I'm a mite surprised that Richard Nowotny's first query has not elicited
more response. Perhaps it is because a lot of birding-aussers simply know
why birds migrate, and so haven't bothered to comment further.
For waders (like stints) it does seem an incredible journey. But then they
are incredible birds. All that fattening up, then atrophying some of their
less-than-essential-for-flight internal organs, then immense (thousands of
kms) hops to and from the breeding grounds - twice a year.
A typical Arctic wader arrives in its northernmost staging point while there
is still likely to be snow on the ground at its breeding site. The last hop
is usually fairly short. I gather that they may feed initially in part on
frozen insects released by the melting snow, which gets them over the period
before the area comes properly alive.
Then they court, lay, incubate, hatch and raise chicks. Often the female
first, then sometimes the male, leaves and the chicks follow. They arrive in
the Arctic in early June and are on their way back again often by late July.
So the question why is a good one.
My response to that has to be because it is hugely advantageous to the birds
to do it. There might be rich pickings on southern hemisphere mudflats -
otherwise they simply wouldn't come here. But there is certainly very little
in the way of breeding habitat if you need a super-abundant food source for
you and your chicks. What southern breeding habitat there is, in any case,
is used by the "locals".
Stints and other Arctic species nest over vast distances, and so are very
thinly spread. There are relatively few predators - Arctic foxes, snowy
owls, a few raptors and skuas excepted. But above all, there is a brief
explosion of food in the form of insects and, later, berries. The latter
form an important component of many of their diets, adults and chicks, and
are pretty scarce on mudflats! So conditions are spot-on for the incredibly
tightly compressed breeding cycle of your Joe-average shorebird. At the
other end of the migration route, the birds need a complete moult, which is
also energy-intensive. That plus the 2 migrations doesn't leave much of the
year left to put their feet up.
I suspect that this amazing lifestyle evolved when there was a lot more ice
- during the last ice age. The migration routes would then have been a whole
lot shorter and "stretched" as the climate warmed. Birds like stints adapted
- those that didn't disappeared.
There are therefore a lot of excellent reasons why vast flocks of stints
should cover southern mudflats in our summer and move to the Arctic to breed
- the hazards of the journey are greatly outweighed by high quality habitat
at both ends. These amazing birds make the migrations of local landbird
species, or seabirds for that matter, look pretty ordinary by comparison!
* * * * * * *
Tony Crocker Ph. ++64 (3) 364 2163
Educational Travel Fax ++64 (3) 364 2057
Centre for Continuing Education, University of Canterbury, Private Bag 4800,
Christchurch, New Zealand.
'I pity the man who can travel from Dan to Beersheba and cry, "'Tis all
barren"' - Laurence Sterne.
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