Adaptations and maladaptations to island living in the Seychelles Warbler

Abstract

The Seychelles Warbler (Acrocephalus sechellensis) was an endangered endemic of the Seychelles islands where, until 1988, the entire population of ca. 320 birds was restricted to the one island of Cousin Island (29 ha). Although warblers can breed independently in their first year, some individuals remain in their natal territory as subordinates, and often help by providing nourishment to non-descendent offspring. The frequency of helping is affected by habitat saturation, variation in territory quality (insect prey availability), and the genetic relatedness between the helper and the offspring. Helping results in indirect benefits from enhancing the reproductive success of close relatives, and direct benefits as improved parental skills and the acquisition of parentage. The overall helping benefits are higher for daughters than for sons, and it is therefore no wonder that most helpers are daughters from previous broods. Furthermore, on low-quality territories breeding pairs raising sons gain higher fitness benefits than by raising daughters, and vice versa on high-quality territories. Female breeders adaptively modify the sex of their single-egg clutches according to territory quality: male eggs on low quality and female eggs on high quality. However, despite the saturated nature of the Cousin population, the possibility of obtaining higher reproductive success on new nearby island, and a well developed flight apparatus, inter-island dispersal by Seychelles Warblers is extremely rare. The Seychelles Warbler is a beautiful example of behavioural and life history adaptations and maladaptations to restricted circumstances.