Fw: How do they do it? Atlantic Flyway 2
VulcanSpirit
Richard & Alison Brunstrom
Mon 3 Oct 2011 13:14
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Birds migrating is one thing, but some butterflies
do it too. We in the UK are used to Red Admirals Vanessa atalanta
visiting us every summer from France, but here a migration really is a
migration. The Monarch Danaus plexippus is a large active butterfly
which occasionally crosses the Atlantic in the jetstream (I've seen them in the
Scillies). Its only foodplant is the poisonous milkweed (Asclepias
spp) which doesn't grow in the UK so it cannot breed there. But in
America it is quite common and covers a vast geographical range, undertaking an
almost unbelievable annual migration. How they do it remains a scientific
mystery.
Here are a couple of fairly feeble photos (my
excuse is that they are timid, and fly fast and high) of Monarchs at Cape
May on their way south.
  Millions and millions of adults overwinter in a
handful of small groves in forests in the Sierra Madre mountains in central
Mexico in huge roosts with tens of thousands of butterflies on an
individual tree. In the early spring they mate, and the males die. The
females then set off north and reaching the Gulf states of the USA where they
lay their eggs on early milkweeds, and die. Several generations of butterflies
then follow the emerging milkweeds northwards until the whole range of the plant
across North America has been covered. In the autumn the adults from all
over eastern and central North America then set off to migrate back to Mexico -
some flying over 3000 miles to do so, and just like the birds some of them use
the Atlantic Flyway, passing through Cape May.
But the truly amazing thing, and so far
unexplained, is 'how do they do it?'. The butterflies 'returning' to Mexico
manage to get back to the same few groves of trees in a forest that
they have never seen before; not only have they never been there before,
nor had their parents, grandparents or even their great grandparents.
Clearly the ability to 'return' is hard wired into their genetic makeup,
but exactly how is as yet unknown. I find it a realy tangible demonstration
of the power of DNA that this tiny insect with a brain about the size of a pin
head is able to navigate like this. Quite extraordinary; almost beyond
belief.
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