Crossbills (15 March 2009)
I have never previously handled a Crossbill
Loxia curvirostra and it was a great surprise when two flew into a mist-net at a feeding
station in Delamere Forest, Cheshire, on 15 March 2009. Crossbill taxonomy is in a state of flux,
but measurements of their bills showed that the birds were what we used to call just Crossbill,
now Common Crossbill in Europe, Red Crossbill in North America, Loxia curvirostra.
Crossbills are found in the breeding season in Delamere Forest (Birds
in Cheshire and Wirral: a breeding and wintering atlas by David Norman on
behalf of CAWOS, Liverpool University Press 2008) and these two are highly likely to be local birds, although neither showed any sign of breeding condition
through cloacal protuberance or brood patch. Winter 2008/ 09 has been very poor for many woodland tree-seeds, with many more birds
coming into gardens and feeding stations, and I guess there was not yet enough
food for Crossbills to get into breeding condition. This might also explain why
these two birds came down to my net, the first ever to do so.
To determine their ages, we need to look closely at the wings in the context of their unusual breeding cycle and complex moult patterns described by Jenni and Winkler (Moult and Ageing of European Passerines, Academic Press, 1994). Crossbills eat little other than conifer seeds (spruce, pine and larch) and their lives are so linked to these crops that they are stimulated to nest by an abundance of food rather than the changes in daylength that induce breeding in most other birds. In European spruce forests they can breed from August to May on occasions, but in English pine plantations the season is shorter. Nests with eggs have been recorded in every month from December to June but mostly from February to April (Ian Newton, Finches, Collins New Naturalist 1972).
The female (below) seems the more straightforward to analyse: I think it is a second year bird, Euring age code 5 assuming she hatched in, say, the first half of 2008. In her post-juvenile moult during summer/ autumn 2008, she moulted all the coverts (lesser, median and greater) and all three feathers of the alula. Two of the three tertials were also moulted, with the longest tertial retained and, unusually for a passerine in Britain, she eccentrically moulted all of the primaries but retained all of the juvenile secondaries 1-6, somewhat browner in colour and more frayed than the newer primaries. The proof that this is eccentric, not regular, primary moult comes from the primary coverts: pcs 1,3 and 4 are 'new' - darker, rounded and more glossy - and the retained 'old' pcs (2, 5-8) - brown, pointed and worn - are the feathers grown in the nest. The extent of wear of the 'old' primary coverts and secondaries is that expected for feathers that might be 9-12 months old.
For readers who have access to Jenni & Winkler's book, this bird looks very much like
their Fig. 597. This was the
only first-year bird they had that had moulted all of its primaries. The only
differences from my bird are that J&W's had retained juvenile pc 1-3 and ss 1-5
but had moulted s6.
British ringers are not very used to looking at eccentrically moulted primaries, but the two generations here are very clear. The bleaching at the exposed tips of, especially, pp 4 and 5 is striking.
The alula feathers of the male are unusual, because the normal pattern for passerines is to moult the small feather (A1) and often retain juvenile A2 and A3, the middle and large feathers; the alula of this bird appears the other way round. Jenni and Winkler also show a Crossbill (their Fig. 595) with juvenile A1 and moulted A2 and A3.
Comparing the two birds, it seems likely that the male was hatched much later than the female: not only has she had time for a much more substantial post-juvenile moult, but his retained juvenile feathers are considerably less worn.
I wish to thank Bob Mulvihill, Powdermill Nature Reserve, Pennsylvania, for helpful comments on the photographs of these birds. Bob also commented, from looking at the photographs in Jenni & Winkler's book, that there appears to be an age difference in the shape of tips to the secondary feathers, with juvenile ss distinctly notched (as with the two birds depicted here) whereas adult ss are not. Is this generally true for Crossbills?