m of a territorial Yellowhammer were identified and recorded during the field surveys. Surrounding crop types, boundary features and type of song post were noted. This data was merged with existing data about land-use, AES-options and watercourses, contained in GIS-layers which were derived from Research Institute for Nature and Forest (INBO), the Vlaamse Landmaatschappij (VLM) and the Province of West Flanders. If necessary data was missing, like small landscape elements and margins that are not under AES management, additional habitat surveys were performed and the lacking habitat features were digitally mapped in GIS based on orthophotos and own field observations. Afterwards, surface area and lengths were calculated in GIS. Habitat and boundary features inside the 150 m buffer of each territory and control point were selected in R (R Core Team 2020) using the ‘sf’ package (Pebesma 2018).

3.2.3.2 Selection of habitat predictor variables The extracted data was aggregated into different categories for analysis using the ‘tidyverse’ package (Wickham et al. 2019). Pooling the data is necessary as some variables were too rare to allow statistical analysis. Collinearity between covariates was evaluated using Pearson correlations coefficients displayed in a correlogram, combined with their significance.

1) Crop types

Information about the type of agriculture in our study area was derived from the Vlaamse Landmaatschappij (VLM). It consisted of a digital map containing all crop types sown in 2019 and harvested that same year. Since 48 different crop types were found inside the territory buffers, they were first grouped in 9 more general crop groups (Table 2). Annex 1 in the Appendix shows all selected crop types and their classification into general crop groups, as well as percentages indicating how much of the total study area (47 ha) is covered by each habitat variable. Starchy crops like wheat, barley and triticale were merged into the crop group ‘CEREALS’, since their overwinter stubbles provide an important winter habitat (Gillings et al. 2005, Bright et al. 2014). Under ‘GRASSLAND’ we grouped (1) temporary grass, primarily cut for fodder, (2) pasture, often grazed by livestock and (3) cover crops, used to manage soil erosion or as green manure to increase soil fertility for subsequent crops. Grasslands provide less invertebrate food than the more natural and flower-rich field margins (Perkins et al. 2002, Buckingham et al. 2006) and lack bare open ground due to their denser sward structure (McCracken & Tallowin 2004). Cover crops with leguminous plants may provide an additional benefit by attracting pollinators and natural enemies of crop pests (Lee-Mader 2015), but the most preferred insects like Syrphid and Lepidoptera larvae and Coleoptera are scarce or absent in the dense vegetation structure (Moreby & Stoate 2001, Dunn et al. 2010). Farm buildings and surrounding stables were combined into ‘FARM’, because also non-crop land-uses may have an influence on the abundance of farmland birds (Sirwardena et al. 2012). Lacking information about farms was completed using a GIS-layer derived from INBO and containing all types of buildings. Crop types occurring in less than 10% of the territory buffers were grouped into the variable ‘OTHER’. Since most spring sown fields were fallow during the bird surveys, we regrouped the crop groups for a second time. Crop groups sown in spring (maize, potatoes, legumes, cabbage crops and root crops) were merged in the variable ‘S_FALLOW’ since they serve as a predictor for likely presence of fallow or stubbles. This summarizes the 48 different crop types into five groups which could be used for analysis (Table 4).

20 Table 2: Description of the different crop groups and their total area present inside the territory buffers

Crop group Description Total area (ha) Potato Potato seed and propagating material 78.62 Maize Grain and silage maize 107.81 Legumes S_FALLOW Peas and bean 17.68 Cabbage crops All brassicas 21.30 Root crops Beets, carrots, onions 45.86 Cereals Wheat, barley, triticale 131.02 Grassland Temporary grass, pasture and cover crops (including bristle oat) Farm Isolated farm buildings and stables 7.14 Other Variables with <10% occurrence 10.33

2) Agri-environment scheme options

Maps with agri-environment scheme options were obtained from the VLM and the Province of West Flanders. A total of nine AES options belonging to four different management objectives were present inside the territory buffers. In order to convert this into workable units, the various AES options were grouped into three units, indicating the effects they may have on Yellowhammers (Table 3). As mentioned before, bristle oat failed to set seed this year. Therefore, it could no longer serve as a winter bird crop, so we added it to the variable ‘GRASSLAND’ (Table 2). Oil radish produces oily seeds which are not preferred by Yellowhammers (Perkins et al. 2007). However, regular observations of a few Yellowhammers on oil radish plots were reported during the monthly simultaneous counts of farmland birds. We decided to include oil radish for the analysis

Table 3: Description of the different AES options and their total area present inside the territory buffers

AES option Description Total area (ha) AES hedge Hedges, shrubs and wood edges with restricted cutting timing. 0.57 Provides shelter and nesting sites. AES margin Field margins including all non-cropped margins under any AES 2.69 management; no fertilizers or pesticides. Includes grass buffer strips (mainly constructed to prevent erosion), nectar flower mixture, floristically ‘enhanced’ grass buffer strips. Provides shelter and food. Winter food plot Fields sown with seed-rich crops, including wild bird seed 4.93 mixtures, oil radish, autumn cereals and spring cereals. Provides food during winter.

3) Boundary units

Yellowhammers typically locate their nest along field boundaries (Bradbury et al. 2000). Small landscape elements, ditches and field margins serve as important foraging and breeding sites. Nesting occurs either on the ground in grassy margins or ditches, or in hedges (Stoate & Szczur 2001, McHugh et al. 2013). A territory site is often chosen in close proximity of the male’s favourite song post, like a tree or solitary hedge (Ferguson-Lees et al. 2011, Dochy 2018b). Therefore, all suitable boundary features that are not under AES management were digitally mapped during habitat surveys. Grassy margins between 2 and 6 meters wide were mapped and classified under ‘Non-AES-Margin’. Their area (in hectares) was calculated in GIS. Hedgerows, shrubs and wood edges were summarized under the variable ‘Non-AES-Hedge’. Solitary trees or tree lines, which may act as a song perch, were grouped

21 under ‘TREES’. Information about all types of watercourses was extracted from the Grootschalig Referentiebestand (GRB) derived from INBO. Unnavigable watercourses and ditches were merged into the variable ‘All_Ditch’ (Annex 1 in Appendix). The length (m) of ‘Non-AES-Hedge’, ‘TREES’ and ‘All_Ditch’ was calculated in GIS.

3.3 Statistical analysis

Model constructions were performed in the statistical program R version 3.6.3 (R Core Team 2020). Visualizations were done using the R package ggplot2 (Wickham 2009).

3.3.1 Territory settlement and the relation with surrounding habitat features

3.3.1.1 Exploratory analysis In an exploratory analysis, the relationships between the various variables were visualized using Pearson’s correlation coefficients with multipanel scatter plots displaying both linear and non-linear relationships between the variables. Multicollinearity between the independent variables is also tested by calculating the Variance Inflation Factor (VIF). If the VIF of a predictor variable is higher than 4, we have to be cautious and probably adjust the model. The distribution of each habitat category per treatment is graphically presented using boxplots. However, these visualizations are not sufficient to draw conclusions. Therefore, a multiple regression model which considers the possible effects of all potentially important variables was constructed.

3.3.1.2 Model building We used a Generalized Linear Model (GLM) to develop a model in which we try to determine which habitat features influence the early territory settlement by Yellowhammers. The presence/absence of a territory is the dependent variable and the independent variables are the set of 10 habitat features (Table 4). Since our data is spatially autocorrelated, i.e. values of a variable sampled at locations close to each other are more similar than values taken from more distant locations, we have to correct for this to avoid increasing type I error rates. This correction was done by adding a spatial autocovariate, which is a distance-weighted function of neighbouring response values (Dormann et al. 2007). It will be included as an additional predictor variable in the original model. The autocovariate is calculated with the use of the ’autocov_dist’ function from the ‘spdep’ package (Bivand 2019). Seven variables were in hectares and three were meter, so we rescaled the explanatory variables using the ‘rescale’ function from the ‘arm’ package (Gelman et al. 2016). This function subtracts the mean and divides each numeric variable by two times its standard deviation (Gelman 2008). Since the presence of a territory is either false or true, we used a binomial distribution and logit-link for the binary data. We applied Likelihood ratio tests (LRT), comparing the model with and without the least significant term. The LRT-test follows a c2- distribution under the null hypothesis.

