Tree species preferences of foraging insectivorous birds in a primeval mountain mixed forest: implications for management

Bird-plant species associations can be an important component of habitat selection in forest birds. We assessed tree species preferences of foraging insectivorous birds in a primeval beech-fir forest in north-west Slovakia, hypothesizing that bird population densities are negatively associated with foraging specialization. We sampled foraging behaviour by random point observations from mid-May until the end of July during 1997-2003. Significant preference or avoidance patterns were found in 16 of 17 bird species. Based on the tree preference index, we distinguished four main foraging specializations: generalists, deciduous specialists, coniferous specialists, and dead wood specialists. Many bird species showed strong preferences for such rare and uncommon tree species as wych elm (Ulmus glabra), sycamore (Acer pseudoplatanus), and Norway spruce (Picea abies). European beech (Fagus sylvatica), hazel (Corylus avellana), and rowan (Sorbus aucuparia) were generally avoided. Birds with low densities tended to be most selective, but that effect was not statistically significant. Population variability was not significantly associated with foraging specialization. We hypothesize that impoverishment of tree species diversity within forest stands could lead to less diverse bird assemblages composed of species specialized on those tree species remaining and of generalist foragers able to adapt to a wide range of foraging substrates.