Hi again Jill et al
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.
Common Terns were always rare on the western shores of Moreton Bay (the most
accessible bits) except in the north, from say Shornecliffe to Scarborough.
Even in winter, you could reliably find small numbers roosting
on the rocks at places like Woody Point, but you would see more out in the
bay from boats. Trouble is, those northern shores have apparently been
nuked - thousands of waders and hundreds of terns reduced to a handfull,
from what I've seen and heard. This decimation seems worse in the north than
further south (someone please correct me if this isn't so), so it's always
possible this is a local effect (eg too many people and dogs) and doesn't
represent what's going on elsewhere. Of course, I fear that may be unduly
optimistic... But if that was so, Common Terns in general may have declined
more in visibility than in numbers.
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?
Frequently!
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.
As Danny has already pointed out, that would be hard to swallow, given the
expected ratios of the different age classes. Globally, second years will be
very much rarer than adults, and rarer than first years (hopefully,
because otherwise it would mean the last breeding effort crashed). It's
possible the proportion of second years could be increased if the adults
have already left. But this wouldn't make sense if numbers of the species as
a whole are high.
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.
In general, judging the darkness of primaries from single photos can be very
deceptive. I'd be wary of such a generalisation. There will be a lot of
variation in the rate at which the outers lose the silvery bloom which makes
them pale, but lighting angle can make a big difference. If you look at lots
of pictures of a single bird, you will probably get very different
impressions from different angles.
I'm also not sure what you mean by that last sentence. A second year bird in
August will be just over a year old (say 14 months), and in first Alternate
with the beginnings of second Basic (the inner Ps), and maybe even some
retained, Juvenal, outer Ps (trashed!). The bulk of the Ps would be
Formative (ie they have been replaced once by a special molt seen only in
first cycle birds). A second year bird now (April, at 22 months age) would
be in second Alternate with second Basic outer Ps and second Alternate
inner Ps. So essentially, a bird which was a second year in August (14
months old) would have molted probably all its feathers again by the next
April, when it is 22 months old. A bird which was a second year in April (at
22 months) would be a third year in the following August (at 26 months).
Just to confuse things more, please note the distinction between "second
year" as in "second calendar year" and "second year" as in "second cycle".
In April, a second calendar year bird would be in its first cycle, and I'd
have called it a first year bird (10 months old).
I don't have a copy of Olsen and Larsson handy, so I can't see the picture
you refer to. But if it was really a third year bird (at 26 months) then you
could argue that its dark primaries would have been darker in the previous
April when it was a second year bird (at 22 months), since they would be the
same feathers. But in fact, second year birds can be identified up to at
least December (at 18 months) by their PALER outer primaries, since these
were replaced more recently than than the outer primaries of adult birds.
Once the outer primaries have been replaced (as they would have by now),
that won't help. The paper attached by Danny does a great job of talking
about ageing for the nominate race, and there's no reason to think it will
be much different for longipennis.
In summary, at present (April) most first year birds (at 10 months) will
look like Basic birds and have retained, Juvenal outer primaries,
though it's quite possible some may be more advanced and harder to tell from
older birds. Second years (at 22 months) will be very hard to tell from
adults, or in some cases from first years.
I hope the above isn't too confusing. Complex Tern would be a better name
for Sterna hirundo!
Cheers, Chris.
Chris Corben.
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