Hi Chris, thanks for entering the
discussion. I couldn't help thinking that among us are some answers,
and each of us lacks some knowledge in the puzzle. I certainly do. And
may I take this opportunity to say that it was you, all those years
ago, who gave me my first insight into how moult works. Thank you. I
have come a long way but the more I learn the less I know.
Taking your thoughts in order below:
Winter numbers in Moreton Bay:
Years ago you told me about the regular presence of Commons in Moreton
Bay (SEQld) in winter (I think you estimated about 500 may have been
present during winter about 20 years ago), and ever since I have been
quoting that and asking people who might ever encounter terns offshore
or on the coast if they see them in such numbers, and no one sees them
like you did back then. Unless they are hiding somewhere and no one is
telling, they don't seem to occur in Moreton Bay as they once did. I
hear of the odd sighting, but no reports of the flocks I was trying to
locate. I have sounded like a cracked record asking people.
The late summer flocks:
This question has also consumed me. We get a peak of Commons and
White-wings in February, March and April on the Sunshine Coast, but
where they come from no one can report. I guess they could be dispersed
in summer into little flocks all around the coast, if the population
does spend the middle summer period in Australia, and that's not to be
taken as a given. But I haven't been able to extract much informaton
about that. All I get is reports of small groups of migrant terns here
and there. The VWSG have banded and flagged them for years, but I don't
believe their numbers equate to our autumn numbers. I think the fact
that many birders are uncertain in identifying the species suggests
that they are not ubiquitous. I sometimes wonder if they are somewhere
out in the Pacific during the summer. But they're not in New Zealand,
they're not around Lord Howe Is, or Norfolk Is, or they would crop up
in reports. Have you ever wondered about this?
Are they really adults?
I'm still thinking about this. I think you are suggesting that perhaps
those Commons which appear to be adults may in fact be 18 months old,
and just beyond the plumage which makes them identifiable as immatures.
And that the breeding adults have already left. Is this correct? And I
am on very unsteady ground with identifying this age group, because I
hadn't yet worked out at what age they become indistinguishable from
adults. I have developed the hunch that the last thing to go in
immatures is the darker primaries, but I haven't been sure of that. The
"adult" birds which are around the coast now have mid-grey secondaries,
like their primaries.
If the above hypothesis holds water, it could help explain why "our
adult Commons right now" don't have breeding caps, before they leave,
though I have seen some with them in previous years prior to migration.
At what age do Commons first attain breeding caps? In "Terns of Europe
and North America by Olsen and Larsson there is just one pic of a 2nd summer immature showing the
nominate species (photo 91). It shows a breeding cap with spotty white
forehead, but what sets it apart is that it has very dark primaries,
which "our adult Commons right now"
don't have. But that photo was taken in August. An August adult in the
same book has primaries which are far paler than the immature of the
same month. If this photo is indicative of longipennis, our 2nd
summer birds should still have darkish primaries.
What would solve some of this question is to tap into people in the
flyway who are seeing Commons on migration, and who can report on their
current state of plumage. Can anyone help here with contacts who might
have the knowledge to assess this? Or to take photos which we can
interpret?
I have hesitantly included birding-aus in this reply, as I am loathe to
bore people with such detail. Nor did I want to exclude people who
might be interested. I am absolutely fascinated by this subject, and
want to know more. Unfortunately I am going off the air from Tuesday
for 6 weeks (thank goodness, I hear you all mutter) and can't say what
kind of internet access I might have.
Cheers,
Jill
Jill Dening
Sunshine Coast, Queensland, Australia
26° 51' 41"S 152° 56' 00"E
Chris Corben wrote:
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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