Cuckoo in the Nest
Although about two-thirds of the family are nonparasitic, cuckoos are infamous for their ability to divert the normal parental energies of a host species to their own ends. The mechanisms by which they accomplish this trickery may be fascinating but the selection pressures causing the evolution of the habit itself are equally intriguing.
There are two sides to this evolution, that of the cuckoo and that of its host, and of the two the former is easier to visualize. Many bird species seem to be limited, in their reproductive capacity, not by the number of eggs they can lay but by the number of chicks they can successfully feed through to independence increasing the number of eggs laid would not increase the number of chicks eventually reared.
Imagine, however, that in a population of birds, a parasitic mutant arises which can somehow foist its offspring successfully onto others. The major limitation of reproductive capacity is at a stroke removed and these mutant females are limited only by the number of eggs they can lay. If she is thus able to increase her fecundity, this mutant will increase in frequency in the population through time and the population will evolve towards parasitic habit.
This scenario need not apply only to cuckoos. Females of such diverse species as House sparrow, starling, Redhead duck, and moorhen have been shown to deposit eggs in nests that are not their own, although these are usually nests of their own, more rarely of other, species. Parasitic females may lie in territories very close to their own and also complete a “legitimate” clutch of their own which they care for in the normal way.
The adaptations of these part-time parasites are unlikely to be as perfect or extreme as those of the specialist cuckoos, but in one study of magpies in Spain, it was unexpectedly found that an introduced swallow nestling repeatedly tried to eject the host eggs from the nest cup, exactly in the manner of a European cuckoo!
One evolutionary “halfway house “between these habits and full-scale cuckoo parasitism is the Black-billed cuckoo. In most years, pairs of this species build a nest and raise a brood of 2-4 nestlings exactly in the manner of any “ordinary” bird. When food is unusually abundant, however, each female also attempts to parasitize the nests of others in addition to raising a brood via her own parental care. Many of these parasitic eggs are foisted upon other Black-billed cuckoos but some are deposited in the nests of other species. What ecological factors have caused the evolution of this mixed policy?
The interesting features of these cuckoos are that they have very large eggs, relative to body size and that these large eggs have an extraordinarily fast development time, hatching in only I z days the shortest incubation period of any bird! Rapidly hatching eggs are of course essential for successful parasitism because the host’s nest will already contain developing eggs when discovered by the cuckoo.
If the cuckoo’s eggs hatch much later than those of the host their chance of success will beery small. In comparison with other bird species showing longer incubation periods, the Black-billed cuckoo’s efforts at parasite-ism are more likely to succeed, and the habit will become fixed in the population.
The more difficult evolutionary problem is understanding just how the host species continues to be fooled by the cuckoo’s tactics. Any female songbird, a unlock or Meadow pipit for example, which accepts a cuckoo’s egg into her nest will leave no offspring that year. Other females, which may be able to discriminate and throw out cuckoo eggs or nestlings, will, of course, leave a reduced but significant number of offspring and so it is precisely these types that we should see in the population. An intensive study of several cuckoo species has shown that, within many species of host, there are discriminating and non-discriminating females.
How then do non-discriminating females remain in the population in the face of their inability to reproduce themselves? One possible explain-nation has been proposed, which does however require that the cuckoo be even more cunning than we imagined. She must ensure that the discriminating birds leaven more offspring than the gullible hosts and one way in which she could do this into visit each nest she laid in and destroying the contents of any that had rejected her offspring.
Cuckoos are very difficult to watch, especially after egg-laying, but there is some evidence to support the foregoing. In an observation on Klaus’s cuckoo, one host nest which rejected the cuckoo egg was quickly destroyed, probably by the cuckoo: the host birds (obviously discriminators) set about building a new nest and laying a fresh clutch nearby.
This nest was also found and destroyed, although this time no cuckoo eggs were laid. Other observations on several species of cuckoo show that it is only the females which commonly eat eggs, alone would predict if they do so only to regulate the evolution of host discrimination.
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