Tweets,

My apologies for sending such a long posting, but I found this one so
fascinating I thought many of you would also be interested. This is an
article which was published in the Dec. 9, 1995 issue of "The Economist",
an unlikely publication for a bird article.

Dave Nunnallee
dnun461 at ecy.wa.gov
Bellevue, WA
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"Cuckosa nostra"

Organised crime is not confined to humans. Birds, too, are capable of
"leaning" on their fellow creatures. In Spain, the cuckoos appear to be
running a protection racket. They are terrorising locals into raising
cuckoo nestlings alongside their own offspring.

In Britain, and other parts of northern Europe, the common cuckoos that fly
in from Africa every spring specialise in parasitising warblers. The
warblers are small, dimwitted birds, apparently unable to work out that the
giant nestlings into whose maws they are shoving so much insect life are
not, themselves, warblers. When the great spotted cuckoos of Spain come back
from their African winter holidays, however, they lay their eggs in the
nests of magpies. These are streetwise enough to be able to look after
themselves. So the Spanish cuckoos have a problem: to stop the mother
magpies from ejecting their eggs as soon as they are laid.

A project organised by Manuel Soler, of the University of Grenada in Spain,
and Anders Moller, of Copenhagen University in Denmark, suggests that the
answer is blackmail. Magpies that accept and raise a young cuckoo are left
to go about their business. The nests of those from which the visitor is
ejected suffer unfortunate "accidents".

Like all the best protection rackets, these accidents rarely have witnesses;
only once have the researchers seen a cuckoo destroy a magpie's nest. But
they have built up an impressive body of circumstantial evidence. First,
magpies rarely eject cuckoo eggs voluntarily. The team looked at 134
parasitised nests over the course of two seasons. Only seven sets of parents
relieved themselves of the parasites. However, six of these seven nests
suffered mysterious damage--with the eggs destroyed or the nestlings killed.

By contrast, only 12% of the magpies that accepted a cuckoo egg suffered
depredation of their nests, and it was of a different kind. In the first
case the wreckage and bodies were left behind, perhaps "pour encourager les
autres". In the second case anything edible was gone, presumably consumed by
crows (cuckoos eat caterpillars).

The team decided to do an experiment. The researchers identified some more
parasitised nests and divided them into two groups. From about half, they
removed the cuckoo egg. The others they visited, but left alone. So did the
cuckoos. Only three of the 28 controls were destroyed--again, apparently, by
crows. However, 16 of the 26 experimental nests got vandalised--and, again,
the bodies were left behind. And when, during the course of a different
study, the ornithologists replaced magpie eggs with plasticine models, they
found that these, too, were pecked at if a cuckoo's egg had been removed at
the same time.

So much for the crime. But what about the organisation? One explanation for
nest destruction could be that the victimised magpies will re-nest, thus
providing a further opportunity for the cuckoo to lay an egg in their nest.
This certainly happens, but it seems not to be the whole story. Some nests
were torn up so late in the season that re-nesting would not have been
feasible. It really does seem that the magpie nestlings are standing hostage
for their parents' good behaviour or, from a selfish evolutionary point of
view, that it is less costly for magpie parents to tolerate an extra mouth
than to risk having their investment destroyed.

The question is how such behaviour could have evolved. This is not as
obvious as it looks. Lessons take time to learn--and, since the cuckoo
mother must patrol her patch to check on the safety of her young--are costly
to administer. And evolutionary theory requires that it is the teacher (or,
at a pinch, her close relations) that reaps the rewards of the
teaching--genes that benefit only other individuals get short shrift from
natural selection. That requires repeated interactions between the same
birds.

This does seem to happen, sinch both magpies and cuckoos are territorial.
The magpies stay put and the cuckoos do, at least sometimes, return to the
same territory year after year, enabling them to benefit from any previous
reign of terror. As to how long it will be before the magpies evolve a
counter-strategy, that remains to be seen. Hoya de Gaudix, the site in
southern Spain where the team has been working, has only recently been
colonised by cuckoos. Dr. Soler and Dr. Moller will be looking keenly for
evidence of magpies striking back.
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