15 October 2013

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...

1 comment:

  1. It could have lost one or more outer primaries during the 'breeding season'. Of all the Goldfinches I have handled only the juveniles seem to show any noticeable wear on PP6-9 (unless you look with a hand lens). Being almost equal in length, they tend to wear very evenly.