Identifying Predators of Eggs and Chicks of Lapwing Vanellus
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Ibis (2008), 150 (Suppl. 1), 74–85 Blackwell PublishingIdentifying Ltd predators of eggs and chicks of Lapwing Vanellus vanellus and Black-tailed Godwit Limosa limosa in the Netherlands and the importance of predation on wader reproductive output WOLF TEUNISSEN,1* HANS SCHEKKERMAN,2 FRANK WILLEMS1 & FRANK MAJOOR1 1SOVON Dutch Centre for Field Ornithology, Rijksstraatweg 178, 6573DG Beek-Ubbergen, The Netherlands 2Alterra, Wageningen University and Research Centre, PO Box 47, 6700 AA Wageningen, The Netherlands, and Dutch Centre for Avian Migration and Demography, Netherlands Institute of Ecology (NIOO-KNAW), PO Box 40, 6666 ZG Heteren, The Netherlands Farmland bird populations in the Netherlands have shown an accelerating decline in recent years, despite extensive conservation efforts including reserves, agri-environment schemes and protection of nests by volunteers. Although agricultural intensification is the main cause underlying these declines, there is a growing concern that the ongoing decline of grassland- breeding shorebirds in recent years is caused or aggravated by increasing predation. Although Red Fox Vulpes vulpes and Carrion Crow Corvus corone are often accused of causing widespread breeding losses, and calls for management of these species are made, very few field data are available on the incidence of predation on grassland shorebirds and the relative importance of different predators. To obtain such data, we identified egg predators using temperature loggers and continuous video recordings of 792 clutches, and chick predators by radiotagging 662 chicks of Black-tailed Godwit Limosa limosa and Northern Lapwing Vanellus vanellus. In total, 22 species were identified as predators of eggs or chicks, of which Red Fox, Common Buzzard Buteo buteo, Grey Heron Ardea cinerea and Stoat Mustela erminea were the most frequent. Eggs were taken primarily by mammals and chicks more often by birds. There was great variation in predation levels and species involved in predation of clutches between sites and years, but less in chick predation. Hence, there was no correlation between predation levels on clutches and those on chicks within the same sites. In sites where more then 50% of clutches were lost to predation, however, nocturnal predators took the larger share. As temporal and spatial variation on a small scale significantly influences predation levels, a site-specific approach based on sound knowledge of the local situation will be more effective in reducing predation on farmland birds than general, country-wide measures. Calculations based on our data indicate that eliminating only one loss factor at a time will often not reverse a local population decline, and provide a strong argument for targeting several locally limiting factors simultaneously instead of focusing on mitigation of predation alone. Keywords: agricultural intensification, conservation biology, farmland birds, habitat management, reproduction. Almost all waders breeding in European farmland that the main cause of this reduction lies in low have shown a decline in population size in the last breeding productivity especially due to agricultural few decades (Thorup 2006). It is generally accepted intensification (Beintema et al. 1997, Kruk et al. 1997, *Corresponding author. Vickery et al. 2001, Wilson et al. 2004, Schekkerman Email: [email protected] & Beintema 2007). In the Netherlands, agri-environment Conflict of interests: The authors declare no conflict of interests. schemes (AES; principal measure postponed mowing) © 2008 The Authors Journal compilation © 2008 British Ornithologists’ Union Predators of Lapwing and Black-tailed Godwit 75 and reserves (principal measures postponed mowing METHODS and higher water tables) have been widely introduced to promote breeding performance of farmland birds, Study sites but with varying results (Kleijn et al. 2001, Willems et al. 2004, Schekkerman et al. 2008a). In addition, Data on depredation of eggs were collected in 10 on a large part of Dutch farmland, clutches are study sites scattered through the Netherlands (Fig. 1; marked and protected from agricultural losses by Teunissen et al. 2005). Sites were selected to cover a almost 20 000 volunteers and farmers (Van Paassen wide range of predation rates (based on data from & Roetmeijer 2006). preceding years) with emphasis on sites with high Despite these efforts the farmland bird population predation losses. They were characterized by dairy has declined at an accelerating rate (Teunissen & grasslands, with relatively high abundance of Lapwings Soldaat 2006) triggering debate on whether agricultural (> 25 breeding pairs per 100 ha) and Black-tailed intensification is the sole cause for the