Posted: Sunday 3rd March 2019 by Twheatear
Comparing winter and summer censuses
Over the past three breeding seasons I’ve been counting every bird on a 1km transect in the Hereford’s Golden Valley (just over the border but in Gwent for BTO administrative purposes). This year the BTO has asked surveyors to repeat the process in winter. Comparing the results shows some surprises.
First, that there is the same number of species present: 32 in winter versus 30-32 in summer. The familiar summer migrants – chiffchaff, willow warbler, swift and swallow – are replaced, not just by fieldfare and redwing from the Arctic, but also by black-headed gull and redpoll, which do breed in the UK, but are less likely to be found in inland farmland in May and June. Mistle thrushes were also noticeably more common – four on the winter transect but one, at most, in the breeding season.
And the number of birds seen or heard is actually greater in winter (226 versus 182 last May/June). That seems surprising, until you look at the numbers. In the breeding season, birds pair up and establish territories, so you’re unlikely to see large groups (the survey protocol is designed to exclude new fledglings). In winter, many species feed in groups, hence totals of 33 blue tits in winter versus 15 in summer, 15 long-tailed tits versus 3, and a group of 4 pied wagtails when in summer I’ve only seen occasional singletons. The most extreme example is starlings: my highest breeding season total is 3, but in winter a single tree held at least 25.
The one resident species whose numbers dropped was the house sparrow: just three in winter, rather than nine or more (in the same stretch of hedgerow) in summer. I suspect that’s because house sparrows nest in colonies, not individual territories, which would explain the greater concentration of individuals at that time of year.