Late

22 Apr 2011 MP

ajohn

All birds in the photo are Black-tailed Godwit.

This is a very informative picture. Most of the birds are in breeding plumage but the larger bird is in non-breeding plumage. Observations and data from trapped birds indicate that wintering birds are larger and moult into breeding plumage later (if at all), so it is likely that this is a wintering bird surrounded by birds from the migrant population. The reason for these differences is not currently known.

If anyone has similar pictures showing flocks of Black-tailed Godwit with large and small individuals, I would be interested to see these.

sdavid

Very useful capture showing the differences bwteen the two populations.

Late

Thank you John detailed comments.

brendank

John, when you say "Observations and data from trapped birds indicate that wintering birds are larger and moult into breeding plumage later (if at all),"

Are you referring only to Black-tailed Godwits or more generally to other shorebirds?

ajohn

Just for Black-tailed Godwits. This is mentioned in the Avifauna. Differences between wintering and migrant populations are not as obvious among other shorebird species.

Late

Hi John & all expert,

I have a question.
non-breeding plumage are bigger than breeding plumage?

I think this bird may be old bird? If not, It can reduce head & mouth size?

ajohn

No, the size doesn't change between breeding and non-breeding. Most growth in birds occurs before they start flying, and there is very little change in size after that. All that changes are the feathers (which are replaced by moult). It is not reliable to age birds according to size, but plumage features can sometimes be used; as far as I can see, there are no features visible to help age this individual.

The point I was making in my previous e-mail was that the wintering Black-tailed Godwits are typically larger than the migrants (which winter further south) AND typically later to gain the breeding plumage. When this bird moults it will probably look similar to the others, but larger.