In the James Bond movie Spectre there’s a reference to the cuckoo bird in two scenes. The evil villain Spectre leers: “I see you! Cuckoo!” when he detects Bond in the classic evil assembly room scene. In the other scene towards the end of the movie Spectre reveals why he considers Bond as a cuckoo. In order to understand the reference we must know the biology of the Common cuckoo bird (Cuculus canorus). The European common cuckoo is a well studied nest parasite. Nest parasites instead of raising their offspring themselves they by-pass parental care duties by laying their eggs inside nests of another species. The behavior is also called brood parasitism.
The famous population biologist Thomas R. Malthus observed that organisms produce more offspring than needed. This leads to increase in population size and more competition for resources. Competition can take place within and between species. The cuckoo egg hatches earlier and the chick grows faster than those of the host. The chick instinctively performs a behavioral adaptation: It expels all competitors out of the nest. The eggs and even the young of the host species can be thrown off the nest. This behavior maximizes the feeding efficiency. The cuckoo chick has an overwhelming begging display which forces its foster parents to feed it frantically. The cost of being parasitized by the cuckoo can be very high.
On the flip side, parasitizing a nest is not an easy job. Female cuckoos have secretive and fast ovipositing behaviors to sneak their eggs into other birds’ nests. They may even have partners in crime: males have been shown to distract host adults away from their nests so that the female can find the window of opportunity to lay her egg in the nest. Before she oviposits she may eat the eggs of the host bird exterminating an entire clutch. Birds therefore frequently mob cuckoos to chase them out of their territories.
Egg mimicry is another way cuckoos have adapted which helps their survival. If the parasitic egg looks very similar to those of the host then the survival chance increases. Some species of host birds can distinguish foreign eggs and eject them out of the nest. If the host bird species lack the ability to recognize eggs then the selection is relaxed and the cuckoo egg mimicry disappears. Would you expel an egg if you can’t be sure? What if accidentally you expel your own egg?
It now appears that warming climate makes it possible for species exchange across the Bering Strait. Cuckoos native to Eurasia have been spotted near Sitka. Similarly Brown-headed Cowbirds is advancing in the opposite direction from America into Asia. This may be bad news for the native birds of in both continents.
Although brood parasitism is obviously harmful, biologists have shown a few cases where it can be beneficial. Based on a 16-year dataset it was demonstrated that parasitized crows’ nests were more successful compared to cuckoo-free nests. Parasitized nests on the average were more likely to produce surviving crow fledgling. Cuckoo chicks secrete a nasty smelling cocktail of chemicals which might be effective in deterring predators. If this is true, then concepts defining species interactions such as parasitism and mutualism which are generally considered independent may be quite interlaced.
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