This year Western Europe had one of its coldest winters in recent memory. As one might expect, this changed bird distribution. The numbers are in from the RSPB's Big Garden Birdwatch, and many species are down compared to last year's results. The Guardian has a summary of the changes.
The survey saw 530,000 people take part and their records showed the average number of birds per garden for already rare goldcrests were down 75% on January 2009, being seen in only about 5,300 gardens. Long-tailed tits were down 27% to an average of one bird per garden, and coal tits down 20%. The long-tailed tithad been thriving due to a succession of mild winters and was one of last year's big success stories.It is not clear from the article what happened to these missing birds. The cold may have pushed them elsewhere, perhaps further south in Europe. They could also have died of cold-related causes, whether hypothermia or starvation. Or perhaps a bit of both. If this count is based solely in the U.K., it would not be possible to answer the question from this count's data alone. (This is one advantage of the Great Backyard Bird Count – it is easier to see movements over a continent-wide scale.) Relocation seems more likely, in my opinion, since the Big Garden Birdwatch showed evidence of a different set of species moving into people's gardens:
This winter's bitter weather had a greater impact because the cold spell was national Kelly explained. Most previous drops in small bird populations caused by cold weather - such as the death of all the Cetti's warblers in Kent after two successive cold winters in the mid-1980s - were only regional.
The snowy winter also led to a huge rise in sightings of countryside birds such as thrushes and finches in the UK's gardens. The number of fieldfares was up 73% on last year, redwings up 185% and song thrushes up 51%, though their numbers are still relatively low compared to garden stalwarts such as blue tits and chaffinches....If you would like to look at the data yourself, it is available online, including in a Google spreadsheet.
Another species apparently doing well was the blackcap, which was up 47% against 2009 levels. The RSPB believes many may have been migrants sheltering from even harsher weather in mainland Europe, though it also speculates that the increased sightings may simply be a result of the blackcaps getting used to eating from bird feeders and becoming more visible.