BTO: Keeping warm during winter’s grip

Small, insect-eating birds like this Goldcrest may find winter a particularly challenging time. 'Picture: Mike Toms/BTO

Small, insect-eating birds like this Goldcrest may find winter a particularly challenging time. 'Picture: Mike Toms/BTO

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The ‘Shortest Day’ may well have passed but there is little indication that the hours of daylight are getting any longer or that winter is about to loosen her grip, writes the BTO’s Mike Toms.

In fact, the first two months of the new year are when winter tends to be at her most bitter, delivering challenging conditions for much of our wildlife, including many of our garden birds.

Small birds can find things particularly difficult during the winter months; the small amount of daylight reduces the time available for foraging and, with favoured food hard to find, birds have to burn off valuable energy reserves in order to maintain their body temperature through the long winter nights. Despite their feathers, small birds lose heat quickly and so either seek shelter in thick vegetation, huddle together for warmth or cram into nest boxes and other cavities in an attempt to keep warm. If you have a nest box camera in your garden then switch it on one evening to see if the box is being used by a solitary roosting blue tit or a huddle of wrens.

Energy reserves used overnight need replenishing come dawn and this is why garden bird tables and hanging feeders tend to be so busy first thing in the morning (and again just before dusk). If you watch your feeding station closely enough over the course of a morning you’ll discover that some birds arrive earlier than others. Robins and blackbirds are the first species up and about, while greenfinches and starlings tend to arrive much later. Work by ‘citizen scientists’, participating in the BTO’s weekly Garden BirdWatch, has revealed that birds with bigger eyes (relative to their body size) get up earlier. This suggests that while our garden birds are under pressure to replenish lost energy reserves first thing, they cannot do this until it is light enough to see what they are doing. Bigger eyes let in more light and enable birds with larger eyes to start foraging that much earlier.

An equally fascinating observation from this study is that urban birds get up later than their country cousins. It is thought that this is because our towns and cities are warmer overnight than the surrounding countryside, typically by a few degrees. Such subtle differences in temperature may have a big effect on how much of their fat reserves birds have to burn off each night. If a robin uses less of its reserves then it will be under less pressure to find food come the next morning, meaning it can remain in its roost that little bit longer.

Interestingly, the BTO has been repeating this study over the last couple of days in an attempt to see what effect light pollution may have on early morning activity. Find out more at www.bto.org/gbw or call 01842-750050 for a free information pack.