Greenish Warbler (Phylloscopus Trochiloides Viridanus): an Overlooked Indicator of Old-Growth Forest?

Ornis Fennica 97: 165–176. 2020 Greenish Warbler (Phylloscopus trochiloides viridanus): An overlooked indicator of old-growth forest? Ülo Väli* & Piia Katharina Vaan Chair of Biodiversity and Nature Tourism, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Keutzwaldi 5D, 51006 Tartu, Estonia. * Corresponding author’s e-mail: [email protected] Received 24 August 2020, accepted 6 November 2020 Effective bioindicators of old-growth forest are important for conservation. The Greenish Warbler (Phylloscopus trochiloides viridanus), a forest-dwelling passerine, has been recorded in old-growth forests; however, its precise habitat preferences are poorly studied. We used opportunistic observations collected by citizen scientists and stand descriptions from a forestry database to analyse its habitat preferences in Estonia, with a focus on the characteristics of old-growth forests. The Greenish Warbler preferred productive spruce and black alder stands but also favoured rare broad-leaved stands. Forest stands were older, less drained and contained more standing and fallen dead trees in warbler sites than in control sites. Positive effects of stand age and soil fertility exhibited the highest average relative importance in generalized linear models, drainage was of intermediate importance and the occurrence of dead wood was least important. The terrain at warbler sites was also more often uneven than that at control sites. The preference for old-growth forests observed in our study makes the Greenish Warbler, despite occupying various forest stands suggestive of plasticity, a good candidate for inclusion in a suite of old-growth forest indicators. 1. Introduction probably the most well-known indicators of biodiversity in old-growth forests (Mikusiñski et al. Bioindicators are species that are matched to spe- 2001, Roberge et al. 2008). In European boreal cific habitat features and are reactive to distur- and hemiboreal old-growth forests, several passer- bances and environmental changes, and they indi- ines, such as the Bluetail (Tarsiger cyanurus), the cate general biodiversity in a particular habitat Red-breasted Flycatcher (Ficedula parva), the (Rohlf 1991, Lambeck 1997, Paoletti 1999, Treecreeper (Certhia familiaris) and tit species, McCarty et al. 2002). Indicators are particularly serve a similar purpose and are abundant and easy useful in habitats where inventories are difficult or to study (Tjernberg 1984, Similä et al. 2006, expensive (Juutinen & Mönkkönen 2004). Many Virkkala & Rajasärkkä 2006, Rajasärkka 2010, species, including various birds (Juutinen & Rosenvald et al. 2011, Pakkala et al. 2014, Lind- Mönkkönen 2004), have been used to evaluate bladh et al. 2020). However, these species may biodiversity in old-growth forests, which develop also breed in younger managed forests (Jokimäki over long periods of time without the influence of & Solonen 2011, Lindbladh et al. 2020). This human activity (Peterken 1996). Woodpeckers are raises a question about their usefulness as an indi- 166 ORNIS FENNICA Vol. 97, 2020 cator (Lindbladh et al. 2020), suggesting the need Citizen science, the involvement of citizens for alternatives. from the non-scientific community in research, The Greenish Warbler (Phylloscopus trochi- provides extensive data that are difficult to obtain loides viridanus) is a passerine inhabiting a variety by professional researchers. A network of ama- of forests across its breeding range from central teurs has contributed substantially to bird conser- Europe to central Asia (BirdLife International vation science (Greenwood 2007). The past few 2017). In Europe, the species breeds primarily in decades have witnessed the expansion of citizen the eastern part of the continent but is expanding science owing to technological developments its range westwards (Thoma & Althaus 2015, (Dickinson et al. 2012). BirdLife International 2017). Little is known While typical projects involving citizen scien- about habitat requirements of this ground-nesting tists mainly involve reporting species distributions species. Lapshin (2004) found that in Karelia, and counts, extensive datasets increasingly allow north-western Russia, the Greenish Warbler explorations of correlative relationships (Dickin- mostly occupies mature spruce and mixed forest son et al. 2010, McKinley et al. 2017; Pocock et al. stands in humid areas, with rich undergrowth and 2018). For example, Weisshaupt & Rodríguez- many wind-fallen trees. Similar forests have also Pérez (2017) recently used citizen data to compare been occupied by the Greenish Warbler in Sweden the habitat use of the Wood Warbler (Phylloscopus (Elmberg 1985). sibilatrix) during spring migration and the bree- According to Suomalainen (1936), the species ding season, Mononen et al. (2018) relied on ob- initially preferred productive mature mixed stands servations of birdwatchers together with airborne dominated by spruce and birch after colonizing laser scanning data to determine the habitat prefer- southern Finland but later spread into less produc- ences of forest birds. tive forests after population growth (Tiainen We used opportunistic observations of the 1980). General bird inventories in the same region Greenish Warbler collected by citizen scientists to suggest that the Greenish Warbler prefers old- analyse its habitat preferences with a particular fo- growth forests (Virkkala & Rajasärkkä 2006, Väli cus on characteristics of old-growth forests in Es- & Laurits 2006, Jokimäki & Solonen 2011, Rosen- tonia, at the western margin of species’distribution vald et al. 2011, Mononen et al. 2018). range. We predicted that sites where the Greenish The species is also associated with uneven Warbler has been recorded (hereafter, referred to landscapes, such as hills and river valleys (Kumari as warbler sites) would exhibit characteristics con- 1954, Rootsmäe & Veroman 1974, Kuus & Leibak sistent with those of old-growth forests (old age, 2018). However, this view is based on limited re- lack of drainage, large volume of dead trees), com- gional observations and few nest findings (Mikel- pared with randomly located sites (hereafter, con- saar 1963, Lilleleht 1963), and quantitative studies trol sites). We also evaluate the view that warbler are lacking. sites exhibit steeper slopes than those of control Despite evidence for associations with mature sites. and old-growth forests, the Greenish Warbler is often not included in lists of species typical to this habitat (e.g. Paal 2007) or species of conservation 2. Material and methods importance (e.g. Lõhmus et al. 2001). At the Euro- pean scale, it is considered a species of least con- 2.1. Compilation of observations servation concern, with no current significant threats (BirdLife International 2017). This might Publicly available occurrence records of singing be explained by geographical variation in habitat Greenish Warblers in Estonia (57°30’–59°40’N; preferences, including more opportunistic habitat 21°45’–28°10’E; Supplementary Figure 1) in use in the core of the distribution where the Green- 2003–2017 deposited by voluntary citizen scien- ish Warbler is rather abundant (Dementev & tists in the biological data management platform Gladkov 1954). However, it may also reflect the PlutoF (https://plutof.ut.ee/; Abarenkov et al. scarcity of studies on habitat requirements and the 2010) were obtained via the online portal lack of sufficient data. eBiodiversity (https://elurikkus.ee/en). Records Väli & Vaan: Habitat preferences of the Greenish Warbler 167 with a precision of £ 200 m between 1st May and 2.2. Habitat analysis 31st July were used. To avoid pseudoreplication, when there were multiple observations within 400 The size of the breeding territory of the Greenish m, only one was retained. The dataset was supple- Warbler is estimated as 0.5–0.8 ha (Cramp 1992). mented with 17 precise records obtained by ÜV in In this study, a circle with a radius of 50 m (0.78 the same period. Analyses of forest characteristics ha) was used as a proxy for the breeding territory were based on 225 total records; terrain uneven- where habitats were described. Habitats were also ness was analysed at 119 warbler sites. described in a 200 m radius (12.56 ha). The larger The Estonian population of the Greenish War- circle could be considered a reference for sur- bler consists of 7,000–15,000 pairs (Elts et al. rounding available habitat, and the difference be- 2019) with a sparse distribution across the country tween the circles with radii of 50 and 200 m would (Kuus & Leibak 2018). The abundance and distri- indicate the habitat preference at a local level. bution differs between years due to influence of However, bird locations are not always correctly weather conditions during spring migration. The estimated by observers in opportunistic datasets. species arrives in Estonia in May, and the main mi- Moreover, the bird moves around in the territory, gration probably occurs in late May and early June and the actual nest site and exact borders of the ter- (Kuus & Leibak 2018), although detailed analysis ritory were not known. Therefore, the 200 m ra- of migration is lacking. Similar arrival dates have dius should be considered a less precise proxy of been recorded in Karelia, where laying occurs breeding territory, rather than a control showing soon after arrival, mostly in the first half of June habitat availability. For comparisons between war- (Lapshin 2004). bler sites and available habitat, a control sample Hence,

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