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.
Nearly 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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