Birds in winter

by Richard Jones

It's no surprise that Britain's smallest bird should suffer in the cold. It's all to do with body mass and surface area ratios.

Long-tailed tit, image courtesy of S. Tranter/RSPBNearly back to normal now, after Christmas and New Year. Sunday saw us with 3-year-old scooting in Dulwich Park. Thankfully there was no wind, because it was blisteringly cold, and the ground was still covered in frost. So when I saw a small bird flitting through the branches of a cherry laurel tree I looked twice. It was a goldcrest.

I've never seen one hereabouts before. Apparently the recent spate of mild winters has helped their numbers increase. Who knows, maybe they'll be vying with the long-tailed tits in my garden next year.

It's no surprise that Britain's smallest bird should suffer in the cold. It's all to do with body mass and surface area ratios. I wonder whether I can get away with a bit of maths on this blog. Imagine a cubic bird — strange design I know, but just humour me. If it is 1 cm along each side, it will have a body volume of 1 cubic centimetre (cm³) and a surface area of 6 square centimetres (cm²). A larger bird, 2 cm along each side, now has a volume 8 cm³, that's eight times as much heat-producing body mass. But its surface area is now 24 cm², only four times as much skin surface through which to lose heat. Keep scaling this up and it's clear that the larger the bird (or other animal), the better insulated against the cold it becomes.

No mammals the same size as the goldcrest are active at this time of year; they switch off their metabolism and hibernate. Good thinking. Birds? They can migrate to warmer winter grounds. Those that stay make a tough choice. As I'm writing this snow is falling outside. I hope we don't get a bitter winter, otherwise the goldcrest's recent gains might be pushed back and I'd like to see them bobbing about on my trees.

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Gardeners' World Web User 08/01/2009 at 19:42

"Recent spate of mild winters " not in north Northumberland im affraid ! Amazing how these little chaps make it through these winter nights.

Gardeners' World Web User 08/01/2009 at 22:28

I feed the birds , as in hang out peanuts and fat balls , and chase the cats but.... there was a big RAT feeding yesterday !! I feel like removing all my feeders , what else can I do ?

Gardeners' World Web User 08/01/2009 at 22:29

And it is jolly chilly here at the moment

Gardeners' World Web User 09/01/2009 at 00:04

Its deinately winter here, have 2 Robins in the garden regularly and feed the family of 4 squirrels all year...

Gardeners' World Web User 10/01/2009 at 11:00

Our garden is full of birds at the moment. We have had a most interesting visitor a strange male blackbird with what looks like a white painted collar around his neck. I believe it is Leucism. Has anyone else seen one like this? He is very tame following us around with no fear.

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