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

The ability to alter primary sex ratios has the potential toincrease a breeding individual's fitness. This is certainlytrue in those cooperative breeders where one sex is both philopatricand helps raise future offspring of its parents. We examinedthe primary sex ratio variation in the cooperatively breedingbell miner (Manorina melanophrys) in southeastern Australiaover six breeding seasons. Male offspring are the philopatricand helping sex in this system and can increase the reproductiveoutput of their parents. Bell miners aggressively defend theirterritory from all interspecific competitors and by doing soallow food resources to dramatically increase. The increasein phytophagous Psyllidae insects (which secrete a carbohydrate-richcoating that constitutes the major component of bell miner diet)leads to a decrease in tree health, often culminating in deathof the tree. Bell miners then move as groups to new areas withlow psyllid abundance, and the cycle repeats. Using this predictabletemporal variation in food availability, we aimed to determinewhether female breeders adjusted the sex ratio of broods toproduce more of the philopatric sex when food resources werehigh and more of the dispersing sex when food resources werelow. Our results provide clear evidence for such facultativecontrol of sex ratio by female bell miners. Newly founded coloniesare characterized by low food availability and a female-biasedprimary sex ratio, whereas colonies more than 1 year old havean increased food availability and a male-biased primary sexratio. We suggest treating forces associated with resource enhancementand competition as opposing sides of a single general principleand suggest that it is necessary to view both the costs andbenefits of philopatric individuals within a variable environment.