Impacts of the wet year on Britain’s birds

2012 preliminary UK Nest Record Scheme (NRS) results from BTO

Full news report and tables from BTO here

Reed warbler. Photo by Mick DrydenThe BTO publishes preliminary results each Autumn to provide an initial assessment on the UK’s bird breeding season prior to the publication of the annual BirdTrends Report the following year. Producing results so soon after the breeding season is only possible thanks to NRS participants’ speedy submission of data.

Record-breaking rainfall

To say it was a wet spring and summer this year is an understatement. UK Met Office rainfall totals for England & Wales in two months were the highest on record, with four times the average falling in April and twice the average falling in June. If the adverse weather affected nesting birds, it also made life difficult for volunteers, resulting in many cancelled field sessions and missed nest visits. However, thanks to the dedication of the nest recorders, over 25,000 records were submitted in time for the preliminary report and 2012 breeding performance figures have been produced for 25 species.

Early breeders advance, late breeders delay

Conditions in February and March were actually warmer and drier than average, and many of the early resident breeders, including tawny owl, song thrush and long-tailed tit, commenced nesting significantly earlier than the five-year average. In contrast, long-distance migrants travelling northwards from their African wintering quarters the following month had to contend with a succession of severe weather systems across Europe and were greeted by cold, wet weather when they finally returned to their breeding grounds. As a result, laying dates of all six species for which preliminary results are produced were significantly delayed relative to the average for the preceding five years. Laying dates of short-distance migrants, blackcap and chiffchaff, which winter in the Mediterranean and therefore return to breed earlier, were no later than average.

Caterpillar-dependent residents struggle

Chaffinch. Photo by Mick DrydenFeedback from lepidopterists suggest that 2012 was a very poor year for moths and species that rely heavily on caterpillars as a source of food for their nestlings experienced a sharp drop in productivity. The average number of fledglings produced per breeding attempt for both blue and great tit was significantly below average, the latter experiencing the second worst season on record. This poor performance was influenced by small clutches and high failure rates during incubation, as well as by a reduction in brood sizes, suggesting that adults were in poor condition when breeding commenced. The number of chaffinch fledglings per nest was at its lowest level since records began in 1966, by some margin. Losses during incubation were high and heavy rainfall may have had a direct impact, wetting the eggs when the female left the nest to feed.

A poor vole year

Common kestrel. Photo by Mick DrydenMany of the lowland owls and raptors feed primarily on volves, which anecdotal evidence suggests were in short supply during 2012, although the extent to which this paucity was influenced by the adverse weather conditions is not yet clear. However, heavy rainfall almost certainly affects the accessibility of these prey items, making it difficult for birds to hunt. Fledgling numbers of all three species for which preliminary trends are produced (kestrel, tawny owl and barn owl) were below average, although this difference was significant only for kestrel.

Flooding and water-logging

Exceptionally high levels of rainfall directly impacted on those species whose nests are vulnerable to flooding. Reed warbler is the most obvious example, demonstrating a significant reduction in every aspect of its breeding success as a result of rising water levels in ponds and lakes, leading to a fall in fledgling production of almost 36%. Increased river flow may also have contributed to the increase in failure rates of dipper nests during incubation.

Poor prospects for fledglings?

Long-tailed tit. Photo by Mick DrydenThe results of the preliminary NRS analyses are broadly consistent with those of the BTO’s Constant Effort Scheme (CES), which calculates breeding success by comparing the numbers of adult and juvenile birds ringed each year at 120 sites across Britain & Ireland. However, there are some species (long-tailed tit, willow warbler) for which CES records a much more marked reduction in productivity during 2012. This difference suggests that young birds may have continued to struggle after leaving the nest, their lack of experience and relatively poor quality plumage making it much harder for them to cope with extreme weather conditions, leading to a drop in survival rates post-fledging.

Long-term impacts

Many of the species for which preliminary trends have been calculated are capable of producing large numbers of offspring each year, and so have the potential to bounce back rapidly following a poor breeding season provided conditions improve. However, the extreme conditions in 2012 resulted from a shift in the jet stream and it is difficult to predict how its position may be influenced by future climatic warming and the melting of the Arctic icecaps. If wet summers become more frequent, then we may witness long-term changes in the numbers of some bird species.

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