May 18, 2016
The brown-headed cowbird is
nothing short of a nightmare for its hosts: If they eject the brood parasite's
eggs from the nest, it punishes them by destroying their entire clutch.
Researchers from the Max Planck Institute for Evolutionary Biology in Plön and
Harvard University have designed a mathematical model for analyzing the
interaction between avian brood parasites and their hosts. The model
calculations show that birds only accept a brood parasite's eggs in their nests
if they are forced to do so by retaliation on the part of the invader.
The mafia hypothesis provides an
explanation as to why some host birds do not remove parasites' eggs from their nests. The cuckoo lays its eggs in
other birds' nests to spare itself the effort of raising its own young.
However, some parasitic avian species like the North American brown-headed
cowbird (Molothrus ater) or the
European great spotted cuckoo (Clamator
glandarius) depredate their hosts' nests out of revenge when the hosts do
not accept their eggs. Under these circumstances, it makes sense for the host
birds to tolerate the additional work involved in raising the cuckoos in the
nest to avoid endangering the lives of their own offspring.
Host birds can have different
reactions to brood
parasites: while some basically accept other birds' eggs, others weed
them out immediately. Others again only accept parasitic eggs when their nests
have already been depredated on one occasion. Which of these behavioural
strategies is most successful depends on the prevailing environmental
conditions and on how often the adversary arises in a population: if there are
a lot of mafia parasites around, it is worthwhile for the hosts to accept the
parasitic eggs without resistance. However, if the parasites do not retaliate,
host birds that remove the cuckoo eggs from their nests immediately are at an
advantage. As a result, the frequency of the different behavioural strategies
fluctuates in regular cycles.
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