Why island birds sometimes move in with strangers

In many animal species, young stay with their parents long after they have become sexually mature. Why individuals delay dispersal and independent breeding is an interesting question from an evolutionary viewpoint because individuals are expected to pass on as many genes as possible to future generations. In the Seychelles warbler, a group-living species, opportunities for independent breeding are limited, and dispersal attempts can be seen as a-best-of-bad-job that result in severe competition and reduced survival. Why, then, do warblers leave if it is good at home?In his recent paper in the journal Frank Groenewoud shows that dispersal of male warblers is the result of eviction by territory owners, but young females sometimes manage to settle in an unrelated group where they are allowed to breed alongside the dominant breeding pair.

Photo 1

A subordinate Seychelles warbler. [Photo Sjouke A. Kingma]

In the 1960’s the entire world population of Seychelles warblers consisted of only 26 individuals. In what became a highly successful conservation effort to bring this species back from the brink of extinction, the vegetation on Cousin Island was restored to its natural state. The population of warblers grew rapidly and now thrives at a stable number of 300-400 individuals. Since there is only space on the island for ca. 100-120 territories, and the warblers do not initiate trips to other islands, juveniles have no other option but to postpone their own reproduction and to wait at home until a breeding vacancy becomes available, either in the home territory or elsewhere on the island.

Despite the fact that delayed dispersal and living with family may be a good strategy for Seychelles warblers, several young birds actually leave home and attempt to find a breeding vacancy in a territory where a dominant bird had recently died. However, since the habitat is saturated and the mortality rate of dominants is extremely low, many young individuals fail to find such a position.

The current study highlights an interesting phenomenon, namely that some subordinates actually settle in unrelated groups. Sociality in birds is frequently explained by kin selection, and groups are often formed by extended families, but settlement in unrelated groups obviously cannot be explained this way. In a recent paper in the journal Frank Groenewoud and colleagues also show that between‐group dispersal is not promoted by improved ecological or social conditions in the new territory and does not result in higher survival. Instead, it appears that subordinate females can lay an egg in the nest of the dominant breeding pair, and have a high likelihood to inherit the territory in the future. Females that dispersed to a subordinate position in a non-natal group therefore had a relatively high life-time reproductive success.

Communal breeding may well explain why between‐group dispersal by subordinates is almost completely female biased. Young males are reproductive competitors, meaning that by accepting new males into the group, the resident dominant male risk losing their female to the newcomer. For that reason, subordinate males appear to be evicted by the dominant male, especially when unrelated. However, female subordinates who lay an egg also share in parental duties, and this may explain why a dominant breeding pair sometimes accepts a subordinate female to join their group. The results of this paper therefore illustrate an additional reproductive strategy that individuals can use when breeding vacancies are limited and that is independent of the benefits of living with relatives.

 

 

Groenewoud F, Kingma SA, Hammers M, et al. (2018) Subordinate females in the cooperatively breeding Seychelles warbler obtain direct benefits by joining unrelated groups. Journal of Animal Ecology 87(5):1251–1263.

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