Why Seychelles Warblers fail to recolonize nearby islands: unwilling or unable to fly there?

Abstract

The Seychelles Warbler Acrocephalus sechellensis is a rare island endemic which, from 1920 to 1988, occurred only on Cousin Island (29 ha) in the Seychelles. Despite the saturated nature of this population and the possibility of obtaining higher reproductive success on new nearby islands, inter‐island dispersal by Seychelles Warblers is extremely rare (0.10%). We test the hypothesis that Seychelles Warblers show an adaptation typical for island birds: a low‐cost reduced‐size flight apparatus. We compared the anatomy of the flight apparatus (wing shape, wing loading, skeletal parts and musculature) of Seychelles Warblers with that of three closely related migratory Acrocephalus species (Eurasian Reed Warbler A. scirpaceus, Australian Reed Warbler A. australis and African Reed Warbler A. baeticatus). Seychelles Warblers do not differ from the migratory warblers in pectoral mass and skeletal attachment area relative to body mass, wing shape and wing loading. Seychelles Warblers show the morphological structures required for sustained flight, but may have the behavioural reluctance to cross what they may regard as extensive bodies of water.

Rescue of the Seychelles warbler on Cousin Island, Seychelles: the role of habitat restoration

“…This is also the case for the Seychelles warbler. Despite the over-production of warblers on Cousin in the last decades, only two warblers (0.13%, N = 1599 banded birds) have crossed the small stretch of sea (1.6 km) between Cousin and Cousine unaided (Komdeur et al, 2004).…”

“…If a colour-ringed bird was missing from its territory, the rest of the island was searched for it. Off-island migration by warblers is negligible (0.13% per annum; Komdeur et al, 2004), so birds that have disappeared were considered to be dead. Breeding activity was assessed by following each female continuously for 30 min and recording whether she was nestbuilding, incubating, or feeding young.…”

“…1). Given the increase of recruits on the one hand, and the presence of a static population and sporadic dispersal of young to other islands (Komdeur et al, 2004) on the other hand, there must be an increase in adult mortality.…”

Extremely reduced dispersal and gene flow in an island bird

“…For instance, the Seychelles Warbler (Acrocephalus sechellensis) shows locomotory structures similar to those found in closely related species that can sustain flight over long distances, but does not manage to disperse successfully to islands with suitable habitats just outside its distribution range (Komdeur et al, 2004). Thus, to investigate the causes of population divergence at a small spatial scale, it is important to be able to tease apart the effects of gene flow from those due to selection and drift at the relevant spatial scale.…”

Sex biased natal dispersal is not a fixed trait in a stable population of Seychelles warblers

“…During the breeding seasons, all territories were checked for the presence of warblers at least once every two weeks. As inter-island dispersal is extremely rare (0.1%, N = 1924, Komdeur et al, 2004) warblers that were missing from their territory and not found on other territories can safely be assumed dead. Dispersal data was based on individuals that were colour-ringed on their natal territory from 1985 to 2005.…”