Thursday, 27 July 2017

25th July 2017: What's happened to Willow Warblers

Another ringing session in the top willows at Billinge failed to produce the sorts of numbers expected for this site in late July. It was made even more disappointing by the fact that it was my earliest start of the autumn (I got up at 04:30) and the conditions were perfect for mist-netting all morning.

The calm and overcast conditions were perfect for mist-netting. Looking in this direction on a clear day I can normally see the major landmarks of Liverpool and the Welsh mountains beyond.
There were a few more Willow Warblers around with a total of 17 (12 new & 5 retraps) being an improvement on the previous visit but almost half (8) were adults which is really unusual. To put that into context I haven't caught anywhere near that many adults in a single July session before, and in 2016 only caught 7 adults in the whole of the month.

Adult Willow Warbler
You can't read too much into a single session so I looked at the ratio of juveniles to adults caught each July over the last 3 years. This is a better guide to productivity than overall numbers as it removes any effect caused by variation in ringing effort between years. The results showed that juveniles have outnumbered adults by a least 5:1 in each of the last 3 years, and in July 2016 the ratio was just shy of 17:1. The average of the ratios for the last 3 years being just over 10:1.

Willow Warblers: juvenile left and and adult right. The juvenile shows the typical lemon yellow washed underparts whereas the adult is much whiter although this adult has more extensive white underparts than many.
Looking at the figures for this July the ratio of juveniles to adults currently stands at just under 2.5:1 which is quite worrying and suggests productivity is well down compared to previous years. There is not much of July left so even if the weather allows me to get out again before the end of the month I would have to catch 43 juveniles and no adults just to bring the ratio up to 5:1 and there seems to be little likelihood of that happening. To bring the ratio near to the average of the last 3 years I would have to catch 133 juveniles and no adults and there is simply no prospect of that. It certainly looks like Willow Warblers have had a very poor breeding season, locally at least. I say locally as most of the birds caught in July will be local breeders and their offspring. 

The prospects for this breeding season had looked quite good at one stage with with migrants, like Willow Warblers, arriving quite early and in good numbers. There is no doubt the weather over the spring and summer has been mixed but it generally seemed to be more favourable for breeding birds than the last couple of years and certainly no worse. To find that Willow Warbler productivity appears to be so low this year is totally unexpected and hard to explain. It will be interesting to see the results of CES and other BTO surveys when they come out later in the year to see how they compare.

Willow Warblers have shown marked differences in regional trends in recent years with the species showing large declines in the south and holding their own or doing better further north. Next month should see a lot of migrant Willow Warblers move through the site and their numbers may give an indication as to how the species has fared in more northerly areas of England and Scotland. Lets hope they have done better further north.

The final capture total of 40 birds (33 new and 7 retraps) was less than expected and continues to suggest that quite a few species have had a poor breeding season locally. Ringing totals for 25/07/2016 (retraps in brackets) were: Goldcrest 2; Blue Tit 5; Great Tit 1; Willow Tit (1); Chichchaff 5 (1); Willow Warbler 12 (5); Blackcap 2; Wren 1; Robin 1; Chaffinch 1; Bullfinch 1; Goldfinch 1; Reed Bunting 1.

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