Resident bird species in urban forest remnants; landscape and habitat perspectives

The aim of this study was to investigate the effects of habitat loss, fragmentation and habitat quality on sedentary forest birds in an urban and suburban environment. The study area was situated in Stockholm, the capital of Sweden, embracing the city centre, suburbs and parts of the rural surroundings. Breeding forest birds were surveyed in 51 forested sample sites (2-700 ha) and five species of resident birds were selected for further studies: willow tit (Parus montanus), crested tit (P. cristatus) and coal tit (P. ater) representing coniferous forest and marsh tit (P. palustris) and nuthatch (Sitta europaea) representing deciduous forest. A spatial landscape analysis was made using a geographical information system (GIS). In 21 of the smaller sites (2-200 ha), a field study was conducted to examine habitat quality parameters like vegetation age, structure and composition, and human-induced disturbance. The probability of occurrence (breeding) of bird species as functions of landscape and habitat descriptors was tested using logistic regression. All investigated species of the Parus guild showed high probabilities of occurrence only in forest patches larger than 200-400 ha, and was not present in patches smaller than 10-30 ha. This meant that patches of presumably suitable habitat (coniferous vs. moist deciduous forest) were left unoccupied. The amount of standing dead and decaying trees provided additional explanation for the distribution of the willow tit. Large areas of urban open land, industrial land use and large bodies of water had a negative influence on the probability of occurrence of several species, which indicate that they were sensitive to isolation. The probability of occurrence of the marsh tit was also influenced by distance to other sample sites with marsh tits. Unlike the Parus species, the nuthatch was breeding in most of the parks and forest remnants. This species prefers mature deciduous forest, mainly oak, which habitat was common in the urban environment. The nuthatch was only absent in some of the smallest (a few ha) forest fragments, with a mean distance between forest patches in the surroundings of over 100 m. The study showed that large forest areas and a high amount of forest in the landscape are important for the investigated resident birds that are not adapted to the urban environment. Vast areas without tree-cover seemed to be poor habitat and/or restrict dispersal. Strips of high-quality habitats, including standing trees with nest-holes, were not entirely absent in the urban and suburban environment.