Extremes of moult in first-year Siskins (23 March 2004)
One of the reasons why I enjoy catching finches is that they can exhibit a very wide range of post-juvenile moults. The Siskins Carduelis spinus visiting my Cheshire garden in the last few weeks have illustrated the extremes. All of the birds shown here are males. Females show exactly the same patterns, but their adult feathers are less strikingly marked so the differences between the generations of coverts can be somewhat more difficult to see.
This male had moulted all of its lesser, median and greater coverts (gcs), and its tertials. At a quick glance it could perhaps be mistaken for an adult, but closer inspection shows that it is a second-year bird (Euring age 5, hatched in 2003) that has retained all three alula feathers, and its carpal covert (only just visible in this photograph). The easiest way of telling the age of this bird was by looking at its tail, because it had moulted five of its innermost tail feathers (two on the left side and three on the right), thus showing a clear difference in shape and wear between the 'adult' type of rectrix, grown during the post-juvenile moult, and the 'juvenile' type, retained from when it left the nest.
Of course, the feathers become more worn gradually during the year, and one should beware of comparing individuals at very different times of year, but all of the birds shown on this page were photographed in January or February 2003 or 2004.
Another second-year male with all new gcs had also moulted the two smaller tertial feathers, but not the longest. The tertial moult was symmetrical, as seen in the close-up photo of both wings in their natural position, folded over the rump. He also had moulted some of the tail, in this case renewing the inner four and the outer two rectrices.
The final example of extremes of Siskin moult is the male shown below. He had not only kept all of the juvenile greater coverts, but had also retained all of the median coverts, and some of the lesser coverts (next to my thumb) as well. This is the least advanced stage of post-juvenile moult that I have noticed this winter.
When the bird was in its normal pose, it was obvious that the retained juvenile coverts showed up as whitish wing-bars, rather than the normal yellowish colour, and some of the unmoulted juvenile lesser coverts were visible as well. I wonder what other Siskins - especially potential mates - make of such an immature-looking bird.
All of this is well documented and illustrated by Jenni & Winkler in their classic book Moult and Ageing of European Passerines, published by Academic Press in 1994 and now, sadly, out of print. There are, however, often differences between the extent of moult that a species undergoes in northern Europe and central Europe, the origin of most of their data. For instance, I have not (yet) seen any Siskin that has moulted any primaries in its post-juvenile moult, as had happened with 1.3% of the Swiss birds, although they found that the proportion varied annually between 0.1% and 5.7% so we might have to wait for a suitable year. Knowing the great variability in moult in this species, it was probably no surprise that Dr Lukas Jenni took the Siskin as the first example during his plenary talk on molt and ageing at the EBBA (Eastern Bird Banding Association, USA)80th Annual Conference (April 2003) at Powdermill Nature Reserve. Also, there is only a small difference (one letter actually) between the European Siskin Carduelis spinus and the North American Pine Siskin Carduelis pinus! Pine Siskins exhibit a similarly wide mixture of renewed and retained coverts and tertials following their first prebasic molt, according to Peter Pyle's Identification Guide to North American Birds (Slate Creek Press 1997).
To put some numbers on the variability in moult, this winter (from 31 January 2004 to 23 March 2004) I caught 176 Siskins in my garden in north Cheshire. Eighty-six of them have been 'adult' birds (Euring age 6) and I have noted the extent of post-juvenile moult on the other 90 'second-year' (Euring code 5) birds, as follows:
In passerines like Siskin there are of course 9 secondary feathers (6 secondaries and 3 tertials) and some authors consider that there are 9 greater coverts: there are, however, clearly ten feathers in that tract, although there is room for debate about whether the innermost is truly a greater covert. Several of these Siskins have had an old gc 10 with new gcs 7, 8 and 9, so the innermost feather is probably moulted out of order with the others, in a similar way to the tertials being moulted separately from the other secondaries.
As always in ecology, observing and recording these differences is the easy part; the tricky bit is to explain them. The life cycle of Siskins is largely dictated by the availability of tree-seeds, especially spruce picea for early breeding, pine pinus for later breeding, and birch betula for autumn. The birds that have moulted least are likely to have been from later-hatched broods that could not find sufficient birch or other seeds in autumn, whilst those that undertook an extensive moult presumably had access to a reasonable food supply. It is known from recoveries of ringed birds that Siskins passing through English gardens in winter are from distinct populations breeding in Scotland and in continental Europe, especially Scandinavia: perhaps these two groups had different success finding food in autumn 2003?
Juvenile feathers are structurally weaker, and - as withGreenfinches - I assume that it is advantageous for birds to replace as many juvenile feathers as possible during their autumn post-juvenile moult. It would be fascinating to be able to test this hypothesis by comparing the winter survival, and perhaps the first year's breeding success, of birds with differing extents of post-juvenile moult.
Finally, some finches are kept, and bred, in captivity, and sometimes these birds escape. In Britain, the law is that captive-bred Siskins have to be close-ringed. I caught an adult male Siskin on 3 March 2003, wearing a close-ring with a number and the inscription 'IOA 00', showing that it was issued by the International Ornithological Association in 2000. Unfortunately we have no information on its origins because the International Ornithological Association has not replied to my letters. However, I caught the same bird again almost a year later, on 22 February 2004. It is interesting that, after whatever period it spent in captivity, it now appears to have adapted successfully to living in the wild.