22 3.3.2 Influence of winter food plots on early territory settlement

In order to test whether Yellowhammer territories are situated close to winter food plots, we created distance matrices in GIS. Information of the winter food plots is obtained from a GIS layer derived from the Province of West Flanders. Two different distance matrices were generated, one with the distance of each territory point to its nearest plot and one with distances to the three nearest plots of each territory. Distances to the nearest plot are the smallest distance between the centroid of the winter food plot and the territory. Winter food plots were sown with seed-supplying crops for a variety of farmland birds. Each crop type used for analysis was defined based on its main component. We distinguish the two following crop types: (1) cereal grain (spring or winter wheat and barley) and (2) oil radish (oily seeds, not favoured by Yellowhammers).

For analysis of the influence of the nearest winter food plot on territory settlement, we used a Generalized Linear Model (GLM) with distance as dependent variable and treatment (Yellowhammer or control), crop type and an interaction between them as independent variables. A spatial autocovariate was added to control for spatial autocorrelation.

A Linear Mixed Model (LMM) was used to analyse the influence of the three nearest plots. LMMs were constructed using the ‘lmer’ function within the ‘lme4’ package (Bates et al. 2015). Fixed effects are treatment, crop type and an interaction between them. Territory identity is included as a random effect. A spatial autocovariate was also included. To calculate the significance levels of fixed effects, we used the Satterthwaite procedure, which immediately corrects the degrees of freedom in an F-test.

Both models followed a Gaussian distribution and used Likelihood ratio tests (LRT) during model selection. Distances were log transformed to meet the normality assumptions.

3.3.3 Patterns of territory distribution

We want to determine whether or not Yellowhammer territories are randomly distributed throughout our study area. To obtain an estimation of the degree of clustering behaviour, we want to test whether the observed population distribution significantly deviates from complete spatial randomness. Besides our collected data, we used historical inventory data on distributions of Yellowhammers from 2003, 2004, 2005 and 2016. If the population is randomly distributed, each subregion of the study area (Fig. 7a) will have the same amount of territories, which can be estimated as the mean territory density multiplied by the subregion area. We call this a homogeneous Poisson process and this will act as the null model in our statistical analysis (Kingman 2005).

Using Ripley's K function1 (Ripley 1977), you can determine whether points have a random, dispersed or clustered distribution pattern at a certain scale. This method is based on the average number of points (here: territories) that can be found within a distance r of a randomly chosen point. The results are plotted in function of the distance r. Besides this, Monte Carlo simulations of the same amount of points under complete spatial randomness2 are plotted and serve as a significance envelope (Besag & Diggle 1977). The more simulations performed, the narrower this significance band becomes. The null

1 The formula of Ripley’s K function is !(#) = '()* , with r = distance, l = density (number of observations divided by study area), and E = number of extra events within distance r of a randomly chosen event. 2 Under complete spatial randomness, the K function is calculated as !(#) = +#,

23 hypothesis of complete spatial randomness is rejected as soon as the Ripley’s K function of our point patterns lies outside the significance envelope.

For our analyses we used the closely related L-function3, in which the variance is stabilized and therefore allows more reliable inferences. Under complete spatial randomness, L(r) = r for all r. Values of L(r) < r indicate a regular point pattern, whereas values of L(r) > r point to a spatially clustered distribution pattern. Significance bands are plotted using global envelopes (Myllymäki et al. 2017). Just like the Ripley’s K function, any excursion of the observed L-function outside the significance bands can be interpreted as rejection of the null hypothesis at the 0.01-significance level.

To draw correct conclusions, we have to remove unsuitable breeding habitat from our study area. If not, the observed clustering could be caused by the clustered distribution of suitable habitat. Yellowhammers are adapted to a small-scaled farmland full of small landscape elements (Dochy 2014) and thus avoid residential areas and broad-leaved woodland as breeding locations. They were excluded using GIS, leaving 4100 hectares (41 km2) of not trivially unsuitable breeding habitat in our study area (Fig. 7b).

In addition, we conducted a nearest neighbour analysis (Clark & Evans 1954) of the significant habitat variables in GIS to infer their distribution pattern inside our study area. A nearest neighbour analysis is part of the analysis tools in QGIS (QGIS 3.12.0-București). The mean of the distance observed between each point and its nearest neighbour is compared with the expected mean distance that would occur if the distribution were random. The nearest neighbour formula4 will produce a result between 0 (clustered) and 2.15 (regular). A value of 1 indicates a random distribution.

For each inventory year as well as all years together, we plotted L-functions using the ‘spatstat’ package of R (Baddeley & Turner 2005). We tested the outcome of the observed L-function against a model of complete spatial randomness that incorporated the irregular form of our study area.

a b

Figure 7: Schematic representation of the study area with indication of unsuitable habitat types for Yellowhammers. 7a: With indication of the occupied territories ranked per year and the division in subregions. 7b: The study area with exclusion of unsuitable habitat as it was imported for the statistical analysis.

3 The L-function is calculated as -(#) = .!(#)/+

23(456) 4 0 = 0 =3(>?@) The nearest neighbour formula is 1 ; where 1 = nearest neighbour value, = mean observed 7.9: < nearest neighbour distance, A = area under study, B = total number of points

24 4 Results 4.1 Yellowhammer territories

A total of 58 different song posts were recorded during the bird surveys. They were merged with the 37 observations of Kristof Goemaere, an excellent local birdwatcher who investigated an overlapping part of our study area, commissioned by the Flemish breeding bird atlas 2020-2023. This led to a total of 95 song posts inside the study area. After clustering, 40 different Yellowhammer territories could be identified within the study area (Fig. 8). The same amount of random points was generated using GIS.

Figure 8: Map of the study area showing all territories used for analysis: 40 Yellowhammer territories (red) and 40 control territories (blue)

4.2 Data exploration

Figure 9 shows the correlogram between all possible habitat variables. ‘Non-AES-Hedge’ and ‘TREE’ show a moderate positive correlation (r=0.44). We did not merge them because Yellowhammers do not breed in trees. ‘Non-AES-Hedge’ also shows a positive relation with ‘Non-AES-Margin’ (r=0.26), ‘AES-Hedge’ (r=0.35) and ‘GRASSLAND’ (r=0.41) and a negative relation with ‘S_FALLOW’ (r=-0.23) and ‘CEREALS’ (r=-0.23). The positive correlation with ‘AES-Hedge’ is not surprising since they are both hedges. We merged them into the variable ‘HEDGES’, because we cannot tell their effect apart. The strongest negative correlation can be observed between ‘GRASSLAND’ and ‘S_FALLOW’ (=-0.45). ‘AES-Margin’ was found in only 10% of the territories, which would be too small for statistical analysis. Therefore, we merged it with ‘Non-AES-Margin’ into the variable ‘MARGIN’. By regrouping the data, we reduced the number of predictor variables from 64 to 10 (Table 4 & Annex 1).

25

Figure 9: Correlogram showing the correlation ‘r’ between all possible habitat features. Significant correlation coefficients (p<0.05) are coloured according to their value.

Table 4: Summary of variables with descriptions used for analysis. Unit: hectare = ha, meter = m

Variable Description Type Unit Treatment Presence/absence of territory Response variable 1/0 (Yellowhammer/control) (binary) CEREALS Spring and autumn sown cereal crop types Continuous ha GRASSLAND Temporary grass, pasture and cover crops Continuous ha S_FALLOW Fallow land Continuous ha OTHER Variables with <10% occurrence Continuous ha FARM Farms buildings and stables Continuous ha MARGIN (Non-) AES-managed field margins Continuous ha Winter_Food_Plot AES field sown with seed-rich crops Continuous ha HEDGE (Non-) AES-managed hedges Continuous m All_Ditch All type watercourses Continuous m TREE Large trees (> 5m) (row or solitary) Continuous m

Pearson’s correlation coefficients between the final covariates showed us that the correlation between them was low to intermediate (varying between -0.45 and 0.44) (Annex 2 in Appendix). The highest correlation was found between trees and hedges (r=0.44). This should be taken into account in the conclusions regarding the analysis, because correlated variables can mask or enhance each other's effect. Trees and hedges seem to be positively correlated with both grassland (r=0.36, r=0.41) and negatively correlated with fallow land (r=-0.23, r=-0.23). This is not surprising, considering the negative correlation between grassland and fallow land (r=-0.45). We can also observe another positive

26 relationship for hedges with margins (r=0.31). Cereals are positively correlated with ditches (r=0.24), but negatively with hedges (r=-0.23). In general, the correlations remain fairly limited and will have no major impact on the model. VIF values ranged between 1.000 and 3.235, meaning there is no problematic multicollinearity between the independent variables.

