Tall, large-diameter trees and dense shrub layer as key determinants of the abundance and composition of bird communities in oak-dominated forests

Increasing human activity is altering the structure of forests, which affects the composition of communities, including birds. However, little is known about the key forest structure variables that determine the richness of bird communities in European temperate oak forests. We, therefore, aimed to identify key variables in these habitats that could contribute to the design of management strategies for forest conservation by surveying 11 oak-dominated forest sites throughout the mid-mountain range of Hungary at 86 survey points to reveal the role of different compositional and structural variables for forest stands that influence the breeding bird assemblages in the forests at the functional group and individual species levels. Based on decision tree modelling, our results showed that the density of trees larger than 30 cm DBH was an overall important variable, indicating that large-diameter trees were essential to provide diverse bird communities. The total abundance of birds, the foliage-gleaners, primary and secondary cavity nesters, residents, and five specific bird species were related to the density of high trunk diameter trees. The abundance of shrub nesters was negatively influenced by a high density of trees over 10 cm DBH. The density of the shrub layer positively affected total bird abundance and the abundance of foliage gleaners, secondary cavity nesters and residents. Analysis of the co-dominant tree species showed that the presence of linden, beech, and hornbeam was important in influencing the abundance of various bird species, e.g., Eurasian Treecreeper (Certhia familiaris), Marsh Tit (Poecile palustris) and Wood Warbler (Phylloscopus sibilatrix). Our results indicated that large trees, high tree diversity, and dense shrub layer were essential for forest bird communities and are critical targets for protection to maintain diverse and abundant bird communities in oak-dominated forest habitats.