Thursday, March 29, 2007

Blackcaps and stuff

Nothing for a while, weather, family stuff, and an exploding tripod, and nothing for a while more as we head off to a birdless Greek Island to paddle in freezing seas and enjoy the site of an island closing down for Easter.

Wintering Blackcaps. I’ve seen a male occasionally during the winter, and a female once. This is the third year in a row they’ve been around. I’ve had the impression that the male spends most of its time somewhere in our small road as I’ve frequently heard it chirping to itself from the bottom of a bush in a neighbouring garden. So presumably the one I saw last winter over by the cricket club was a different one.

It seems to me that many birdwatchers have a resident Blackcap in their garden. So just how many are there? Say that there is one for every 30 houses – 1 per 100 people, so that’s 500,000. And say only a quarter of total habitation is suitable so that’s 125,000. And they are quite widespread.

Now for the surveys. According to one in Worcester . the BTO 1978/1979 survey found a total of 1714 of which Worcester had 39. A more recent survey found in Worcester found 245 males and 115 females – almost a 10 fold increase, which if replicated in the national survey would give 17,000. I think I saw a BTO survey which gave the figure of 10,000.

So there’s a ten-fold difference between my crass estimate and the survey. I know mines a rough guess, but how many are there actually here in Sawbridgeworth, a town of c11,000 for instance? If there were 10 – quite likely , that would be 1 per 1000 – a population of 50,000 across the country.

The generally accepted version is that this is a German/Eastern European population that has developed a separate migration route. But how has this happened? One theory is that this is a genetic mutation that has become propagated by virtue of its success (as the climate warms, and gardens more hospitable), and a whole new population has arisen.

Theories are easy, but evidence is difficult. Fortunately the ever excellent P. Berthold has got some – the offspring of British-wintering Blackcaps naturally orient themselves WNW in autumn, whereas offspring of Southern German Blackcaps naturally orient SW.

So presumably Yellow-Browed Warblers, Pallas’s and Dusky Warbler’s are doing the same in their wintering populations in the UK.

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