How do edge effect and tree species diversity affect bird diversity and avian nest survival in Germany´s largest deciduous forest?

Habitat fragmentation is a major driver of species loss. Here we test the hypotheses that high tree diversity in a large deciduous forest enhances bird diversity and nest survival. We further expect that forest edges support higher bird diversity when different habitat types adjoin, whereas nest predation is not higher, because the large forest area mitigates potential edge effects. We studied how edge-centre differences and tree diversity (beech-dominated vs. tree-species rich) affect the bird community and survival rates of ground breeding birds’ nests based on an artificial nest predation experiment in the Hainich National Park, Germany. We surveyed birds three times during the breeding season. We selected six forest stands with low tree diversity (i.e. dominated by beech) and six with high tree diversity (i.e. tree-species rich). Each forest stand contained four bird survey plots (plot 1: 0–30 m, plot 2: 60–90 m, plot 3: 120–150 m and plot 4: 180–210 m distant from edge; altogether 48 bird survey plots). Additionally each plot corner contained one artificial ground nest baited with one Blue-breasted Quail egg and one plasticine egg for eight days of exposure in the middle of the breeding season. Bird abundance and diversity were higher in the first 30 m of the forest. Bird diversity, including ground breeding birds, was also enhanced by higher percentages of bushes, which can provide enhanced food supply, perches as well as sheltering. Nest predation showed no edge effect, supporting the idea that small area of forest fragments causes more important negative effects than the edge in large forest remnants. Predation rates were higher in tree-species rich stands compared to beech-dominated stands, probably due to greater diversity and density of mammalian predators. Edge effects shaped the bird community composition and positively affected abundances of tree and shrub breeding birds, but did not affect ground breeders and the nest predation of ground nests. Shrub breeders accumulating in forest edges might, however, suffer more from nest predation in forest fragments. In conclusion, bird diversity and avian egg predation were affected by both forest edges and tree diversity in surprisingly different ways.