decline. There Godwits (> 10 breeding pairs per 100 ha), or a mix is increasing concern (particularly among the large of grassland and arable land, with high numbers of group of volunteers and farmers) about the effect of Lapwings and few or no Godwits. In all sites volunteers predation on eggs and chicks on population size and searched for nests and protected them against whether predation negates the positive effects of agricultural losses when needed. protective measures. Although there is little evidence Data for only Black-tailed Godwit chicks were that predation losses of wader nests have increased collected in four additional sites where the effective- strongly in recent decades (about 0.5% annually; ness of a new AES for Godwits was studied (Fig. 1; Teunissen et al. 2005), this discussion is often Schekkerman et al. 2008a). The main purpose of this emotional as (some) volunteers are losing all ‘their’ AES was to create a mosaic of different types of clutches to predation, and hence focus strongly on grassland by postponing mowing dates or grazing by the magnitude of predation losses and the identity of cattle. Each site consisted of an experimental plot key predators. It is the belief of many people involved managed according to the AES and a nearby control in the protection of farmland birds that Red Foxes that was farmed conventionally. Vulpes vulpes and Carrion Crows Corvus corone are In two study sites, Arkemheen and Lange Rypen, we the primary predators, as suggested by Pienkowski expected low levels of predation based on observa- (1984) and Parr (1993). Experimental removal of tions in previous years by local volunteers, whereas Red Fox and Carrion Crow has, however, shown in all other sites much higher levels had been observed. variable results for the survival of Northern Lapwing In most cases volunteers assumed that either Red Vanellus vanellus clutches (Bolton et al. 2007): some Fox (Bontebok, IJsseldelta, Langezwaag, Noordmeer) sites showed an increase of clutch survival whilst other or Carrion Crow (Sudermarpolder) or both (Leende, sites showed reductions. This was partly explained Ruinen, Soest) were the main predators, as the by differences in predator densities among sites, but animals or their tracks were seen in the area. another possibility is that in those sites other predators were active. These predators may require different Identifying predators of eggs management techniques. So identification of key predators and quantifying their impact are not only Identifying predators of eggs is difficult, because in essential for objective debate but also for effective many cases the nest contents are removed completely management. (Green et al. 1987), although sometimes predators In this paper, we present nest survey and chick can be identified from bite marks on the eggs (Green radiotracking data for Lapwing and Black-tailed et al. 1987, Bellebaum & Boschert 2003). We used two Godwit Limosa limosa from 10 sites across the methods to identify predators of eggs. To distinguish Netherlands over 4 years in order to: (1) quantify between diurnally and nocturnally active predators the spatial and temporal variance in the frequency of we placed temperature loggers (Tinytag, Gemini nest and chick predation, (2) identify the range and Data) in 545 nests to record nest temperature every relative frequency of predators of wader nests and 3 min, enabling us to assess the time of a predation chicks, (3) explore the variation across sites in nest event from the sudden substantial drop of nest predator species, and (4) explore the relative impact temperature followed by the nest tracking environ- of different causes of nest and chick loss on overall mental temperature as a consequence of the incubating fledging success. bird permanently leaving the nest (Fig. 2). The thermal © 2008 The Authors Journal compilation © 2008 British Ornithologists’ Union 76 W. Teunissen et al. Figure 1. Study sites where data were collected to determine the identity of predators of eggs (predation study) and chicks (predation and agri-environment scheme (AES) studies). Background shaded areas show relative predation levels in farmland in 2000 based on data collected by volunteers from 90 000 clutches (Teunissen et al. 2005). Darker shading indicates higher predation rates. probe was placed between the eggs and the logger predicted by floating eggs in water; Van Paassen et al. was buried beside the nest. Secondly, 247 nests at six 1984), and (2) if after the expected hatching date no sites were continuously recorded with infrared video small eggshell fragments were found in the nest, cameras (black-and-white bullet camera) with a indicating hatching. In all of these cases videotapes VHS time-lapse recorder (Sanyo time lapse TLS- were examined to identify the predator. In four of 1960) set to