Figure 10 shows a boxplot of the distribution of the habitat variables between both treatments. Their presence seems to be (slightly) higher in Yellowhammer territories, except from ‘FARM’. The variables ‘All-Ditch’, ‘HEDGE’ and ‘TREE’ are expressed in meter and show the distribution of their length for both treatments. The other variables are expressed as the proportion present inside a territory buffer, which is calculated by dividing their area (in hectares) by the area of a territory buffer (= 7.065 ha).

Figure 10: Boxplot of the distribution of each habitat category per treatment. Unit of All_Ditch, HEDGE & TREE = meter; unit of rest = percentage of total territory area

4.3 Territory settlement in relation to habitat variables

The GLM comparing Yellowhammer territory sites to unoccupied random sites showed significantly non-random selection of habitats within territories. According to Likelihood ratio tests, the variation in territory selection was explained by a model including three habitat variables (Table 5). A positive relationship between Yellowhammer occurrence and the extent of hedges (c2(1)= 18.906, p<0.001), ditches (c2(1)= 15.741, p<0.001), margins (c2(1)= 10.163, p=0.001) was significant in our model. All other habitat variables were not significantly higher in either occupied territories or at control points. The autocovariate is highly significant, which indicates strong spatial autocorrelation among the territories in our study area.

27 Table 5: Results of backward elimination using Likelihood ratio test. P-values with an asterisk indicate significance. Degrees of freedom was always 1.

Step LR (Chi2) Pr (>Chi2) 1 - Other 0.014 0.905 2 - Cereals 0.227 0.633 3 - Grassland 1.101 0.294 4 - Farm 0.932 0.334 5 - Winter food plot 2.212 0.137 6 - S_Fallow 1.653 0.198 7 - Tree 1.670 0.196 8 Margin 10.163 0.0014 ** Ditch 15.741 <0.001 *** Hedge 18.906 <0.001 *** Autocovariate 20.318 <0.001 ***

Parameter estimates of the significant variables are shown in table 7. The probability of territory occupations will be larger with increasing length of hedges and ditches and increasing area of field margins (Fig. 11)

Table 6: Parameter estimates (logit scaled) of the Generalized Linear Model comparing unoccupied and occupied Yellowhammer territories.

Estimate Std. Error Intercept - 0.013 0.688 Margin 7.433 3.197 Ditch 5.241 1.740 Hedge 5.273 1.765

28

Figure 11: Effect of habitat variable on Yellowhammer territory occupation. The vertical axis is labelled on the probability scale. Black indicates the average length/area. Grey indicates the standard error.

4.4 Influence of winter food plots on early territory settlement

Figure 12 shows histograms of the difference in distance to the nearest plot (left) or three plots (right) between Yellowhammer territories and random territories. Vertical lines refer to the mean distance to a plot. The exact value of each mean distance and its standard deviation is given in table 8.

a b

Figure 12: Histogram for each crop type of distances between a territory and (a) the nearest winter food plot or (b) the three nearest winter food plots

29 Table 8: Mean distance (m) to the nearest and three nearest winter food plots

YELLOWHAMMER CONTROL

Cereals 467 ± 342 738 ± 378 NEAREST PLOT Oil radish 446 ± 322 450 ± 226

THREE Cereals 773 ± 416 1036 ± 473 NEAREST Oil radish 651 ± 289 810 ± 284 PLOTS

4.4.1 Nearest winter food plot

The results of our GLM for the difference in distance to the nearest winter food plot showed a significant effect of treatment (c2(1)=16.94, p<0.001), Yellowhammer territories are closer to the winter food plots than random control points. Both crop type (c2(1)=0.12, p=0.728) and its interaction with treatment (c2(1)=1.84, p=0.175) had no significant effect on distance.

4.4.2 Three nearest winter food plots

We subsequently modelled the mean distance of the three nearest plots between the two treatments. According to the results of our LMM, treatment has a significant effect on the mean distance (F=25.42, p<0.001). No significant effect was found for crop type (F=0.19, p=0.656) and the interaction term (F=0.023, p=0.878).

4.5 Territory distribution

L-functions of all inventory years are shown in Figure 13. These L-functions allow us to infer the type of population distribution as being random, clustered or dispersed at different spatial scales. For each graph, at three different distances (r=500, r=1000 and r=1500) we looked for evidence against complete spatial randomness. The results are presented in Table 9 and show that in none of the years, the population distribution was regular. Yellowhammer territories are randomly distributed in 2003, 2004 and 2005, whereas in 2016 and 2020, the overall distribution patterns were found to be clustered. When evaluating all observations over the years, clear clustered distribution patterns were found. However, we have to be aware of a certain bias caused by the fact that the same territory locations may be used more than one year.

According to the nearest neighbour analysis, hedges (Rn=0.53) and margins (Rn=0.61) are neither fully clustered nor completely random. Yet, they tend more towards a random distribution than a clustered distribution. Ditches on the other hand exhibit a more or less random distribution (Rn=1.21).

30 Table 9: Summary of the results from the L-functions for all inventory years at three different scales. C = clustered distribution of Yellowhammer territories, R = random distribution of Yellowhammer territories

Year r = 500 r = 1000 r = 1500 2003 R R R 2004 R R R 2005 R R C 2016 C C C 2020 C C C All C C C

Figure 13: L-function for the distribution patterns of each year. When Lobs(r) falls outside the significance band (shaded and demarcated by Lhi(r) and Llo(r)), H0 is rejected. If Lobs(r) lies above the significance band, the pattern is clustered. If Lobs(r) lies under the significance band, the pattern is regular.

31 5 Discussion 5.1 General

For the implementation of effective conservation measures, it is crucial to determine species-specific habitat associations and behaviour. By the comparison of occupied and unoccupied habitat patches at the territory scale, we aimed to investigate which habitat features are of importance for Yellowhammers and need to be protected from further degradation. Our results suggest habitat preference by Yellowhammer for areas with a higher proportion of hedges, ditches and margins. We can thus conclude that field boundary features and nearby grassy margins play the most important role in the early settlement process. They provide the two most crucial requirements to guarantee successful breeding: a safe nesting place and invertebrate-rich foraging habitat. Pooling the data made it impossible to set the effect of AES-managed habitat features apart from the effect of non-AES- managed ones.

Our data indicates that Yellowhammers settle in closer proximity to winter food plots than expected by chance. This difference was not related to the crop type of the plots (cereal grains or oil radish) or an interaction between crop type and territory occupation, although this could be the result of the small sample size of oil radish plots. Unfortunately, we could not collect enough resightings of colour- ringed Yellowhammers to determine how territory settlement proceeds through spring.

Analysis of territory distribution confirms that in none of the inventory years Yellowhammer territories were regularly distributed. In 2003 and 2004, territories were randomly distributed. In 2005, clustering appeared from 1300 meters onwards, although this is unlikely to be caused by conspecific attraction. Instead, our results indicate clustering to be present from 400 - 500 meters onwards for the territories in 2016 and 2020. This distance is higher than we hypothesized, but we believe that Yellowhammer males do not need to hear each other all the time in order to form territorial aggregations. In general, suitable habitat showed a random distribution and thus could not explain the clustered territory distribution (Annex 3 in Appendix). We were not able to get a clear view of the pattern of territory settlement and therefore we could not make clear inferences about the possible mechanisms driving conspecific attraction.

5.2 Habitat features determining territory selection

In line with the hypothesis, we found that boundary features typical to small-scale agriculture have the most significant impact on territory selection. More specifically, territory selection by the Yellowhammer in West Flanders was influenced by the extent of hedges, margins and ditches. Our results correspond to previously found insights into the nesting and foraging preferences of Yellowhammers (Bradbury et al. 2000, Stoate et al. 2001, Whittingham et al. 2005, Goodwin et al. 2013, McHugh et al. 2016). Yellowhammers preferably build their nest in the dense vegetation along field boundaries, either on or close to the ground. Those field edge habitats comprise a combination of features including the shrubby vegetation of the hedge, annual or perennial herbaceous vegetation at the hedge base, and often a ditch. Foraging is preferred in invertebrate-rich patches with a large edge-to-area ratio which increases the accessibility to prey (Atkinson et al. 2005). Field margins and well-vegetated banks serve as suitable foraging habitats, especially early in the breeding season.

