| Popis: |
Increasing deer populations across the northern hemisphere have been cited as one of the factors potentially contributing to declines in the populations of some woodland bird species, particularly species that are dependent on the shrub layer. This thesis examines the effects of browsing by deer on the use by birds of a regenerating deciduous woodland system eastern England, and investigates the mechanisms by which impacts operate. Using a replicated experiment, over the course of four years, habitat use by birds in exclosures without deer was compared with that in paired control plots. The latter plots were browsed by roe deer and Reeves's muntjac throughout the year (and fallow deer in winter). A range of ornithological survey methods were used to compare the habitat types, including point-counts, standardised mist-netting and radio-tracking. During the breeding season, the most marked negative responses to deer were shown by birds that are dependent on dense understorey vegetation, such as dunnock Prunella modularis, nightingale Luscinia megarhynchos and garden warbler Sylvia borin. Pertinently, browsing by deer has the potential to reduce habitat quality for woodland birds. Territories of nightingale became confined to vegetation where deer were absent and male blackcaps Sylvia atricapilla established territories earlier and were in better body condition there. Unbrowsed young re-growth (prior to canopy-closure) was particularly important during the post-breeding period for a range of species, presumably in response to foraging resources and protection afforded. Similarly, the first ever examination of responses of woodland birds to deer in winter, showed that several common resident ground-foragers, such as robin Erithacus rubecula, were negatively affected in winter having shown no effect in the breeding season. |
