Complete Goldfinch moult and other eccentric finch moults

It's long been known that some finches will undergo a limited moult of their primaries as part of their (partial) post-juvenile moult. It's not always easy to spot, but over the last couple of days we've seen some good examples in my Lizard garden. This Greenfinch is easily aged as a first-year bird (juvenile) by virtue of its relatively fresh plumage and brown-centred, pointy primary coverts. But you'll notice that it's in primary moult, with one feather still growing and another fully-grown inside it. The moulted feathers are not only fresher, but have much blacker centres, greyer tips and the yellow portions on the outer web are brighter than the juvenile feathers. But note that in this type of eccentric moult, the corresponding primary coverts aren't replaced.

The difference in the colouration of the moulted/unmoulted feathers is obvious here

This same pattern of moult is seen in several other finch species, including Goldfinch, Siskin and, rarely, Linnet. Two Goldfinches bizarrely caught at the same time today also showed this same moult pattern ,but had already finished moulting. So whilst they didn't have any obvious growing feathers, the contrast in the two ages of feather is still obvious enough. Both birds had just moulted two primaries in the centre of the feather tract.

Note the difference in the base colour of the feathers and also the degree of wear
on the white tips of the new feathers and those inside them

The second bird was very similar. Note how the two new feathers not only have
much 'smarter', unworn white tips, but also have much brighter yellow outer webs,
forming a brighter yellow patch in the wing.

Interestingly though, we did also catch a Goldfinch that was just finishing a complete moult, having moulted all flight feathers (primaries AND secondaries), primary coverts (seemingly) and tail. We know this was a first-year bird, as it was ringed as a 3J (complete juvenile) on 26th May. This is a much rarer pattern of moult, but unfortunately we don't have any photos of this bird!

Apart from garden ringing, we've also been helping out a Masters student at University of Exeter, helping to fit PIT tags (transponders) to Blue and Great Tits. This is part of a study looking at the effects of supplementary feeding on fitness in birds. PIT tags allow birds to be recorded remotely at feeders, checking themselves in and out of a feeding aviary as they pass through an aerial.

Although it looks clunky, the PIT tag on the right leg (on the left here) is very light,
and acts like a bard code which scans as the bird enters the aviary.

Finch frenzy and a moult muddle

The last few days has seen a continued influx of autumn migrants into The Lizard and a couple of calm mornings has allowed us to capitalise on this. In my relatively small garden (below, with the school playing field behind), with just one 30' net, we've been busy catching Goldfinches coming to nyger seed and have caught over 140 in the last two days. Add to this a few Greenfinch, Starling, Chaffinch and single Siskin and Pied Wagtail and it's been busy!

Most of the Goldfinches have been young birds (and oddly more females than males), distinctive with their pointed tails, juvenile primary coverts and unmoulted greater coverts (although most had moulted them all). A surprising number were still in their first juvenile plumage as well, which seems a bit late. Most of the (presumed) adults caught were just finishing primary moult, with just two already done. There were then the odd one or two birds where ageing wasn't quite so simple.

The bird below was just finishing primary moult, with just the outer two primaries (including the vestigial outer one) to go. But is the unmoulted primary here really old enough? If this was a moulting adult, then this primary would have been grown as part of the bird's main moult a year ago. The other option is that this is a juvenile bird (from an early brood perhaps) undergoing a complete moult, in which case this primary would have been grown in the nest a few months ago.

So how can we decide on the age of this feather? Comparing it to known juveniles is one option... The bird below is a known juvenile, with a 'good' tail and dull primary coverts, and appears to show primaries with a similar amount of wear to our mystery bird.

Another juvenile (below) shows even browner primaries though, despite the poor quality photograph - iPhones don't do macro! So if our mystery bird has a fresher unmoulted primary than this juvenile, then it too must be a juvenile right?

Note as well on this juvenile has also moulted some primaries, but not all of them. The outer three (ignoring the vestigial outer feather) appear rather brown with worn tips, whereas the inner feathers are much fresher, darker-centred, with brighter yellow outer webs and with more obvious unworn white tips.

To be sure we'd really need to compare to a known adult in active moult and see how worn the unmoulted outer primaries really are at this time of year. But this does indeed look like a juvenile undergoing a complete moult, which means that it's rather unsafe to assume that other birds finishing primary moult are adults...