Gap-crossing decisions of woodland songbirds in Scotland: an experimental approach.
Recent declines in woodland birds in Britain have been linked to increasing habitat fragmentation. To understand the effects of fragmentation, data on avian dispersal across woodland gaps are essential but often lacking. We used song thrush Turdus philomelos mobbing calls to attract songbirds across gaps ranging from 5 to 120 m in width and along comparable woodland edges. The habitat was coniferous plantations with a mosaic of spruce (Picea), larch (Larix), pine (Pinus) and Douglas fir (Pseudotsuga menziesii) located at East Loch Lomond, Loch Ard Forest and Achay Forest in the west of Scotland, UK. Responses were modelled against distance using generalized linear models. Such models have clear applied value for connecting fragmented landscapes. We also calculated response indices and compared these with bird morphology. The gap-crossing results were applied to a real landscape in central Scotland and landscape metrics were calculated to judge how perception of habitat connectivity varies interspecifically. The chaffinch Fringilla coelebs and the robin Erithacus rubecula both responded more readily across gaps than through woodland. There was no difference between gap and edge response for the coal tit Parus ater, while the goldcrest Regulus regulus responded more readily along edges than across gaps. Maximum gap-crossing distances ranged from 46 m (goldcrest) to 150 m (chaffinch). There was a positive linear trend between mass of bird and the difference in the maximum response for gap and control experiments. Likewise, there was a positive curvilinear relationship between wing area and the difference in probability of response between gap and control experiments at 50 m. These results may be interpreted in terms of manoeuvrability and ability to escape avian predation. For the central Scotland landscape, the perceived number of patches in the landscape decreased exponentially with increasing gap-crossing distance, while the median patch size and mean patch fractal dimension increased linearly with gap-crossing distance. Our results showed that an experimental approach using playback can be used to obtain data on avian gap crossing and the results applied to real landscapes to visualize interspecific differences in habitat perception. This has practical management applications, especially for designing forest habitat networks to maximize avian biodiversity, and potentially could help reverse the recent declines in woodland birds.