32 5.2.1 Hedges

The presence of a hedge showed a strong significant association with territory occupation in our model. According to Lack (1992), the maximum abundance of species and numbers of birds is achieved at a hedge density of 60-80m/ha. This optimum equals the hedge length in our results where territory occupancy is maximum (Fig. 11). Next to a safe nesting site, hedges also provide physical shelter and roost sites (Hinsley & Bellamy 2000, Herzog et al. 2005). The importance of a hedgerow agrees with earlier research about breeding habitat preferences (Batary et al. 2010, Dunn et al. 2010, McHugh et al. 2013). According to previous studies of Yellowhammers, hedgerow presence was linked to an increased breeding population density at both local and national scales (Bradbury et al. 2000, Whittingham et al. 2005). However, a lot depends on the structural complexity of a hedgerow and its associated habitat. Hedge size (length, height, width) and the presence/abundance of arboreal vegetation in the hedge have been shown to positively affect bird diversity (Hinsley & Bellamy 2000, Batary et al. 2010). The optimal hedges for Yellowhammers are rather low (1.40 m) and wide (1.20 m) and are not too neatly trimmed (Lack, 1992). McHugh et al. (2016) observed Yellowhammers to have a strong preference for hedges that had been cut between January and March. They believe that frequently cut hedges develop closely interlinked branches which reduce the incidence of predation when nesting or roosting. In addition, high contrast between the flat platform of cut hedges and a nearby song post strengthens the visibility of Yellowhammer males when singing. But largescale and synchronized cutting of hedges should be avoided at all time to maintain the avifauna of agricultural landscapes. Thoughtful hedge management is crucial to meet the requirements of both nature conservationists and land-users (Ferrarini et al. 2013). As long as hedge management is based on the best knowledge available, the ecological and economic value (e.g. bioenergy production) of hedgerows can be united (Sauerbrei et al. 2017).

Hedgerows also act as important linear landscape elements providing safe cover for both local and larger-scale movements (Hinsley & Bellamy 2000, Clarysse 2003). In a fragmented landscape, hedgerows may facilitate birds to access new habitats or foraging resources which would otherwise be too risky or too far away to use.

Predation risk to foraging adults plays an important part in habitat selection. Yellowhammers prefer foraging sites with lower vegetation densities, since this enhances detection of predators, like Sparrowhawks (Accipiter nisus) (Anderson 2014). As important is the proximity of physical structures to seek cover and protect them from attack. Hedgerows offer ideal protection from predation and Yellowhammers have shown to select foraging areas close to hedges (Whittingham & Evans 2004). In addition, the hedge base itself, and particularly the tussocky grasses, are also used as foraging habitat. The woody component and hedge base are relatively stable microclimates and are largely protected from agrochemical inputs. They can provide shelter for invertebrates and are especially important for those overwintering (Dover 2019).

Prior to breeding, Yellowhammer males need an appropriate song post to display themselves and attract a mate. A noticeable hedge may serve as a suitable song perch, although trees are also popular. Contrary to the hypothesized association, our model did not find a significant difference in the length of tree-lines between occupied and unoccupied sites. This was consistent with Bradbury et al. (2000), although other authors did find the presence of trees to be important in determining Yellowhammers settlement patterns (Whittingham et al. 2005, Siriwardena et al. 2012). Since there is a strong correlation between the presence of trees and hedges, it is impossible to determine whether one or both are important. During the bird surveys, Yellowhammer males were often seen singing and displaying themselves in a tree. Especially shrubby hedges with trees were most desired (pers. obs.).

33 We believe that the presence of a suitable song post in general is more important than the type of song post. Eye-catching terrain elements such as isolated trees, bushes, wires or wooden posts are liked to be used as a strategic lookout or song perch.

5.2.2 Field margins

The positive relation between Yellowhammer territories and margins might imply that territory site selection is influenced by the quantity of nearby spring and summer foraging habitat. Yellowhammers are known to forage in grass field margins (Perkins et al. 2002, Douglas et al. 2009, Dunn et al. 2010, Anderson 2014). Other studies also found a positive relationship between foraging habitat and territory selection of Yellowhammers and other farmland birds. Both Burgess et al. (2015) and McHugh et al. (2016) found that Yellowhammers were more likely to locate territories in areas containing enhanced margins. In the Czech Republic, Salec et al. (2019) found that Ortolan Bunting (Emberiza hortulana) territories were associated with a larger area of intermediately vegetated patches and the presence of bare ground. Cirl Bunting (Emberiza cirlus) territories in the UK were positively correlated with scrubs and rough grassland (Stevens et al. 2002).

Unlike grassland, which did not determine territory selection in our model, field margins have a more heterogeneous vegetation and less dense sward structure. The unsuitability of grassland (both pasture and temporary grass ley in our model) is the consequence of extensive intensification including increased grazing pressure, mowing frequency and fertilisation, and reseeding after ploughing (Gossner et al. 2016). These activities result in a simplified sward structure offering limited foraging and shelter opportunities for many invertebrates (Plantureux et al. 2005). The reduced fertiliser and spray drift into field boundaries is likely to help maintain the perennial field margin vegetation hosting higher abundances of invertebrates which are crucial as chick food (Perkins et al. 2000, Hof & Bright 2010, McHugh et al. 2013). Defining the diversity of insect orders living in margins is beyond the scope of this study. Previous research demonstrated margins as having a beneficial impact on Araneae (Clough et al. 2005), Symphyta larvae (Cole et al. 2012) and Carabidae (Cole et al. 2008). These insect orders or families are known to be part of Yellowhammer’s diet (Buckingham 2005, Macleod et al. 2005, Douglas et al. 2012)

Margins are especially important as food resource early in the breeding season. According to Douglas et al. (2009) their suitability as a foraging habitat can decrease in favour of crops. The seasonal increase in vegetation height in margins may reduce food accessibility and consequentially a decline in margin use in late summer. From mid-June crops become well established and arthropods migrate into the field from overwinter refuges in the boundaries (Anderson 2014). The row pattern in which the crops were sown ensures easy accessibility. At the end of the breeding season, cereal fields are more frequently visited to collect semi-ripe grain as additional food for both adults and nestlings (Stoate et al. 1998, Morris et al. 2004). In general, good quality habitat close to the nesting site is important to reduce the energetic costs of foraging excursions to parents and reducing the time nests are left unattended (Anderson 2014).

34 5.2.3 Ditches

An interesting result from our analysis is the positive association between Yellowhammer territories and the presence of a ditch. It confirms existing theories and observations about the settlement of territories along streams in West Flanders (Dochy 2018a). Since Yellowhammer is no water-loving species by nature, there is rather an indirect link between the presence of ditches and territory occupation. In the fast decaying mixed arable landscape, ditches are still flanked by small landscape elements like hedges, scrubs and trees. Typical trees alongside ditches in farmland are pollard willows which serve as suitable song posts and are rich in insects from early spring onwards. Vegetated banks provide an ideal nesting habitat, well hidden from predators and in close proximity of suitable foraging habitat. The rough herbage along a ditch acts as refugia for invertebrates, and might be especially of importance in dry periods.

In pasture, trampled ground by livestock contrasts with the well-vegetated banks (Anderson 2014). This bare earth may lead to an increase in accessibility of invertebrate prey hiding in the structurally complex banks, and so increasing the suitability of ditches as foraging habitat. Ditch vegetation alongside arable fields may also be more biodiverse and containing more food. The extreme edges of silage fields alongside ditches, but also hedges, may be unable to be harvested as they are less accessible to machinery. As a result, seed heads get the chance to develop and set seed (Atkinson et al. 2005). In addition, less pesticides might be sprayed round the edges of a crop (Moonen & Marshall 2001). Both mechanisms have the potential to increase the availability of food resources and hence making field boundaries in general more attractive to Yellowhammers.

5.2.4 Methodological remarks on determining habitat preferences

We were unable to detect a possible preference for AES-options. The management of AES-hedges was focussed on the maintenance of already existing hedges, which explains its correlation with hedges which are not under AES-management. AES-managed margins on the other hand were too scarce inside the territory buffers to include them as a separate explanatory variable.

A radius of 300m is often used to define territory buffers (Stoate et al. 1998, Morris et al. 2001, Goodwin et al. 2013). However, these studies usually examine Yellowhammer behaviour during the whole breeding season. Prior to nesting, the size of avian breeding territories is typically smaller (Møller 1990). Since we have studied Yellowhammer’s early territory preferences, we chose to use a radius of 150m.

