" The Life of the Robin" is an account of David Lack's ground-breaking study of the truth about the life of the Robin, away from all the sentimentality that surrounds it. There's my version of it on the left, and the modern version on the right. I remember hearing that many readers of the book when it first came out were horrified when they read of the tiny life-span of the average Robin, and the dinky survival rate of the young birds. Many , I read, flatly refused to believe a word of it. Even professionals in the field thought the analysis in the book had to be wrong, though on what basis I've no idea. Wishful thinking seems to be a strong contender. Failure to take notice of what actually happens in the natural world. Failure to simply look around them. So ..I was pleased to read this review of the book ... "How long does a bird live?" is one of the questions ornithologists are always being asked and there is not a simple answer. There is a great difference between the various types of bird and there is a great difference between the average length of life and the greatest length of life.Many years ago David Lack was addressing an audience of learned ornithologists on the subject of his studies on Robins. He said that the average life expectancy of a Robin was a little over one year. He was laughed off the stage. Everybody knew that they have had a Robin in their garden for years and they were sure that it was the same one. But he was right. And it takes only a moment’s thought for anyone to work it out. No need for complicated modern maths - anybody can do it. There are about as many Robins this year as there were last year (populations usually fluctuate about an average); so if there was a pair in your garden last year then there is a pair this year. But in the mean time that original pair has had, say, two broods of five young - that is ten new Robins. But by the start of the next breeding season there are only two. So: 2+10=2 So how many have died? Obviously, ten. So out of twelve (2+10) birds, ten have died - that’s more than 83%. Ringing studies have shown that about 60% of the adult Robins die each year, so of the original two adults only 0.8 of a bird will be alive this year (yes, 0.8 of a bird is nonsense but we are talking about averages here), so for us to have two birds again this year 1.2 (2 minus 0.8) birds must have entered the population. In other words, those ten young have resulted in only 1.2 adults - that’s an 88% mortality. Much of that ‘infant mortality’ happens in the nest or shortly after fledging. Ringing studies have shown that the first year mortality of young once they have fledged is about 72%. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When I talked about " failing to notice what is happening in the natural world" , that's the sort of thing I meant. It's pretty obvious to anyone that most birds have a varying but reasonably stable population, and if the young have a high survival rate, and the adults as well, the population should increase .. unless they're all bogging off to some magic fairyland of infinite dimensions. And very often it's things that aren't there that we miss. An obvious example, and very topical, is migrant birds leaving the country. We all notice faithfully when we see the first one... but the last one is a different matter entirely.. because by the time we notice they're gone, we've missed the "going" . And with the Robins, we don't notice all the young that die even before leaving the nest, we don't notice them getting eaten as fledglings,we don't notice them dying of starvation and cold in the winter and we therefore assume that they're all reaching adulthood ... and then we don't notice the contradiction that follows. If most of them are surviving, where are they ? Then there's even bigger and unnoticed questions...... Why aren't there more of them? Or fewer? Why this particular population level? Why aren't there so many flies that we can hardly see through the darkness ? Why aren't there so many ants that we crush 200 of them at every step ? Why are there only a few types of Starling, but hundreds of sorts of Warbler? Why are all Starlings ( and almost all species) astonishingly identical ? Why are the crow family either dowdy or gaudy ? These are the sorts of questions that nobody ever thought of asking. Until recently. Grrrrrr. Music Time here at M-B Towers ... I think I put this on here ages ago, but another go won't hurt. Klee... Zwischen den Jahren .....
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AuthorThat's the author up there ... I was young and sprightly then. Archives
October 2022
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