Variations in bird populations in a broad leafed woodland: 1975 to 1998

Variations in passerine populations were examined at a single Common Bird Census location over 24 years. Long term trends in the populations of Robin Erithacus rubecula, Blackbird Turdus merula, Wren Troglodytes troglodytes, Blue Tit Parus caeruleus and Great Tit Parus major were investigated. The statistical method proposed by Bulmer[1], which overcomes the problem of regressions to the mean, was used to investigate density-dependent variation (DDV). Little evidence of DDV was found and so long-term population trends could be evaluated. The commonest species seen were the Wren and Robin. Significant increases were seen in breeding pairs of Great Tit (p=<0.001) and Wren (p=0.048) whereas those of Blackbird decreased (p=0.006) and there was no significant change in breeding pairs of Blue Tit (p=0.090) or Robin (p=0.871). Selection pressures on the populations studied were probably reduced by the consistently mild winters and feeding at bird tables in the gardens near the study area in winter. The CBC technique is recognised as the most accurate method of estimating the size of breeding bird populations. This analysis shows that the CBC data collection method, combined with relatively simple statistical techniques, can be used to accurately interpret local trends in breeding bird populations.