35 5.3 Influence of winter food plots on early territory settlement

We found that Yellowhammer males mark their territory closer to winter food plots than expected by chance. We believe that fields with winter food also provide an increased supply of food during the breeding season. Fewer pesticides are usually used on winter food plots, which increases the supply of insects and seeds from field herbs in the spring and summer period (Moreby & Southway 1999, Lambrechts et al., 2007). However, according to the rules of this AES-option, the farmer is free to harvest or plough its field from the 15th of March. The subsequent land management like cropping, pesticide use, etc., is completely dependent on the farmer’s will. Elsewhere it was also observed that in spring more territories were occupied near wintering habitat. In England, Whittingham et al. (2005) found that winter habitats play an important role in determining where Yellowhammers locate their territories in spring. In Flemish Brabant, where core populations of Yellowhammer still occur, Lewylle & Veraghtert (2010) discovered a significant correlation between the number of territories and the area of winter food in a 25 km2 grid. More territories were observed as the surface area of winter food increased. Moreover, plots with overwinter cereals seemed to create a ‘pull effect'. In the immediate vicinity of plots with winter food high numbers of territories were observed, both around new and older winter food plots. In our study area, Dochy (2018a) noticed in 2016 a slight population increase in the areas where annual winter food plots were located. New territory settlements were not necessarily located on the plots themselves, but spread over a wider area within approximately 1 kilometre.

Despite the fact that Yellowhammers prefer cereal crops over oil radish (Holland et al. 2006, Kleijn et al. 2014), we did not find a significant difference in territory distance between these two crop types. The smaller sample size of oil radish plots could have accounted for an undetected difference. Another possible explanation might be a high variability in plot quality. Unfortunately, we could not consider this since we lack detailed information about the specific crop type of each plot and its evolution of food quality and quantity throughout the winter. The quality and quantity of the winter food provided can vary greatly with plant type, subsequent management and across space and time (Vickery et al. 2009, Hinsley et al, 2010). In order to provide sufficient food until the end of the winter, it is important that many seeds remain of good quality for as long as possible. The success of a winter bird crop is determined by many factors, e.g. the amount of shade, soil moisture, weather (heavy rain and wind cause lodging which accelerates rotting), etc. Quantity of seeds differs with crop type. Summer cereals ripen later than winter cereals and can theoretically provide food until the end of the winter. Some crop types such as buckwheat (Fagopyrum esculentum) and rapeseed (Brassica napus) drop their seeds at the end of the summer, making them unavailable to farmland birds during the winter period, whereas bristle oat (Avena strigosa) can provide enough seed to cover the hungry gap if sown before the 1st of August (Kleijn et al. 2014, Coelembier et al. 2015). Damage due to other species (e.g. common pheasant (Phasianus colchicus), roe deer (Capreolus capreolus), brown rat (Rattus norvegicus), mice, etc.) can dramatically alter the food quantity. To conclude, plot quality at the end of the winter period could be more explanatory than the crop type itself.

We have to note that the locations of winter food plots have already been pre-selected to be as effective as possible. Therefore, winter food plots have been constructed within a 'winter perimeter' of 2 km around the known breeding territories of the past five years (Dochy 2018a). Our study area was completely imbedded inside the winter perimeter, so we would expect that the selective placement of winter food plots does not have influenced our results. However, when we look closely to distribution of winter food plots and other habitat variables (Annex 3 in Appendix), it seems that winter food plots are mainly situated in a zone with a higher proportion of suitable habitat features

36 like hedges. We did not specifically test for a correlation between winter food plots and surrounding breeding habitat, although AES-hedges showed a weak correlation with winter food plots (Fig. 9). It is therefore not excluded that the location of Yellowhammer territories in itself is an artifact.

We recommend further studies to take plot quality into account and to correct for a possible correlation between winter food plots and surrounding habitat features. Large-scale colour-ringing of birds and field observations by dedicated birdwatchers are necessary to determine if birds settle first or closer to the winter food plot where they mainly resided. We acknowledge that this is a labour- intensive method. Another method can be tracking of tagged individuals, although this depends heavily on the available technology.

5.4 Clustered territory distribution

In a heterogeneous environment, like real life, species will strive to select breeding habitats with optimal conditions. The spatial distribution of a territorial bird species, like the Yellowhammer, is subject to both first- and second-order effects. First-order effects are variations in abundance resulting from variabilities in habitat suitability. By contrast, second-order effects refer to patterns that originate from interactions between individuals, such as competition or social behaviour (Johnson 1980, Cornulier et al. 2006). The observed distribution of a species will ultimately be the result of a combination between these first- and second-order effects (Scott & Lee 2013).

First-order effects are rather easy to investigate and therefore they are often the only effects considered in habitat selection studies. If only first-order effects are included, habitat quality is the driving force behind the difference in fitness between individuals. This is assumed to be negatively density-dependent, meaning that a high territory density will negatively influence the reproduction success and population growth due to an increased competition for resources between conspecifics. Negative density-dependent factors are found to be present and to some extent important in Yellowhammer populations. Dunn et al. (2015) showed that higher territory densities in anthropogenically modified environments result in lower fledging success, parental provisioning rates and nestling growth rates. Despite the existence of negative density-dependency, we found in two of the five investigated years (2016 and 2020) a clear deviation of complete spatial randomness in favour of clustered distribution patterns of territories in our study area (Fig. 13, Table 9). This is the first time that clustering of Yellowhammer territories is found. Attraction between conspecifics was already found for closely related sister species like Corn Bunting (Emberiza calandra) (Sanza et al. 2012) and mainly Ortolan Bunting (Emberiza hortulana) (Vepsäläinen et al. 2007, Berg 2008, Dale & Steifetten 2011). For the territory distribution pattern of 2020, we have another strong argument in favour of clustering, namely the highly significant spatial autocovariate in our habitat preference model (Table 5). Its significance means that the result of our test is strongly determined by nearby points. These nearby points have a higher chance to give similar results, meaning a higher probability of adjacent points to be all occupied or unoccupied, which in turn implies that territories are clustered. In this section we were particularly interested in to what extent conspecific attraction is important in determining the distribution patterns of Yellowhammers. We made use of the L-function because this method allows to exclude unsuitable habitat. The L-function assumes first-order effects to be stationary (Ripley 1977). Since we only have habitat data of 2020, the obtained distribution patterns of 2003-2016 are hard to interpret. Under stationary first-order effects, each part of the study area is assumed to have an equally likely probability on being colonised by Yellowhammers. In consequence, the observed distribution pattern is independent of habitat availability and has to be the result of

37 second-order effects such as conspecific attraction. However, we only performed a rather rough exclusion of unsuitable habitat. Since we added no additional information in our analysis, it could be possible that at least a part of the observed clustered distribution can be explained by other parameters, such as the presence of a suitable song post or field margins to forage. We have tried to refute this by performing a nearest neighbour analysis of the three significant variables (hedges, ditches and field margins) accounting for territory selection. They all tended more towards a fully random distribution and therefore cannot explain the clustered territory distribution (Annex 3 in Appendix). However, to be able to fully omit the effect of habitat suitability, more detailed knowledge on habitat quality in the study area and its evolution over multiple years is required. To estimate the impact of conspecific attraction more precisely, you have to examine the pattern of settlement in their breeding areas and investigate whether the probability of occupation of a certain patch is higher if the neighbouring patch is already occupied, or whether the occupation of territories through time is random.

According to the L-functions, clustering appeared from 400 meters onwards in 2016 and from 500 meters onwards in 2020. This is a further distance than we hypothesized, but we don't rule out that Yellowhammers can hear each other up to 500 meters on very calm mornings and evenings. There is no literature available about the hearing range of Yellowhammers, neither of other bunting species. We believe that new settlers are attracted by the songs of conspecifics and preferentially settle in proximity to established territory owners, forming territorial aggregations (Stamp 1988, Muller et al. 1997). It might be enough that Yellowhammer males only hear each other once in a while and not every day.

The L-function of all territory locations occupied in the inventory years shows a very strong degree of clustering. A possible explanation might be that Yellowhammers prefer a traditional location as territory. Yellowhammer is largely sedentary in Europe, wintering in the same general areas as occupied in the breeding season (Bradbury et al. 2000, Whittingham et al. 2005, Dochy 2018a). Only the northernmost part of their range is vacated in winter (Byers et al. 2013). During the breeding season the couples are strongly territorial. The connection of a Yellowhammer male with its traditional territory could be greater than the current suitability. However, a traditional territory does not need to be taken by the same bird every year. Sons or former neighbours can also settle in vacant traditional territories where they remembered a male singing in previous years. This theory has already been formulated by Dochy (2014), but no strong evidence has been found ever since. It requires extensive and detailed information on territory locations of consecutive years. This information is as yet unavailable. Annual inventory, as was done by Vepsäläinen et al. (2007) for Ortolan Bunting, is necessary to verify the existence of traditional territory sites in Yellowhammer populations.

