Animal Behavior/Intraspecific Brood Parasitism

Intraspecific Brood Parasitism
Brood Parasitism conventionally involves the laying of one’s own eggs into the nest of another species, most commonly seen in species such as the cuckoo and brown headed cowbird. Most often fatal to the offspring of the host and detrimental to the host parents that must satisfy the begging behavior of chicks far greater in size than often themselves. Intraspecific brood parasitism however is a common occurrence in ducks and allies, though not unheard of in other families of birds.

Several hypotheses have been put forth as to why IBP occurs, including limitation of suitable nesting sites, or intrinsic individual limitations such as lack of experience rearing offspring or poor physical condition. Still more hypothesize that the parasites maintain their own clutch but lay eggs in host nests because they have lost eggs of their own, to spread the risk of predation, or that her physical health is well enough that she can increase her fitness by laying more eggs,. And still more hypothesize that if IBP is linked to kin selection; since colony formation and female philopatry is common in ducks then the host female could gain indirect fitness through the rearing of kin offspring. It is argued, however, that the loss in direct fitness of the host is not compensated enough by the increase in indirect fitness of the host.

A recent study by Tiedemann aimed to investigate if age linked with relatedness would prove a reasonable hypothesis for IBP in the Common Eider Somateria mollissima. It is already well known that clutch size in ducks decreases with higher age. This study looked at several microsatellites within two separate eider colonies. What they found was that genetic relatedness between the parasite and host were indeed greater than when compared to the relatedness of the entire microsatellite area. Important to note though is that there is no direct correlation between nest vicinity and genetic relatedness. Two important conclusions were drawn, “The average age of parasitized females is higher than the age of nonparasitized females, (and) the percentage of nests with alien (parasite) eggs increases with the age of nesting females”. From this study Tiedemann predicts that elderly eiders that cannot produce as many eggs as they may be able to incubate gain a significant indirect fitness through IBP by close genetically related individuals. This idea of increased fitness in both parties was postulated early but this is the first study of its kind to demonstrate experimental evidence that IBP in ducks at least could be age-dependent and that cooperation between generations of females may increase the total fitness in both individuals.