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Structural diversity in forests favors avian diversity – but not the endangered Wood Warbler

Occupancy dynamics of the Wood Warbler Phylloscopus sibilatrix assessed with habitat and remote sensing data. Huber, N., Kéry, M., Pasinelli, G. 2017. IBIS. DOI: 10.1111/ibi.12472. VIEW

The Wood Warbler Phylloscopus sibilatrix is a migratory passerine that has suffered long-term declines in many European countries, especially in Western Europe. Unfortunately, the reasons for this negative population trend are not yet understood. Thus, a research project at the Swiss Ornithological Institute examines factors that may affect the strong fluctuations in population size.

Here we studied the occupancy dynamics of the nomadic ground-nesting passerine to better understand the mechanisms behind its presence or absence on the breeding grounds. For identifying appropriate conservation measures, the following questions were of high interest:

  • Which forests provide suitable breeding grounds for the Wood Warbler?
  • Which factors favor occupancy and colonization, and which factors, in turn, lead to local extinction from former breeding grounds?

To answer these questions, we analyzed 80 1-km squares in Northern Switzerland that have been surveyed since 1999 in the course of the Swiss common breeding bird survey. Each year, experienced observers have recorded the presence of the Wood Warbler (and other species) three times during the breeding season.

We analyzed this dataset using dynamic occupancy models (MacKenzie et al. 2003, 2006). This modern approach provides several advantages to traditional models. First, it allows studying local colonization and extinction through time, and thus does not assume that the distribution of a species remains constant over time. Furthermore, dynamic occupancy models consider that a species may not be detected during a survey although it was in fact present.

We analyzed a broad range of habitat characteristics, i.e. topography, forest extent and structure, soil, seed masting (as a proxy for rodent abundance) and weather conditions during settlement. Apart from a diverse set of nationwide remote sensing data, we used the remote sensing technique LiDAR (light detection and ranging) to describe forest characteristics with a high level of detail (Fig. 1). LiDAR is particularly valuable because forest structure often determines habitat suitability for forest-dwelling species. Field data on forest structure is often limited at large scales, while the three-dimensional structure of forests can be quantified over large areas at a high level of detail with the help of LiDAR.

Pasinelli et al fig 1Figure 1 Reconstruction of terrain and forest structure based on LiDAR. Pink points show the shape of the terrain, green points represent the vegetation structure. Each colored point corresponds to a reflection of the laser light emitted and caught by the aircraft flying above the forest. Top: three-dimensional view with undulating terrain, ground vegetation and trees; bottom: two-dimensional cross-section of another forest illustrating vertical stand composition (note the bush and shrub layer below the canopy at the right end of the figure) © Christian Ginzler.

The results of our study show that forest characteristics are of overwhelming importance for the settlement dynamics of the Wood Warbler. Suitable breeding grounds are characterized by broadleaved forests in fairly steep areas on nutrient-poor soils, by canopy trees of similar and intermediate height, by high canopy closure and sparse ground vegetation cover (Fig. 2). These findings corroborate previous studies conducted at the territory scale. In addition, our analyses at the 1-km scale showed the relevance of rather extensive broadleaved forests with relatively low amounts of forest edge for the occupancy dynamics of the Wood Warbler. These aspects could not be addressed in previous small-scale studies at the territory scale. Thus, an important conclusion from our study is that habitat requirements need to be addressed at different spatial scales. With the help of LiDAR and other remote sensing data, forest structure can be described with a high level of detail (forest area, forest edge length, canopy closure, vertical diversity). This allows identifying suitable forest stands for the study species.

Pasinelli et al fig 2Figure 2 Two examples of Wood Warbler habitats in northwestern Switzerland. At first glance, the forests look quite different, but both show the characteristics of suitable habitats for this species, that is, a grassy ground vegetation, no or little shrubs and bushes, an open stem space below the relatively closed canopy consisting of quite a lot of trees. Left picture: © Alex Grendelmeier, right picture: © Gilberto Pasinelli

Moreover, the Wood Warbler appears to be an area-sensitive species, showing high occupancy probability and low extinction probability in squares featuring large tracts of forests with relatively low forest edge length, that is, a high proportion of forest interior.

In contrast to many other bird species, the Wood Warbler prefers homogeneously structured forests. Since many conservation efforts in forests aim at developing open and diversely structured forest stands to promote biodiversity, the Wood Warbler will hardly benefit from such conservation efforts. Thus, specific conservation measures are required that should focus on providing and maintaining the Wood Warbler’s preferred forest structure, specifically in extended forests. In Switzerland, forests located on slopes and on nutrient-poor soils should be prioritized for conservation.

References and further reading

MacKenzie, D.I., Nichols, J.D., Hines, J.E. & Knutson, M.G. 2003. Estimating site occupancy, colonization, and local extinction when a species is detected imperfectly. Ecology 84: 2200-2207. VIEW

MacKenzie, D.I., Nichols, J.D., Royle, J.A., Pollock, K.H., Bailey, L.L. & Hines, J.E. 2006. Occupancy estimation and modeling. Interferring patterns and dynamics of species occurrence. Burlington, Mass.: Academic Press.

More information on the Wood Warbler project here and here

Image credit

Featured image: Wood Warblers Phylloscopus sibilatrix can be easily distinguished from congeneric warblers by song, calls and plumage coloration © Michael Gerber

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