Monday, July 21, 2008

How long do birds live?

Times Online (Derwent May)

The writer Derwent May published an article on The Times, which I read traveling up to North Wales recently. I tore it out to read later as we were just arriving at my station. I thought that it might be of interest, given how much bird life we have in trees and on the docks. Here's an extract but the full article can be read at the above address:

If a razorbill can live to 41 when we thought it only lived to about 13, we may have to rethink our view of avian lifespan

A razorbill, it is reported today, has lived for 41 years, in spite of being buffeted by the winter waves in the Atlantic for a good half of every one of those years. Is that normal? In fact how long do birds generally live?

We know that there have been many other long-lifers. The oldest bird known is a sulphur-crested cockatoo, which was over 80 when it died in London Zoo in 1982. Fulmars, another seabird, have been found alive after 40 years. A golden eagle that spent its life winging over the Highlands reached 32.

Until recently we hardly knew anything about birds' ages. It is ringing them, usually when they are still in the nest, that has taught us that some of them live for so long. Their date of ringing is recorded, and when they are found again with the little numbered ring still on their leg - washed up dead on the shore, or re-ringed when they return to their colony to nest - we know their age.

But really these razorbills and cockatoos are exceptions. All they tell us is how long birds have the capacity to live; the truth is, very few of them get there.

We can see this if we stop and realise that in most summers, under stable conditions, there are just the same number of birds around as there were the previous year. That means that a number of birds equivalent to the whole baby boom of last year must die. And they do. Most of the new birds born in any year die. So each year the population is made up of many old birds, and a small number of new ones.

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