At first sight the presence of a section about cuckoos might seem out of place here. Mostly a host and its parasite come from very distant systematic groups (mammals and bacteria, fish and tapeworms, plants and viruses). In contrast, brood parasitism usually occurs between quite closely related species and even between members of the same species. Yet the phenomenon falls clearly within the definition of parasitism (a brood parasite 'obtains its nutrients from one or a few host individuals, normally causing harm but not causing death immediately'). Brood parasitism is well developed in social insects (sometimes then called social parasitism), where the parasites use workers of another, usually very closely related species to rear their progeny (Choudhary et al., 1994). The phenomenon is best known, however, amongst birds.
Bird brood parasites lay their eggs in the nests of other birds (Figure 12.2), which then incubate and rear them. They usually depress the nesting success of the host. Amongst ducks, intraspecific brood parasitism appears to be most common. Most brood parasitism, however, is interspecific. About 1% of all bird species are brood parasites - including about 50% of the species of cuckoos, two genera of finches, five cowbirds and a duck (Payne, 1977). They usually lay only a single egg in the host's nest and may adjust the host's clutch size by removing one of its eggs. The developing parasite may evict the host's eggs or nestlings and harm any survivors by monopolizing parental care. There is therefore the potential for brood parasites to have profound effects on the population dynamics of the host species. However, the frequency of parasitized nests is usually very low (less than 3%), and some time ago Lack (1963) concluded that 'the cuckoo is an almost negligible cause of egg and nestling losses amongst English breeding birds'. None the less, some impression of the potential importance of brood parasites is apparent from the fact that magpies (Pica pica) in populations that coexist with great spotted cuckoos (Clamator glandarius) in Europe invest their reproductive effort into laying significantly larger clutches of eggs than those that live free of brood parasitism (Soler et al., 2001) - but those eggs are smaller in compensation. The presumption that this is an evolutionary response to the losses they suffer due to the cuckoos is supported by the fact that magpies that lay larger parasitized clutches do indeed have a higher probability of successfully raising at least some of their own offspring.
holo- and hemiparasitic plants the ecological importance of brood parasitic birds
Highly host-specific, polymorphic relationships have evolved among brood parasites. For instance, the cuckoo Cuculus canorum parasitizes many different host species, but there are different strains ('gentes') within the cuckoo species. Individual females of one strain favor just one host species and lay eggs that match quite closely the color and markings of the eggs of the preferred host. Thus, amongst cuckoo females there is marked differentiation between strains in their mitochondrial DNA, which is passed only from female to female, but not at 'microsatellite' loci within the nuclear DNA, which contains material from the male parents, who do not restrict matings to within their own strain (Gibbs et al., 2000). It has long been suggested (Punnett, 1933) that this is possible because the genes controlling egg patterning are situated on the W chromosome, carried only by females. (In birds, unlike mammals, the females are the heterogametic sex.) This has now been established -though in great tits, Parus major, rather than in a species of brood parasite (Gosler et al., 2000). Females produce eggs that resemble those of their mothers and maternal grandmothers (from whom they inherit their W chromosome) but not those of their paternal grandmothers. Of course, if female cuckoos lay eggs that look like those of the species with which they were reared, it is also necessary for them to lay their eggs, inevitably or at least preferentially, in the nests of that species. This is most likely to be the result of early 'imprinting' (i.e. a learned preference) within the nest (Teuschl et al., 1998).
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