Seasonal timing of reproduction in a tropical bird, the Seychelles warbler: A field experiment using translocation

Reproduction of the Seychelles warbler (Acrocephalus sechellensis), a single-island endemic species living close to the equator, is characterized by a pronounced annual rhythm. The bird usually raises only one or two clutches of one egg each per year. Observational data suggest that seasonal changes in feeding conditions are an important proximate factor controlling reproduction. This hypothesis could be tested by transferring breeding pairs from Cousin Island to islands previously unoccupied at the same latitude. These islands, Aride and Cousine, have a higher food supply but similar vegetation and climate. After translocation to Aride Island, with high food availability year-round, individual breeding pairs prolonged their reproductive season (some bred year-round), increased the annual number of broods, and improved the reproductive success per nest-building attempt. Annual production of yearlings per pair on Aride was on average 23 times higher than that of the same pair on Cousin before the transfer. After translocation to Cousine Island, where food availability varied seasonally and was intermediate between Cousin and Aride, individual pairs increased the annual number of broods, but reproductive success per nest-building attempt remained the same. Annual production of yearlings per pair on Cousine was on average 5 times higher than that of the same pair on Cousin before the transfer. Thus the experiments, controlled for group size, breeding partner, breeder age, and experience, showed that food supply can be an important proximate factor in the timing of reproduction in the tropics. The differences in reproductive timing and success by warblers on the three islands are due not to genetic differences but entirely to differences in environmental conditions.