One of the fascinating aspects of bird study is observing and recording changes in bird populations. Since 1989 I have censused the breeding populations of Willow Warbler and Chiffchaff on a woodland and scrub section of Leighton Moss RSPB Reserve.Chiffchaff have seen a marked rise in the population from just 1-2 pairs in the 1980’s increasing to 4-6 pairs in the 2000’s and to a peak of 18 pairs in 2014. By contrast Willow Warblers have fluctuated but overall have remained reasonably stable throughout with 26 to 35 in the 1990’s and the 2000’s and 27-30 over the past five years.
We had concluded that the change in habitats basically the maturing of the Hawthorn/ Ash scrub favoured the Chiffchaff and was the main reason for the recorded increase . However I thought it would be interesting to look at the Groups ringing statistics of these two closely related species to see what they reveal.Figure 1 shows the annual ringing totals of new birds of each species from 1981 to 2015. It shows that Willow Warbler ringing totals have fluctuated quite widely, probably to some extent due to varying ringing effort which is mainly related to weather conditions, but the peaks have been similar throughout with peaks of 640 in1985 and 645 in 2014. By contrast although there has been some small fluctuations in the Chiffchaff ringing totals the main trend has been upward especially recently. From a low of only 9 in 1981and 82 to a high of 437 in 2014 strongly suggesting an increasing population breeding in and migrating through our area.