Hi Jill et al
I don't know about Common Terns in winter in NE NSW, but in summer I have
seen (eg at Ballina) the same sort of mix as in Moreton Bay - ie mostly
adults and a minority of 1st year birds with a few second years while you
can still tell them apart.
When I lived in Brisbane, I didn't regard Common Terns as at all uncommon in
SE Qld in winter - in fact they were commonly encountered in flocks of tens
to hundreds in Moreton Bay. Almost all of these would have been 1st year
birds, based on the plumages they showed. The species was certainly much
less common than in summer, but I wouldn't be so confident this applied to
1st year birds.
There was also a phenomenon which I used to see every year, of large
collections of migratory terns (especially Common and White-winged Blacks)
out along the ocean coasts in March - April. I wondered if these had moved
out from the estuaries prior to migration. But I also felt there were more
birds in the region as a whole than in summer. If that was the case, then
the additional birds would have presumably been migrants, and therefore more
likely to be adults, diluting the proportion of first years.
Another point to be aware of is that it doesn't necessarily follow that
birds you are seeing in partial (or even full) alternate plumage must be
adults. They could conceviably be younger birds, and it's always possible
the real adults have already moved north. Danny should be able to throw
light on that question.
Good to see someone interested in these questions!
Cheers, Chris.
Chris Corben.
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