Words: Kate Risely, of the British Trust for Ornithology (BTO)
The tiny, jewel-like Goldcrest is a bird whose fortunes I have been following with interest. One of our smallest species (Firecrest is of very similar stature), weighing just 6g, and a year-round resident, Goldcrest populations can crash after hard winters – of which we have had a few in recent years. The winter of 2008/09 was the coldest for 10 years, with particularly cold spells during December and early February.
Numbers of Goldcrests reported to BirdTrack were noticeably lower at the start of 2009 than in the previous two years, leading us to believe that the results from the 2009 Breeding Bird Survey (BBS) would show a marked drop in numbers – and so it proved.
Goldcrest populations had been climbing since the start of the BBS in 1994, possibly due to a run of mild winters, and by 2008 numbers of this tiny bird had reached their highest levels since 1994. However, the 2009 BBS results revealed that the breeding season Goldcrest population fell by more than half between 2008 and 2009, very probably due to mortality during the winter months.
If we look further back into the BTO’s records, we see that Goldcrest populations have experienced similar fluctuations in the past. Numbers were very low following the winter of 1962/63, climbed to a peak in the early 1970s, and crashed again, before showing smaller fluctuations up until the 1990s, when numbers began to rise steadily.
The large year-to-year changes, with numbers bouncing back after crashes, reflect the high breeding potential of this species. Goldcrests, which generally nest in conifers, can lay up to 12 eggs in a clutch (though 6-8 is more common).
These eggs, though barely larger than peas, can represent one and a half times the bodyweight of the female – a huge reproductive effort. Remarkably for such a small bird, Goldcrests breed right up to northern Scandinavia – although their chances of winter survival are even lower in the far north, and many Scandinavian birds migrate to the UK in the autumn.
The fact that such diminutive birds can survive a gruelling sea crossing is amazing, and in the past people simply did not believe that these birds could migrate at all. Observations of Goldcrests arriving on the UK’s eastern shores in the company of larger migrants led to the somewhat far-fetched belief that they hitched a lift in the feathers of Woodcocks or Short-eared Owls.
Looking at the BirdTrack reporting rates for 2009, it is clear that, as well as numbers of breeding birds being lower than previous years, the influx of birds from Scandinavia in the autumn was also far smaller than usual. One explanation for this could be that Scandinavian breeders were also hit by the hard winter of 2008/09, resulting in smaller numbers migrating in the autumn.
After the poor year of 2009, Goldcrest populations would normally be expected to bounce back. However, the winter of 2009/10 was even more severe than the previous year – the average temperature for the period December 2009 to February 2010 was 2°C below the long-term average, making it the coldest winter since the 1970s! We don’t yet have the BBS results from 2010 to show us how this winter affected Goldcrest breeding numbers, but indications from BirdTrack seem to show that numbers during the breeding season were roughly similar to 2009.
By the time you read this, Goldcrests should be flooding into the country, filling the coastal scrub in places like Norfolk, Lincolnshire and Northumberland, and no doubt closely scrutinised by those hoping to find a rarity amongst them.
It will be interesting to see the shape of the BirdTrack reporting rate graph for winter 2010 – will there be the normal peak in October? Or will the 2009 pattern, where numbers hardly rose at all, be repeated? And for the sake of the birds, I hope that this coming winter is less severe than the last two.