Facultative control of offspring sex in the cooperatively breeding bell miner, Manorina melanophrys

The ability to alter primary sex ratios has the potential to increase a breeding individual's fitness. This is certainly true in those cooperative breeders where one sex is both philopatric and helps raise future offspring of its parents. We examined the primary sex ratio variation in the cooperatively breeding bell miner (Manorina melanophrys) in southeastern Australia over six breeding seasons. Male offspring are the philopatric and helping sex in this system and can increase the reproductive output of their parents. Bell miners aggressively defend their territory from all interspecific competitors and by doing so allow food resources to dramatically increase. The increase in phytophagous Psyllidae insects (which secrete a carbohydrate-rich coating that constitutes the major component of bell miner diet) leads to a decrease in tree health, often culminating in death of the tree. Bell miners then move as groups to new areas with low psyllid abundance, and the cycle repeats. Using this predictable temporal variation in food availability, we aimed to determine whether female breeders adjusted the, sex ratio of broods to produce more of the philopatric sex when food resources were high and more of the dispersing sex when food resources were low. Our results-provide clear evidence for such facultative control of sex ratio by female bell miners. Newly founded colonies are characterized by low food availability and a female-biased primary sex ratio, whereas colonies more than 1 year old have an increased food availability and a male-biased primary sex ratio. We suggest treating forces associated with resource enhancement and competition as opposing sides of a single general principle and suggest that it is necessary to view both the costs and benefits of philopatric individuals within a variable environment.