In cooperatively breeding species, sexually mature individuals may delay natal dispersal and become subordinates, helping a dominant pair raise offspring. To understand how cooperative breeding evolved, it is important to determine the mechanisms leading to delayed dispersal. Adult sex ratio (ASR) variation may affect dispersal by limiting breeding vacancies available to the more abundant sex, and cooperative breeders often have a more biased ASR than noncooperative breeders. However, no studies of cooperative breeders have related ASR at both the local and population level with dispersal. Using the long-term Seychelles warbler, Acrocephalus sechellensis, data set, we tested the influence of population-wide and local ASR, and density, on natal dispersal of yearlings. Our ASR - density hypothesis predicts that the probability of natal dispersal is lower when the ASR is biased towards the sex of the focal individual, but only when the population density is high. Dispersal was associated with population density and population-wide ASR in males, but not in females; males were more likely to delay dispersal when ASR was male biased and density was high. Our findings illustrate a complex association between demographic factors and cooperative breeding and suggest that individuals alter their dispersal behaviour in response to the demographic composition of the population.

Senegal hosts hundreds of wintering Eurasian Spoonbills Platalea leucorodia every year. To contribute to the paucity of knowledge on their distribution and ecology, this study aimed to compare the occurrence and behaviour of spoonbills at the three most important sites in Senegal: Djoudj National Park, Technopole of Dakar and Palmarin Reserve. Based on monthly counts in 2017 and 2018, Djoudj reached peak numbers of more than 2500 birds in April. The number in Palmarin peaked at 500 birds in October. In both cases this well exceeds the 1% threshold for the East Atlantic Flyway population, but Djoudj can hold up to a sixth of the population. Technopole is an intermediate site that rarely hosts more than 50 individuals. Based on the scanning of behaviours throughout the day, we show that foraging activity was high early in the morning and decreased towards noon. At Palmarin, Spoonbills spent more time resting (68%) than at Djoudj (50%), allocating less time to foraging (16%) compared with Djoudj (29%). If this indicates favourable wintering habitat at Palmarin, the growing population of Eurasian Spoonbills from Europe may increasingly rely on the Palmarin site during the coming decade. Received 25 Apr 2022, accepted 21 Sept 2022.