Cooperative breeding in birds  (Page 6/7)

From an evolutionary perspective, as a bird gets older the cost of reproduction increases. At some point in the bird’s lifecycle it will no longer be beneficial for it to breed its own offspring. Older females can thus maintain greater reproductive benefits by increasing the reproductive success of their relatives, rather than breeding themselves (Williams 1957; Hamilton 1966). In this manner they are still able to pass their genes on to a limited extent, similar to the sibling helpers in Micronesian kingfishers.

Often the dominant female is deposed by related individuals. In 57% of displacement cases among Seychelles warblers, the new dominant female was identified as being a daughter, sister, or niece of the deposed female (Richardson et al. 2007). Since deposed females tend to help the nest they were originally breeding in, this observation indicates that in most cases the grandparent helper ends up increasing the reproductive success of a closely related warbler. In accordance with Hamilton’s theory, this behavior maximizes a grandparent’s gene dispersal and as a result, is evolutionarily favorable.

However, reproductive fitness benefits of helping in these cooperatively breeding warbler communities include more than just the chance to indirectly pass on genes. Subordinate females also have the chance to breed offspring of their own. Unlike other bird communities, such as the Micronesian kingfisher where helpers completely forfeit the opportunity to reproduce, 44% of Seychelles warbler helpers, during every breeding season, are able to directly increase their reproductive fitness by having offspring of their own (Richardson et al. 2002).

In light of these advantages, it is strange that deposed females ever choose to become floaters, a role that provides neither direct nor indirect reproductive fitness. This mystery is accounted for by considering the quality of resources available in different territories. As illustrated by [link] , it has been observed that the quality of territories on which ousted breeding females become subordinates is consistently higher than the quality of territories where they become floaters (Richardson et al. 2007). Thus, similar to how ecological factors determine the level of cooperative breeding in the Azure-winged Magpie, the probability a displaced warbler will adopt a helping role is dependent upon whether or not the territory holds enough resources to support an additional member.