Thursday, 2 March 2017

Ageing Goldfinches gets more interesting.

This is a post for aficionados of moult, ageing birds in the hand and Goldfinches in particular but hopefully it will be of interest to others too. First year Goldfinches normally only have a partial post-juvenile moult in the UK and this usually involves the replacement of the body feathers along with most of the upper wing-coverts and tertials, and may also include some of the tail feathers. However, first year birds from populations in southern parts of Europe frequently have a more extensive post-juvenile moult which involves more of the upper wing coverts and can include some or nearly all of the flight feathers. Whether any actually have a complete moult, like adults, is open to question but if some do it may only be a fairly small proportion. It is probably better to think of it as an unknown proportion as it is an aspect of moult that is difficult to study so may be more common than the literature suggests.

Much of the literature on this subject is quite dated now and many ringers in the UK will have had experience of first year birds that have undergone a more extensive post-juvenile moult than was the case say 20 or 30 years ago. When I started ringing it used to be relatively rare for Goldfinches and Greenfinches to replace all their greater coverts as part of the post-juvenile moult but things have changed and it is much more common these days. That tendency for a more extensive post-juvenile (pj) moult hasn't stopped there and some Greenfinches also started replacing a few primaries as part of their pj moult. The inclusion of some primaries in the pj moult of British Goldfinches followed and there have been recent reports of first year Goldfinches undergoing a complete moult like adults. In a couple of cases that I am aware of the birds were ringed when they were in juvenile plumage and were considered to have undergone or were just finishing a complete moult when they were retrapped later in the year. I don't know how thoroughly these birds were scrutinised when they were retrapped but one of the reports suggested the extent of the moult was noticed at the time of recapture rather than being assumed from an age discrepancy on checking the original ringing details.

So where am I going with this and why is it interesting. Well I caught a Goldfinch yesterday that had undergone a very extensive post juvenile moult and it only just fell short of a complete moult. It had undergone the most extensive pj moult I have ever seen in the species and is the sort of bird that could get overlooked unless all the feather tracts are carefully examined. It had replaced nearly everything and the few feathers it hadn't replaced were not very obvious and are easier to see in the photographs than they were in real life. I had to turn the bird so the feathers caught the light just right and also played with the exposure setting on the camera to be able to show the differences between the moulted and unmoulted feathers. So lets have a look at it ......

I have included this photo to show that the moult limit in the primaries was not that easy to see. The outermost primary in the image is just that bit browner but remember this image is larger than life size. Also, and more importantly, the bird was held at just the right angle to the light to try and make the difference stand out.
Close the wing a little bit and adjust how the feathers catch the light and the difference is a bit easier to see.

Zoom in even closer and you can see the outer feather is a little more worn in addition to being brown but as the wear is relatively slight and the feather is not that bleached it rules out the possibility of it being an adult that has arrested its moult.
Here I have to admit that I thought the bird had replaced all of its primary coverts on the right wing when I examined it and only noticed that it had just replaced the inner 6 on reviewing the photographs. The contrast between the new and the old feathers wasn't as obvious in real life as it appears in this heavily cropped photo. Because of this oversight I didn't check the primary coverts on the left wing.
Zoom in a little more and it doesn't make the difference that much easier to see. The 7th and 8th primary coverts were unmoulted and the 9th (if Goldfinches do have a 9th primary covert) is so small its very difficult to see never mind age.

So here we have the left wing. I had to show it this way up as I can't use the camera in my left hand. The two outer primaries in this image just look a a tiny bit browner and the shape formed by the primary tips isn't quite right.

Crop in closer and you can see the outer two primaries in the image are browner, unmoulted juvenile feathers.
Zoom in further and the difference in colour is still obvious but the old feathers are only a little more worn. So it has moulted asymmetrically but only by 1 feather.
I was a bit less certain about the age of the tail feathers although I do think all the tail feathers have been replaced. There only appeared to be one generation of tail feather as there was no detectable difference in the intensity of the black between any of the feathers. They were too fresh looking for juvenile feathers although the 3rd, 4th and 5th tail feathers were a little bit more worn and pointed than I would have liked but then we have to bear in mind that it is March, if only just, and the tail feathers of adults are starting to show signs of wear at this time of year. If these were unmoulted juvenile feathers I would expect them to be far more worn and pointed and a little less glossy too. It is fair to say that it is not the most convincing adult type tail but then it doesn't look like a totally convincing first year tail either. 

This image shows all the tail feathers and the outermost (6th) tail feather on both sides appears to be very slightly less glossy black but this is an artefact caused by the light acting on the angle of the tail feathers. I just couldn't keep the tail totally flat so some of the feathers are in a slightly different plane and that affects their appearance. The shape of the feathers is just within the range shown by adult type feathers and I do have photographs of a known age adult with a similarly shaped tail. It was the appearance of the tail that initially set the alarm bells off in my head and caused me to have a really good look at this bird.

In this image we get a much closer look at the tips of the tail feathers and that wear. The tips of the feathers on the left side of the tail are slightly less worn than those on the right but it is marginal and not that unusual. The wear is fairly even within each half of the tail and supports my view that there is only one generation of tail feathers. The black portion of feathers  5L, 6L, 4R, 5R and 6R appears to be less glossy black than the other feathers but again it is simply an artefact of the light and is also a camera focus and depth of field issue rather than it being anything age related.

The tertials were interesting in that there was some uneven wear of the white tips with the middle tertial being a bit more abraded than the other two. They are all the same glossy black so they have all been replaced but the white tip of the middle tertials are showing more signs of wear. The white tips of the tertials in this bird are a little more worn than you would expect to see on an adult, even though they have been replaced like those of an adult, but that is probably because a first year undergoing an extensive partial post-juvenile moult will have started moulting earlier than an adult undergoing a complete moult. The replaced feathers on this bird could be up to a month or more older than those of a fully moulted adult and therefore should show more signs of wear. This difference in age of the new tertials on this bird compared to those of an adult may also explain why the tail is as worn as it is.

Here is an image of the right wing which clearly shows all the feathers are new apart from the outer primary and primary coverts as detailed above.

And here is the whole bird and it is a nice male it is too, not that its sex has any bearing on the extent of the pj moult.

So why is any of this interesting and why does it matter? Well, if it becomes the norm for some first year Goldfinches to have a complete moult then those that do will be indistinguishable from adults and potentially make it unsafe to age any Goldfinch as an adult. The error rate at the present time is likely to be small but it is unknown quantity and may not be as small as we would like it to be. Opinions will vary as to the level the error rate needs to reach before ageing Goldfinches as adults is considered to be too unreliable but we have to prove to what extent it is happening first. Unfortunately finding this evidence is intrinsically difficult for reasons I won't bore you with now but the fact that these juveniles become indistinguishable from adults is no small hurdle in itself.

The best evidence will come from retraps and some of that evidence may already be out there in the form of age discrepancies between the time of ringing and recapture. An indication that some birds are having a complete moult will come from birds that have been ringed when they are in juvenile plumage and and are aged as an adult when recaptured later in the birds first year of life. If this type of discrepancy becomes more common it potential points to more first year birds having a complete moult as opposed to errors in data recording and ageing. So what may initially appear to be a cock up could actually be a useful piece of evidence.

Another reason the change in the extent of post juvenile moult matters is that it is probably linked to climate change. Research is showing that some summer visitors are arriving earlier and a range of species are breeding earlier. The factors that are influencing those changes are likely to be involved with the tendency for some species, such as Goldfinches, to have a more extensive pj moult. Monitoring pj moult in a systematic way across a range of species could prove to be worthwhile and could be another useful indicator for demonstrating the effect of climate change.

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