Not for all years a clustered distribution of Yellowhammer territories was observed though (Fig. 13, Table 9). In 2005, clustering appeared from 1300 meters onwards, although this is unlikely to be caused by conspecific attraction. We believe that the absence of clustering can be attributed to the lower population size. After their population crash, Yellowhammer breeding abundances have been gently rising since the start of protection programs in West Flanders in 2002 (Dochy 2018a). It is possible that in the early days (2003, 2004, 2005) Yellowhammers did not manage to hear each other due to their lower population size (and hence population density). Alternatively, due to unknown density- dependent processes, it is possible that the benefits of population aggregation only outweigh the costs once a certain number of individuals are present. This could be the case for extra-pair copulations, but requires detailed research of the genetic similarity between nestlings and the neighbouring males.

38 Overall, we cannot make clear inferences about the mechanisms behind conspecific attraction. This requires intense follow-up of the pattern of settlement during the breeding season. We did not manage to perform these labour-intensive surveys and therefore recommend it for further research.

5.4.1 Methodological remarks on cluster analysis

When performing the cluster analysis, we exclusively focused on the territories inside our study area. However, when we look at the location of the territories on a map, it is striking that the majority of the territories are in a long band along the French border (Annex 3 in Appendix). This is contrary to scattered groups of territories we would expect under clustering. This longitudinal pattern can have arisen by the distribution of suitable habitat features. Despite hedges and margin tend toward a random distribution, they seem to have a higher concentration alongside the French border. The same goes for winter food plots. Furthermore, it is very likely that the population in West Flanders is the edge of a bigger French Yellowhammer population. In France, they are still widespread, although the population trend is also declining (Beaudoin et al. 2019). Unfortunately, there is no largescale inventory of Yellowhammer territories in France. The most nearby study was conducted in Eecke, about 10 km from our study area. They found a density of 1.3 territories/km2, which is high compared with other regions in France (Beaudoin et al. 2019). We cannot distinguish if the Flemish territories along the French border are still present because of clustering and/or site fidelity or because of the higher proportion of suitable habitat. This requires cross-border research.

5.5 Implications for management

The reduction of habitat diversity in agriculture is a major ‘overarching’ mechanism driving farmland bird population declines. An increased knowledge of the preferred breeding habitat features of Yellowhammers in a small-scale landscape has clear implications for conservation prescriptions. Our results suggest that conservationists wishing to enhance local populations of Yellowhammers should provide suitable habitats from the start of the breeding season. Field margins should be managed in conjunction with adjacent boundary features, especially well-managed hedges with hedgerow trees and a tussocky vegetation base. They both create complex structures that maximise safe nesting opportunities for Yellowhammers and create habitats for a range of invertebrates. Field margins and hedges are already included in agri-environment scheme options. Ditches on the contrary are not yet protected. We recommend to include ditches, flanked by small landscape elements and well-vegetated banks, as AES-option since they are proven to be one of the selection criteria in Yellowhammer’s territory choice. To encourage more birds to breed during the summer, also the amount and proximity of preferred wintering habitats should be considered. We recommend to improve the environment in a radius of 500 meters around the existing territories. This distance corresponds to the scale on which territory clustering occurs, so eventually a connected area of suitable habitat will arise. Farmers are an important factor in conservation management and need to be sensitized and stimulated to implement AES-options. At least, these measures are not only of benefit to Yellowhammers, but also to a wide range of farmland species, including many of conservation concern.

39 5.6 General proposal for future research

In order to asses inter-annual variation in the selection of territories by Yellowhammers, there is need for a broader study that extends to multiple years. We recommend the use of Bayesian statistics to account for spatial data and a small sample size. Annual inventory of territory locations may give a decisive answer about a possible preference for the same territory sites over multiple years. We propose not to limit this to West Flanders but to do an inventory across the border as well. Project TEC! has already proven that cross-border cooperation is possible and will only benefit the survival of the Yellowhammer (Dochy 2018b). We further recommend to annually monitor the suitability of breeding habitat features (hedges, field margins, small landscape elements alongside ditches), and in particular AES-options. By comparing their evolution with the rise and fall of territory clusters, management measures can be better tailored to the Yellowhammer’s needs.

40 6 Conclusion

This research aimed at studying the process of early territory settlement by the threatened Yellowhammer (Emberiza citrinella), a bird species tied to small-scale agriculture. Based on a model in which territory occupation was estimated based on all habitat features occurring within 150m of the territory centre, we found that hedges, field margins and ditches have a significant influence on territory selection. They provide the two most crucial requirements to guarantee successful breeding: a safe nesting place and invertebrate-rich foraging habitat. Hedges serve as a notable song post, as well as providing safe nesting opportunities. Field margins have a more heterogeneous vegetation and less dense sward structure than grassland. They are a preferred foraging habitat since they offer a higher availability and accessibility of invertebrate prey, especially early in the breeding season. Ditches are rather indirectly linked to territory selection, since Yellowhammer is no water-loving species by nature. They are often flanked by the increasingly disappearing small landscape elements and well-vegetated banks, providing opportunities to forage as well as nest.

Study on the influence of winter food plots on early territory settlement showed that territories were established in closer proximity to winter food plots than expected by chance. Differences in distance between occupied territories and control points could not be explained by the type of winter bird crop (cereal grains or oil radish) or an interaction between crop type and territory occupation.

The Yellowhammer territories showed aggregated distributions and this clustering behaviour is assumed to be positively density-dependent. New settlers are attracted by the songs of conspecifics and preferentially settle in proximity to established territory owners. Cross-border research is needed to distinguish if the concentration of Flemish territories along the French border is still present because of clustering and/or site fidelity or because of the higher proportion of suitable habitat.

Overall, these results suggest that conservationists aiming to conserve the sedentary Yellowhammer should consider the quality, quantity and positioning of its preferential nesting and foraging habitats, as this is likely to improve the territory capacity of the surrounding habitat.

41 7 Summary 7.1 English summary

In recent decades, farmland birds have a hard time in almost all of Europe and most of the species show alarming and permanent population declines. The continuous decline is mainly driven by an intensification in the agricultural sector, which in turn is conducted by European agricultural policy. Especially granivorous species, those with a substantial seed component in their diet, have become victims of this dramatic habitat change and consequentially population declines. Both during winter and breeding periods they struggle to survive. The switch from spring-sown to autumn-sown cereals have led to a large reduction in the area and quality of overwintered stubble fields, their preferred overwinter habitat. Besides depletion of winter food, finding suitable breeding habitat is becoming increasingly difficult. Invertebrate-rich field margins and small landscape elements, like hedges, scrubs, trees and pools, have to yield to the ever-increasing intensification of agriculture.

In order to halt, stabilize or even reverse the population declines, so-called agri-environment schemes (AES) were created. They are designed to encourage farmers to protect and enhance the environment on their farmland for five years by paying them for the provision of environmental services. AES- options include the provisioning of extra winter food by sowing winter bird crops, the protection of small landscape elements, floristically enhanced arable margins, etc.

The Province of West Flanders applies already applies these measures in their Species Action Plan for the Yellowhammer (Emberiza citrinella). This plan aims to maintain and enhance the biodiversity in West Flanders in order to regain resilient populations. The Yellowhammer has been chosen as the flagship species of this project. The Yellowhammer is a threatened passerine bird form the bunting family Emberizidae. A relict population still occurs in the Westhoek, in western Belgium near the French border. During the winter they reside in group and exclusively feed on starchy cereal grain. In February, male Yellowhammers leave the winter flock to search for a suitable breeding territory. During the breeding season, Yellowhammers add invertebrates to their diet, particularly as food for their growing chicks.

To protect the species in the long run, it is not only essential to ensure overwinter survival, but also to improve their breeding conditions. In order to improve the current conservation measures, it is important to know which habitat features influence the early territory selection and thus deserve extra protection.

The study was carried out in a 47 square kilometres area centred around Heuvelland, situated in the Province of West Flanders. The region is characterized by mixed agriculture, with cropland and pasture bounded by ditches, hedges, tree lines, fences or grass margins. Between the end of February and mid- April 2020, bird surveys were carried and singing Yellowhammer males were mapped. These observations were later clustered into territories, leading to a total of 40 Yellowhammer territories present inside the study area. As many random points were generated to allow comparison between occupied and unoccupied territories. All habitat features in a radius of 150 m of the territory centre were extracted from digital layers or recorded during field surveys. They were finally grouped into 10 variables used for analysis.

