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B6) Bird-Forest Relationships

Multi-scale assessment of bird-forest relationships

Ilse Storch
Doctoral researchers: Marco Basile (2016 - 2020) & João Manuel Cordeira Vale Pereira (since 2019)

University of Freiburg, Faculty of Environment & Natural Resources, Institute of Forest Sciences,
Chair of Wildlife Ecology and Management

Background

Birds are commonly-used indicators of biodiversity: they are popular, well studied, and comparatively easy to survey.  Old-growth forest stages, and bird species associated with them, are of particular conservation concern and are believed to be negatively affected by forestry. Studies of local bird species richness in managed versus natural forests, however, have reported ambiguous results. Because of their mobility, bird assemblages are likely shaped not only by local forest structure, but also by the surrounding landscape. Abundance of forest specialists correlates with forest structure; yet, structure is an insufficient predictor of species occurrence and hence, of species richness. The first doctoral researcher, Marco Basile, assessed how retention elements across the landscape influence occurrence and abundance of forest birds.Describing patterns of bird-habitat relationships, however, cannot reveal the mechanisms shaping local bird communities. Therefore, in his doctoral project, Joao Pereira is currently focusing on functional responses of birds to forest structure, namely their trophic linkages (food resources and predation pressure), with the goal of shedding light on the ecological processes producing the numerical patterns observed by Marco. Finally, in a future doctoral project, by making use of the full multi-year series of repeated bird surveys on all ConFoBi plots, B6 plans to analyse the spatio-temporal dynamics of the bird assemblage across the study landscape.

 

Research questions and hypotheses

B6 addresses the overall hypotheses that 1) the effects of stand-scale retention measures for biodiversity on forest bird communities are moderated by the surrounding landscape; and 2) that patterns in the occurrence and abundance of forest birds in relation to multi-scale forest structure can be explained by their trophic relationships. In particular, we expect that:

  • The occurrence, abundance, and richness of bird species at the scale of forest stands (plots) are explained by local forest structure and by the surrounding landscape;
  • Structural heterogeneity, thought to be favoured by retention practices, promotes invertebrate food availability for late-successional forest birds, while nest predation risk is essentially the product of landscape-level drivers.
  • Increased habitat suitability, driven by trophic mechanisms, is associated with a shift in community composition favouring old forest specialists.
  • There is significant inter-annual variation in the abundance and richness of bird species, which is moderated by stand- and landscape-scale forest structure.

 

Approach, methods and linkages

B6 assesses the structural correlates of avian biodiversity at multiple spatial scales by relating plot-scale presence, abundance and species richness (overall, functional groups and red-listed species) of birds to the abundance, heterogeneity and spatial distribution of structural elements at plot to landscape scales. B6 will tri-annually collect point count data on the full bird assemblage at all 135 ConFoBi study plots and their surroundings. Data will be analysed using multivariate modelling approaches. Currently, the foraging study focuses on two groups of forest birds, representing a large portion of the breeding bird assemblage, and on the corresponding prey: 1) ground foragers and ground-dwelling invertebrates; 2) canopy foragers and canopy-dwelling invertebrates. Ground-dwelling invertebrates are sampled with pitfall traps (with B2, B3 and B4), and canopy-dwelling invertebrates with a combination of caterpillar frass collectors and blacklight traps (with B5). Abundance of prey items will then be related to habitat features, and to abundance of corresponding bird species. Nest predation rates relative to structural habitat features will be assessed by camera trapping of previously identified natural nests as well as artificial nests.  

B6 provides repeated bird count data for all 135 ConFoBi plots to the ConFoBi data pool; cooperates in invertebrate sampling with projects B2-5, and for the predation study with A2. B6 makes use of forest structural data and remote sensing products from A1. Models of multi-scale bird-forest relationships will also be validated using existing data from the Cost Action BOTTOMS-UP database.

 

Findings

During the first three years, B6 has repeatedly surveyed birds at all 135 ConFoBi plots to relate forest bird abundance and species diversity to habitat structure, i.e. different levels of retention, the abundance of microhabitats and dead wood, and landscape patterns.

So far, B6’s first PhD student Marco Basile has assessed the association of forest bird species with specific microhabitats, as well as the co-occurrence of birds, bats, and invertebrates with microhabitats. Analyses for individual species showed, for example, that woodpeckers select cavity trees not by their absolute size, but their relative dimension: it doesn’t matter how big the tree is, but only that it is bigger than the others. Analyses of the contribution of landscape patterns to plot-scale bird species diversity showed that higher habitat heterogeneity, often resulting from an increase in the share of broadleaved-forest in the landscape, are related to higher bird abundances, richness and diversity. Distance between forest patches is associated with lower bird richness and diversity and with the overall species turnover. Further, in a meta-analysis, B6 found that various bird guilds show different responses to retention forestry. In cooperation with other ConFoBi projects, B6 assessed the potential of diverse forest management regimes for securing tree microhabitats and bird abundance under climate change, as well as the implementation of socially optimal biodiversity levels in multiple-use forest management plans.

 

Perspectives

In the future (years seven to nine), B6 plans to analyse the temporal and spatial dynamics of the forest bird community in order to evaluate its resilience with regard to retention and habitat trees. Finally, B6 aims to provide forest managers with quantitative threshold values for the abundance and configuration of stand-scale and landscape-scale structural habitat elements in favour of forest bird species and communities and contribute expertise on birds to recommendations for optimizing retention forestry approaches and measures for overall biodiversity conservation.