Big birds, and others (25 January 2003)

A female Sparrowhawk Accipiter nisus was caught at the Norton Priory roost on 23 January 2003, quite early on so it was light enough to take some photos. This is an interesting example for all sorts of reasons. It is an unusually grey bird for a non-adult female, but we should beware because I think Baker's Non-Passerine Guide exaggerates the expected differences between the sexes. That book describes adult males as 'blue-grey' and females as 'slate-brown' while Ian Newton, who has handled hundreds, says in his Poyser Sparrowhawks book 'plumage differences between the sexes are slight'. This one was also the smallest female Sparrowhawk I have ever handled, falling just outside the range quoted by Baker or Newton, with a wing of 221 mm (Baker gives 222 -256 as the range), a tarsus length of 57 mm (59-64) and a weight of 230g. Although a small female, she was still substantially larger than the top of the male range (188-212 wing, 51-55 tarsus, weight (BWP) 110-196g). I did wonder if it could have been a continental bird which might have a different range of sizes, but Sparrowhawks are reported to be bigger, on average, the farther east you get. Dave Okill has very helpfully looked through the data from Fair Isle and Shetland, where the trapped Sparrowhawks are from the continent. The majority of males were 195 - 205 mm, with the largest at 209 mm (6 birds), 210 mm (2 birds), 211 mm(3 birds) and one bird of 216 mm which was left unsexed. Females too are not especially large, mainly 230-245 mm with four at 250 mm and two at 253 mm.

A similar photo, taken with flash, is somewhat better-focussed but the colours are less true to life:

Next, it is an interesting example for ageing as it is a 'Euring 7' (hatched in 2001). It is obviously not a first-year (5) bird as they are barred with buffy tips all over, but this bird has two generations of contour feathers in its wing, and, the clinching point, still some of its juvenile inner median coverts, normally hidden under other feathers and so well protected that they do not usually moult them in their first full moult (last summer). I had to part its feathers, between the tertials and the rest of the wing, to take the photo below, showing the buff tip very obvious on one feather, and several others that are lighter-grey and more worn and frayed edges. Although I have called them inner median coverts, I am not really sure exactly which tracts of feathers they are: Baker does not know either, as he describes  it as 'old feather with buff tip usually retained between tertials and scapulars'!

Plus, the eye of females starts out pale yellow in juvenile and gradually gets darker yellow, eventually going orange-ish in birds several years old, especially around the edges of the iris. The hints of orange flecking can be seen on the outer part of this bird's iris. All in all, a very instructive bird to have in the hand.

Another big bird caught at Norton Priory on 31 December 2002 was a Tawny Owl Strix aluco. The first three pictures are just to enjoy: that of its back shows the camouflage rather well, and those of its head are quite nice to see.







But the really tricky image is that of its wing. We had great difficulty in ageing it, partly because of our inexperience with the species and partly because we could not interpret Baker's Guide very well. Baker writes about a 'terminal band', which is supposed to mean at the end of a feather, but his drawings do not show any: perhaps it should be subterminal band? Whatever it is called, we are supposed to be looking at the pattern towards the end of the feathers, and the shape of the tips of the feathers.

At the time we ended up thinking it was a juvenile (3, on 31 Dec 02) but now, looking at this photo, I think this is wrong. The first secondary seems to be of a different generation from the rest (bigger and darker in colour) but we did not notice that on the other wing so perhaps it was just one feather accidentally moulted. But any bird that accidentally moults a feather grows a new one of the next generation - that's why we get so many first-year finches (especially) with a mixture of pointed and rounded tail feathers - yet this one seems to be similar in patterning to the other secondaries. Also, the innermost three primaries look paler than the rest. Tawny Owls have an unusual moult pattern, starting in the middle of the primaries (p5/6, counting from the inside out), and moulting a variable number of feathers depending on how well they are feeding. They suspend wing moult in September-October with some feathers unmoulted, wherever they have got to, and re-start from the point of suspension next summer. So they can have up to three generations of primary (and secondary) feathers that ought, in principle, to be easy to distinguish, but ...

I find Starlings Sturnus vulgaris interesting birds. There are not many species that have three-and-a-half pages in Svensson. Of course, juveniles, as well as adults, have a full moult in their first autumn, so there are no moult limits in their wings, but there are still differences in the shape and markings of the feathers between the ages and the sexes. Oddly, the adult feathers tend to be more pointed than those of first-years, the opposite way round to almost all other species.

The colours of the eye and bill are the easiest ways to sex them. These two photos, taken this week, show two of the extremes. The upper photo is a male with a clear dark eye and the bill turning to blue as its breeding season hormones start to run. The lower photo of a female shows the obvious white outer ring in the iris and a very plain bill - they go pinkish as they get into breeding condition. This male is in almost complete 'summer' plumage and the female still has all of her winter 'spotty' feathers. But I find that many of them are nowhere near as straightforward, and this subject is worth returning to later in the year.

David Norman.