Based on a model in which territory occupation was estimated based on the habitat variables, we found that hedges, field margins and ditches have a significant influence on territory selection. They provide the two most crucial requirements to guarantee successful breeding: invertebrate-rich

42 foraging habitat and safe nesting opportunities. Next to a safe nesting site, hedges also provide roost sites and protection against predators. The hedge base, and particularly the tussocky grasses, are also used as foraging habitat. A noticeable hedge may serve as a suitable song post to display themselves and attract a mate. However, a lot depends on the structural complexity of a hedgerow and its associated habitat, therefore thoughtful hedge management is crucial. Field margins are a preferred foraging habitat. Unlike grassland, they have a more heterogeneous vegetation and less dense sward structure. Reduced fertiliser and spray drift into field boundaries is likely to help maintain the perennial field margin vegetation hosting higher abundances of invertebrates which are crucial as chick food. Since Yellowhammer is no water-loving species by nature, the link between ditches and territory occupancy is rather indirect. The well-vegetated banks of ditches might act as a refugia for invertebrates and as nesting habitat. Small landscape elements, like trees and scrubs, are gradually disappearing, but still persist along ditches.

Fields sown with winter bird crops are an effective conservation measure to help Yellowhammers and other bird species survive the winter. We investigated if those winter food plots have an influence on early territory settlement. By colour-ringing Yellowhammers, we tried to find out if these individuals would settle in the neighbourhood of the plot where they were caught. Our results show that Yellowhammers in general settle in closer proximity to winter food plots than expected by chance. This difference was not related to the crop type of the plots (cereal grains or oil radish) or an interaction between crop type and territory occupation. We suspect that quality of the winter food plot is more determinant than its crop type and therefore recommend further studies to take plot quality into account. Unfortunately, we could not collect enough resightings of colour-ringed Yellowhammers to determine how territory settlement proceeds through spring.

Finally, we wanted to investigate if the distribution of Yellowhammer territories deviates from spatial homogeneity. Besides our collected data, we used historical inventory data from 2003, 2004, 2005 and 2016 on population size distribution patterns of the Yellowhammer in our study area. These data were used to check for territorial aggregations using the L-function, a derivate of Ripley’s K. The Yellowhammer territories showed a clustered distribution in 2016 and 2020, whereas in the other years the territories were randomly distributed. We suspect this clustering behaviour to be positively density-dependent. New settlers are attracted by the songs of conspecifics and preferentially settle in proximity to established territory owners. We were unable to unravel the mechanisms behind conspecific attraction. Intense follow-up of the pattern of settlement during the breeding season might give a decisive answer. In addition, cross-border research is needed to distinguish if the Flemish territories along the French border are still present because of clustering and/or site fidelity or because of the higher proportion of suitable habitat.

To conclude, the probability of territory occupations will be larger with increasing length of hedges and ditches and increasing area of field margins. Yellowhammers settle closer to winter food plots than expected by chance. Yellowhammer territories showed aggregated distributions and this clustering behaviour is assumed to be positively density-dependent.

43 7.2 Nederlandstalige samenvatting

In bijna heel Europa hebben talloze akkervogels de laatste jaren te maken gehad met een serieuze afname in populatiegrootte. Deze voortdurende afname wordt vooral gedreven door een intensivering van de landbouwsector die op haar beurt gedirigeerd wordt door het Europese landbouwbeleid. Vooral zaadeters zijn het slachtoffer geworden van nieuwe wetgevingen en technologische vooruitgang in de landbouwsector. Zowel tijdens de winter als het broedseizoen hebben ze moeilijkheden om te overleven. De omschakeling naar het gebruik van wintervariëteiten die reeds in de herfst worden ingezaaid, hebben gezorgd voor een sterke afname in het oppervlak aan kwalitatieve graanstoppels, niet toevallig het favoriete overwinteringshabitat van akkervogels. Naast de afname van wintervoedsel wordt ook het vinden van een geschikt broedhabitat steeds moeilijker. Insectenrijke perceelsranden en kleine landschapselementen zoals knotbomen, struiken, hagen, houtkanten en poeltjes, hebben te leiden onder de voortdurende landbouwintensivering.

Om het tij te keren werden de zogenaamde beheerovereenkomsten in het leven geroepen. Landbouwers kunnen een contract van vijf jaar afsluiten waarbij ze zich ertoe verbinden beheerpakketten in functie van natuurbehoud uit te voeren tegen betaling van een vergoeding door de overheid. De beheerovereenkomsten bevatten o.a. het inzaaien van vogelvoedselgewassen, het in stand houden van kleine landschapselementen, de aanleg en onderhoud van bloemrijke graslanden, etc.

De provincie West-Vlaanderen past deze maatregelen reeds toe in hun Soortactieplan voor de geelgors (Emberiza citrinella). Dit plan heeft als doel om de biodiversiteit in West-Vlaanderen te behouden en versterken en zo populaties opnieuw veerkrachtig te maken. De geelgors werd daarin geselecteerd als symboolsoort voor het behoud van een kleinschalig landschap. De geelgors is een bedreigde zangvogel uit de gorzenfamilie Emberizidae. Van deze zeldzame broedvogel resteert in West-Vlaanderen enkel nog een populatie in de Westhoek langs de Franse grens. Tijdens de winter vormen ze groepjes en voeden ze zich bijna uitsluitend met zetmeelhoudende zaden van granen. In februari verlaten de mannetjes de wintergroep op zoek naar een geschikt broedterritorium. Tijdens het broedseizoen voegen geelgorzen insecten toe aan hun dieet. Die zijn vooral belangrijk als voedsel voor de kuikens.

Om de geelgors op lange termijn te beschermen is het niet alleen van belang om ervoor te zorgen dat ze de winter overleven, maar ook om hun broedgebied te verbeteren. Om de huidige beschermings- maatregelen efficiënter te maken, is het belangrijk om te weten welke habitatkenmerken een invloed hebben op de vroege territoriumkeuze en dus extra bescherming verdienen.

Deze studie werd uitgevoerd in een 47 km2 groot gebied rond het Heuvelland, gelegen in de provincie West-Vlaanderen. Deze regio wordt gekenmerkt door gemengde landbouw waarbij akker- en weiland omringd worden door grachten, hagen, bomenrijen, omheining of grasranden. Van eind februari tot en met midden april 2020, trokken we twee keer per week naar het veld om zingende geelgorsmannetjes in kaart te brengen. Deze waarnemingen werden later geclusterd tot territoria. In het totaal konden er 40 verschillende geelgorsterritoria in het studiegebied geïdentificeerd worden. Vervolgens werden er evenveel controlepunten gegenereerd zodat we een vergelijking konden maken tussen bezette en onbezette territoria. Alle habitatkenmerken in een straal van 150 meter rond het territoriumcentrum werden geëxtraheerd uit digitale kaarten of opgetekend tijdens het veldonderzoek. Uiteindelijk werden zij gegroepeerd in 10 variabelen die gebruikt konden worden voor analyse.

44 Op basis van een model waarin territoriumbezetting geschat werd op basis van de habitatvariabelen vonden we dat hagen, perceelsranden en grachten een significante invloed hadden op territoriumselectie. Alle drie vervullen zij de twee meest cruciale eisen voor een geslaagd broedsel, namelijk een foerageergebied rijk aan invertebraten en een veilige nestmogelijkheden. Naast een veilige nestplaats bieden hagen ook een slaapplaats en bescherming tegen predatoren. De onderkant van een haag, en dan vooral de kruidachtige begroeiing, worden ook als foerageergebied gebruikt. Een uitstekend deel van een haag of struik kan dienen als zangpost om op te vallen en een partner aan te trekken. Maar de aantrekkelijkheid van een haag is sterk afhankelijk van zijn structurele complexiteit en nabijgelegen habitat. Doordacht beheer van hagen is dus van cruciaal belang. Perceelsranden vormen een geliefkoosd foerageergebied. In tegenstelling tot grasland hebben zij een meer heterogene vegetatie en een minder dichte graszodestructuur. Perceelsranden hebben vaak minder te leiden onder bemesting en pesticidegebruik. Hierdoor kunnen meerjarige planten ontwikkelen die hogere abundanties aan insecten huisvesten. Die proteïnerijke insecten zijn broodnodig voor een snelle groei van de kuikens. Omdat de geelgors van nature geen waterminnende soort is, is de link tussen grachten en territorium- selectie eerder indirect. Stevig begroeide oevers zouden fungeren als refugia voor invertebraten en als nestplaats. Kleine landschapselement zoals bomen en struiken verdwijnen zienderogen, maar houden stand langs grachten.

