Not being familiar with the wetlands of SE Qld, I don't really know the
answers to Trevor Fords questions about why curlews turn up first at his
roosts and godwits last. But since I've just spent a month in Roebuck Bay
(North-west Australia) trying to work out such things for the waders up
there, I thought I'd have a few guesses anyway:
Why are the curlews first to arrive? I suspect that digestion pauses might
have something to do with it. There is a rather elegant series of papers
about Whimbrels in West Africa dealing with digestion pauses (published in a
book called Homewards Bound, which is wonderful bedtime reading if you want
to know how waders go about building the reserves for a long migration, or
how dutch experts go about studying waders). The Whimbrels studied sometimes
had problems in that they caught prey faster than they could digest it. This
meant that they were obliged to spend a fair bit of time standing around
doing nothing but digesting their food; they couldn't take in any more until
they had emptied their stomachs of what was there. Most wader species don't
have this problem to the same extent as they digest their prey quite
quickly; worms can be passed through the gut rather fast and even a small
bivalve can pass through a Red Knot in 20 minutes. West African Whimbrels
(and north-west Australian ones for that matter) eat a lot of large crabs,
which take a much longer time to process. Eastern Curlews in NWA also seem
to feed mostly on large crabs, and towards the end of low tides, when there
is still plenty of exposed mudflat, it is pretty normal to see Whimbrels and
Eastern Curlews loafing (presumably digesting) and ignoring potential prey
that isn't far away. I would guess this might be why those SE Qld Eastern
Curlews stop foraging before other waders. Why they chose to loaf and digest
on a high-tide roost (instead of doing so on the flats for as long as
possible) is a mystery to me.
Why do godwits arrive last, and only on very big tides? It sounds like they
have a preferred high-tide roost (or roosts) elsewhere which they only
abandon when they become water-covered on very high tides. In Roebuck Bay,
where roosting waders usually have a wide choice of roost sites, Bar-tailed
Godwits and Great Knots show a strong preference for open muddy or sandy
roosts (with as little vegetation or driftwood as possible) in which they
can see approaching predators from a long distance. On really big tides,
when these favoured beaches become water-covered or very narrow, Barwits and
Knots will fly several km to remote saltmarsh roosts behind mangroves,
unlike smaller fry (including Red-necked Stints, Grey-tailed Tattlers, Terek
Sandpipers and Greater Sand Plovers) which are much more relaxed about
roosting on narrow beaches (or on rocks and mangroves, in the case of
Tattlers and Tereks) and will often stick to the beaches after all the knots
and godwits have flown inland. I'd guess that the missing preferred roost of
the Barwits and Great Knots in the SE Qld site will also turn out to be a
relatively exposed and open site.
Danny Rogers
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