Multiparasitism: why do interspecific brood parasites lay eggs so frequently in already parasitized host nests?

Obligate avian brood parasites delegate parental care to unrelated foster parents. Therefore, the main way a brood parasite female can increase the probability of her offspring surviving until independence is to select the most appropriate nests, particularly those without a previously laid parasitic egg so as to minimize competition with another parasitic nestling. Brood parasitic females usually lay one egg per host nest. However, evidence of parasitic females avoiding laying in previously parasitized nests is lacking. In this context, "multiparasitism" can be defined as parasitic eggs laid in a nest by different females or by the same female (with the latter known as "repeated parasitism"). Repeated parasitism has been well documented in only two parasite species, the cuckoo finch (Anomalospiza imberbis) and the great spotted cuckoo (Clamator glandarius). Usually, multiparasitism and a high number of parasitic eggs per nest results from a high density of brood parasites, mainly in non-evictor, but also in some evictor brood parasites. Multiparasitism is widespread, having been reported in all well-studied nest-sharing brood parasites and in all except one well-studied nestmate-killing species. Both multiparasitism and number of parasitic eggs per nest vary widely depending on various factors, specifically the brood parasite species, the host species, the area or habitat, and the year. Given that, in multiparasitized nests of nestmate-killing species, only one parasitic nestling can survive per nest, multiparasitism should be scarcer and number of parasitic eggs per nest lower in these species than in nest-sharing ones, a prediction that receives support from data included in this review. Multiparasitism exerts a strong negative effect on the reproductive success both of brood parasites and of hosts in most brood parasite-host systems. The size of the host relative to parasite is a crucial factor affecting the survival of both the brood parasite and the host nestlings. That is, nestling mortality of both is higher in smaller than in larger host species because the latter can provide a greater amount of food to the nest. Three different hypotheses have been proposed to explain why multiparasitism is much more frequent than might be expected from the negative effect it exerts on brood parasite success: first, multiparasitized nests are less concealed than singly parasitized nests; second, competition is strong for host nests because of scarcity of available host nests at the appropriate stage and/or a very high abundance of brood parasites; and third, multiparasitism is an adaptation that results from an active decision by the parasitic female. The second explanation is the most widely accepted, but a female laying in a previously parasitized nest has frequently been interpreted as the parasite making "the best of a bad job", given the scarcity of unparasitized host nests. The third explanation (multiparasitism as an adaptive strategy) has been suggested based on two contrasting arguments: first, because production of parasitic fledglings is higher in multiparasitized than in singly parasitized nests; and second, because the presence of several parasite eggs increases the likelihood of acceptance of these eggs by the host.