birding-aus

Common Tern observation

To: Chris Corben <>
Subject: Common Tern observation
From: Jill Dening <>
Date: Sat, 11 Apr 2009 13:52:45 +1000
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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