Cuckoos and other brood parasites:
Nature of the cuckoo duck - David
Attenborough - BBC wildlife
http://www.youtube.com/watch?v=4Mb0GOITRUU&feature=fvwe2
From Nature's Camouflage, by Edith
Banks,
"An interesting sort of mimicry is found
in cuckoos: not in the birds themselves, but in their
eggs. As is well known, many cuckoos lay their eggs in
the nests of other birds which then rear the baby cuckoo as if it were their
own. Not all cuckoo species do this. Some build a nest and raise their own
young, and these provide an interesting comparison with the parasitic cuckoos.
The non-parasitic species usually lay quite
large, plain, white eggs. The parasitic species almost all lay smaller eggs,
and these are coloured and patterned to mimic the
eggs of the birds who unwittingly adopt them. Many different birds are used as
foster-parents by cuckoos, but generally, in any one region, a cuckoo species
parasitizes one particular bird, and the cuckoo's eggs specifically imitate the
eggs of that bird.
If the nest belongs to a large, intelligent
species, such as a Crow or a Magpie, the egg mimicry is very good. Smaller
birds are not so intelligent, and can be duped just as easily by an egg which
is only an approximate copy of their own. Most can detect that an egg is an
alien one if the pattern and colour are very
different, and will eject it from the next. But they
do not seem perturbed by an egg which is larger than their own. Often the
pattern and colouring are the same but the cuckoo's
eggs are half as large again as those of the foster parent.
It may even be that birds prefer larger eggs.
Experiements with Herring Gulls (which, incidentally,
are not parasitized by cuckoos) , have shown that this
is so for them. If a model of the Herring Gull's egg, correct in colour, shape and pattern but three times larger than
normal, is offered to a female gull, she will brood it in preference to her
own. The egg is acting as a 'super-stimulus'. The large, fake egg compels the
gull to incubate it far more strongly than one of her own, much smaller eggs.
The same mechanism is at work when small
birds which have hatched a cuckoo's egg continue to feed the nestling even
though it grows to twice their size. A tiny Reed Warbler will persist in
feeding even though it may have to perch on the back of the grotesquely large
cuckoo chick in order to reach its beak. Go to http://www.isle-of-wight.uk.com/jeffbrett/cuckoo.jpg to see what the possible difference in size between
the cuckoo chick and its much smaller foster parent.
So powerful is the sight of the cuckoo
chick's open beak that another small bird passing by will respond to it,
although it has never seen the chick before. The passing bird will suddenly
stop and push into the greedy mouth a morsel of food which was destined for its
own young ones.
A cuckoo which is fostered by the more
intelligent Crows and Magpies needs to be more restrained. IT has to share the
nest with the foster bird's own young (those that parasitize small birds just
push the other eggs out of the nest as soon as they can). But since these
larger birds are less gullible the cuckoo chick needs to mimic one of their own
nestlings in order to be fed. Such cuckoo chicks have evolved colouring on the tops of their heads, on their backs and
inside their beaks which mimics that of the other young birds. Their
undersides, however, look quite different, but this does not matter since the
parent bird never sees them from below."
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A cuckoo's egg in the nest of a Meadow Pipit; the egg is only an approximate copy of the foster bird's own since small birds are easily duped. (NSP, T.D. Bonsah) |
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Brown-headed cowbirds, located in the continental
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Figure 4. Brown-headed Cowbird nestlings grow rapidly, frequently outcompeting the host's nestlings for food and parental care. This adult Common Yellow-throat is feeding a cowbird fledgling that's more than twice its size. Photo by John Gavin, as it appears at http://birds.cornell.edu/conservation/tanager/images/cowbird.jpg . |
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SO, in a nutshell --
I. Cuckoos do not produce their own nests,
but lay eggs in nests produced by other species.
II. Eggs laid by cuckoos resemble those of
"foster" parents in size and coloration pattern, but are generally
larger. Larger size allows for earlier hatching. Sometimes, foster parents can
tell, in their own "birdy" way, that
something is not right with their nest, so they will toss out one egg. Because
of hyperstimulation by the cuckoo's egg, they are
more likely to toss out one of their own.
III. Cuckoo nestlings show similar "begging"
behavior for eliciting food from foster parents. Cuckoos start getting fed even
before the foster parents' young are hatched.
IV. Cuckoo nestlings push foster nestlings
out of the nest, so the cuckoo nestling is often the only remaining one in the
nest. (This is not pleasant to watch.)
V. Cuckoos are often found along borders of
different communities, so native bird populations, already reduced because of
fragmentation of their habitats, are getting clobbered.
References:
Payne, Robert B. and Laura L. Payne. 1997. Brood parasitism
by cowbirds: risks and effects on reproductive success and survival in indigo
buntings. Behavioral Ecology 9(1):64-73.
Payne,
Robert B. 1998. Brood parasitism in birds: Strangers in the nest. Bioscience
May 1998.