Sunday, 14 March 2010

Brackenhurst - Sat 13 March

What a beautiful morning. Blue sky and singing birds everywhere, though still not much sign of new leaves and I'm still yet to see a daffodil. It'll be very interesting to see how spring progresses when everything seems so late. Anyway, it was a great morning to be out, even if the mild weather seemed to discourage many birds from visiting the feeding station. As always, a slow morning means a chance to scrutinise birds more carefully than usual.
We finished on 23, 10 retrap and 13 new, about half of which were Yellowhammers. 4 retrap Yellowhammers were all adults and this gave us a good opportunity to check them against the recently published aging and sexing criteria published by Jenny Dunn and Chris Wright. We found that birds showed many of the features described but not all and by continuing to check birds carefully we hope to be able to establish which are the most helpful for us. We also looked carefully at alulas as it would seem that many first year birds show a moult limit with the tiny feather having been replaced, and the larger two not. This is potentially very useful as it could prove to be one of very few hard and fast aging criteria available. The smallest feather of the young bird is fringed with a greenish-yellow and has a dark grey brown centre that matches the replaced lesser coverts and this contrasts with the larger two feathers which are a more bleached brown with a buffy yellow fringe. There is no doubt that this is subtle and on some birds it is difficult to determine, however, on others it is fairly easy to detect with a bit of practice. The following picture show two fairly typical birds (well, perhaps slightly more obvious individuals!) and as can be seen the adult birds also tend to show a more attenuated large feather, whereas that of the the first year bird is blunter. However, like so many Yellowhammer criteria, there is some overlap and such a feature should never be used in isolation.
We also caught a first year Blackbird with asymmetric moult limits. Firstly there were two old greater coverts on the left wing, but on the right wing the outermost had been replaced, but not the second. This was possibly a result of accidental loss, but the tertials were also odd making us wonder if it was just an odd bird. On the left wing only the smallest tertial had been replaced but on the right wing exactly the opposite - only the largest two. Also below is a picture of a typical first year tail, fairly narrow and pointed.
I am starting to prepare a crib sheet for aging and sexing the common winter birds we catch as a group and at some point we'll hopefully be in a position to illustrate this with the photos too. So there is nothing unusual about this Robin at all, it is just a typical young bird.
Away from the nets, we recorded one flock of over 600 Fieldfares which was fairly spectcular. It was being ineffectively hassled a Buzzard, which was in turn being hassled by a Carrion Crow. Redpoll, Siskin and Linnet all flew over, Lapwings were displaying and two different Great Spotted Woodpeckers were drumming.

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