Akkers ingezaaid met vogelvoedselgewassen zijn een effectieve maatregel om geelgorzen en ander vogelsoorten de winter te helpen overleven. Wij onderzochten of deze wintervoedselveldjes een invloed hebben op de vroege territoriumvestiging. Door geelgorzen te kleurringen probeerden we uit te zoeken of individuen zich zouden settelen in de buurt van het veldje waar ze werden gevangen. Onze resultaten tonen dat geelgorzen zich in het algemeen dichter vestigen bij de wintervoedsel- veldjes dan we zouden verwachten op basis van toeval. Dit verschil was niet te wijten aan het gewastype op de plots (graan of bladrammenas) of een interactie tussen het gewastype en territoriumbezetting. We vermoeden dat de kwaliteit van wintervoedselveldjes bepalender is dan het gewastype zelf. Daarom raden we verdere studies aan om plotkwaliteit in rekening te brengen. Jammer genoeg konden we te weinig gekleurringde geelgorzen terugzien om te bepalen hoe territoriumvestiging verloopt doorheen de lente.

Als laatste wilden we onderzoeken of de verspreiding van geelgorsterritoria afwijkt van ruimtelijke homogeniteit. Naast onze zelf verzamelde data hebben we ook gebruik gemaakt van inventarisatiegegevens uit 2003, 2004, 2005 en 2016. Met behulp van de L-functie, een afgeleide functie van Ripley’s K, werden er clusteranalyses uitgevoerd. Geelgorzenterritoria vertoonden een geclusterde verdeling in 2016 en 2020, terwijl de territoria in de andere jaren willekeurig verspreid waren. We vermoeden dat clustergedrag positief densiteitsafhankelijk is. Geelgorzen op zoek naar een territorium worden aangetrokken door het gezang van hun soortgenoten en vestigen zich bij voorkeur in de buurt van reeds gevestigde territoria. We waren niet in staat om de mechanismen achter het aantrekkingseffect van soortgenoten te ontrafelen. Intensieve opvolging van het vestigingspatroon tijdens het broedseizoen zou een beslissend antwoord kunnen geven. Daarnaast is grensoverschrijdend onderzoek nodig om te onderscheiden of de Vlaamse territoria langs de Franse grens nog steeds aanwezig zijn omwille van clustering en/of traditionele plekken of omwille van het hogere aandeel aan geschikte habitat.

Samenvattend, de kans op territoriumbezetting is groter met toenemende lengte van heggen en grachten en toenemende oppervlakte van perceelsranden. Geelgorzen vestigen zich dichter bij de wintervoedselveldjes dan verwacht wordt door toeval. Geelgorsterritoria vertoonden geaggregeerde verdelingen en dit clusteringsgedrag zou positief densiteitsafhankelijk zijn.

45 8 Acknowledgements

Writing this final note means that my master’s thesis and my Biology study in general has come to an end. It was a long, but exciting journey. Despite the setback we suffered from COVID-19 which forced me to leave the original topic, everybody kept believing in the new project. It taught me that life is full of unexpected twists and turns that you should embrace rather than condemn. During my fieldwork in Heuvelland, I discovered that some magnificent and biodiverse places are left in the otherwise fully urbanised Flanders. These scarce places are home to the symbolic Yellowhammer. I am very grateful to get to know this beautiful species better. I hope that I have been able to contribute some valuable information to the Yellowhammer project. Although this thesis bears my name, it has been the result of the contribution of many people. I want to show my gratitude to the following people, as this master thesis would not have come about without their much-appreciated input.

I would like to thank my supervisor Luc Lens, for giving me the opportunity to work with this intriguing bird species. I would like to express my sincere gratitude to my co-supervisor Luc De Bruyn for the continuous support. His scientific expertise served as a valuable guidance throughout the entire process. I am thoroughly grateful to my tutor Olivier Dochy for his confidence and constructive feedback. His love and commitment towards the Yellowhammer still intrigue me. I would like to thank the bird ringers, Miguel, Stefaan and especially Norbert for their voluntary efforts in ringing and tagging our Yellowhammers. Next, I would also like to thank Ghent University for lending their spotting scope. Thank you to Pierre for lending me his old service bike and Jeroen to be the right person at the right time when I faced car trouble. Thank you to photographer Rini Lamboo who allowed me to use his exquisite picture of a Yellowhammer as the cover image.

Thank you to my family and friends, for their inexhaustible support and understanding. A special reference is in place to my parents for their moral and financial support. Without their encouragement, this journey would have been a lot longer. A special thanks to Lies to be my prop and stay all the time. Thank you for your love and for always making me feel good. Thank you to Simon, Hanne and Dries to forgive me trying their patience from time to time. Finally, I would like to thank my friends who supported me through the entire process. Not only by deliberating over our problems and findings, but also for the entertainment and breaks when needed.

I also place on record, my sense of gratitude to one and all, who directly or indirectly, have lent their hand in this research.

Thank you!

Bram Catfolis

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56 10 Appendix

Annex 1: Pooling of the 64 habitat features into 10 variables used for analysis. For each variable, the percentage of its area relative to the entire study area is given. Units are hectare (ha) and meter (m).

Habitat variable Category Habitat feature Name Area % Name Area % Definition Area % Unit S_ARABLE 48.58 POTATO 15.53 Potatoes (not 15.17 ha early) Potatoes 0.20 ha (propagating material) Potatoes (early, 0.15 ha harvesting after 19/6) MAIZE 19.97 Grain maize 3.62 ha Silage maize 16.35 ha LEGUMES 3.48 Broad and field 1.18 ha beans Green beans 1.24 ha Peas 1.05 ha CABBAGE CROPS 3.50 Cauliflower 1.09 ha Broccoli 0.32 ha Kohlrabi 0.08 ha Savoy cabbage 0.007 ha Brussels sprout 1.99 ha coal ROOT CROPS 6.10 Carrot 0.63 ha Onion 0.50 ha Sugar beets 4.60 ha Fodder beets 0.37 ha CEREALS 15.87 CEREALS 15.87 Winter wheat 13.17 ha Winter barley 2.65 ha Triticale 0.05 ha GRASSLAND 25.30 GRASSLAND 25.30 Grassland 23.65 ha Natural 0.06 ha grassland with minimum activity Seed grasses 0.37 ha Grazed non- 0.32 ha agricultural land Grass – clover 0.51 ha Grass – ha Fabaceae (other 0.02 than clover) Mixture of 0.01 ha leguminous plants Perennial alfalfa 0.36 ha OTHER 2.56 OTHER 2.56 One-time 1.16 ha occuring food

57 crop (12 crop types) Unspecified crop 0.12 ha - small farmer Hops 0.09 ha Fibre flax 0.60 ha Ornamental 0.04 ha trees and shrubs Grapevines 0.55 ha FARMS 5.10 FARMS 5.10 Main buildings 4.33 ha Other stables 0.77 ha and buildings MARGIN 0.67 AES-Margin 0.30 Grass buffer 0.18 ha strips (mainly to prevent erosion) Floristically 0.04 ha ‘enhanced’ grass buffer strips Nectar flower 0.08 ha mixture Non-AES-Margin 0.37 Non-AES 0.37 ha managed field margin Winter_Food_Plot 2.58 Winter_Food_Plot 2.58 AES fields with 0.09 ha wild bird seed mixtures AES fields with ha bristle oat 1.69 AES fields with 0.11 ha oil radish AES fields with 0.41 ha autumn cereals AES fields with 0.28 ha spring cereals HEDGE AES-Hedge AES-managed / m hedge AES-managed / m shrub AES-managed / m wood edge Non-AES-Hedge Non-AES- / m managed hedges, shrubs and wood edges All_Ditch All_Ditch Ditch / m Watercourse / m (production potable water) TREE TREE Large trees (> / m 5m) (row or solitary)

58 Annex 2: Correlation matrix of the independent variables used in a Generalized Linear Model explaining Yellowhammer’s territory settlement. On top the Pearson’s correlation coefficients. On bottom, the bivariate scatterplots, with a fitted line.

59 Annex 3: Map of the total study area showing the Yellowhammer territories, control territories and habitat variables.

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