Hi Everyone,
I was in phone conversation this morning with Peter Madvig, giving him
directions to tern watching in Noosa, and afterwards thought that some
tern behaviour which he found interesting might also be of interest to
subscribers. To me it's normal behaviour, but many birders don't watch
terns arriving to their night time roosts.
I won't assume that this behaviour is ubiquitous, but on the Sunshine
Coast for some years we have noticed that most of the terns arrive after
sunset. . Very often the estuary empties of terns, especially migratory
terns, in the couple of hours leading to sunset. The birds go out to sea
for a final feed for the day. Let's say that maybe a quarter may have
arrived back at the night roost by sunset, though that varies according
to weather conditions and perhaps other reasons (like how far they have
to go for a feed).
By sunset the migratory terns (Littles, White-wings and Commons) are
arriving in flocks of 10, 20, 30, and sometimes as many as 100, but
always in flocks. By contrast, the Cresteds fly in as individuals or
loose groups, but not as flocks. The migrants fly in at altitudes of
around 50/100m, sometimes higher. At this stage we are always counting
them against the sky, as they fly over the mouth of the Noosa River into
the estuary.
About 10 minutes after sunset the Cresteds continue to arrive as before,
but the migrants suddenly stop coming. We learned from our Caloundra
survey days that at this stage they are actually still arriving, but are
gathering outside the rivermouth into larger groups. They wing and
wheel, all the time seemingly struck by indecision about how to enter
the estuary. I have concluded that the change in behaviour is connected
with failing light. As light fails, the birds change their manner of
entry through the rivermouth.
Then suddenly a river of terns begins, flying low (maybe about 1-2m
above the waterline) in a thin line, using the fading light of the
western horizon reflected on the water to guide them to their night
roost.At this stage we start to count terns against the water
background, which is usually shining with reflected light from the
horizon. As before the migrants arrive in flocks, dropping low just
outside the river mouth and spreading into a thin line to fly into the
estuary. Usually we can survey until 40 mins after sunset, but by that
stage we are straining through our binos to count against the darkening
water. We usually give up, knowing that terns are still arriving.
It's a poor year for migratory terns on the Sunshine Coast. Right now
should be the peak of the migration here, and yet last night we only had
6650 estimated migratory terns (estimating in 10s and often
counterchecking). Most, at least 90%, were Commons. In past years you
could put 30,000 in front of that number and not blink.
Also, we had the first Double-banded Plover of the season yesterday.
Cheers,
Jill
--
Jill Dening
Sunshine Coast, Queensland, Australia
26° 51' 41"S 152° 56' 00"E
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