Repeated experimental removals unveil sex and age-specific dispersal strategies in a social passerine bird

Abstract

Context Sex and age are frequently proposed as drivers of a number of behavioural and demographic patterns that can have important consequences for population dynamics including access to mates, sexual selection, parental care and lifetime productivity. Sex and age might also be important in shaping the movement patterns and colonisation processes of social species moving into vacant habitat. Such information is critical for the management of strongly interacting species such as the noisy miner (Manorina melanocephala), which structure ecological communities through aggressive exclusion of other taxa from areas that they occupy. Aims In Manorina colonies, young females are usually dispersive, while males remain in the natal colony as the philopatric sex. Following removal of individuals from an area, we aimed to determine whether female-biased dispersal, particularly of young females, would result in a more equal sex ratio and a younger age structure in the recolonising population. Methods These predictions were tested by anatomically ageing and sexing 1856 noisy miners that had been experimentally culled in two regions of New South Wales, Australia, to reduce the aggressive impact of this species on other native species. Key results Prior to removal, noisy miner populations were significantly male-biased in both regions (57% and 60%); however sex ratios after each of two removal episodes no longer differed from parity. Immature birds were a dominant feature (65%) of recolonising populations in both regions, however, the age structure of recolonising populations was different in each region, mostly likely due to the respective timing of culls during the year. Furthermore, the culling response in terms of age-specific sex ratio varied between regions. After the final cull, the sex ratio of mature birds had fallen to parity in one region but had become even more male biased (68%) in the other region. There was no sex-ratio bias among immature birds before or after culling. Conclusion These results confirm the expectation that immature birds are more likely to be colonisers, but the expectation of greater female dispersal was equivocal. Implications The differences in response between regions may reflect variation in population density, landscape connectivity or seasonality, highlighting challenges when implementing culling programs for conservation management.