THE EFFECTS OF SEX, AGE, AND SOCIAL STATUS ON ANNUAL SURVIVAL IN THE SPLENDID FAIRY-WREN

Survival is a critical life-history trait, and among cooperative breeders survival may be linked to the evolution of social organization. We used multi-state models in the program MARK to estimate apparent survival in the Splendid Fairy-wren (Malurus splendens), a cooperatively breeding species in which most pairs are assisted by male offspring from previous generations. We examined survival as it relates to sex, age, and social status (nestling, auxiliary, breeder), and quantified the probabilities of transition between social states. The best-supported model was one in which survival rates differed by social state, survival of auxiliaries and breeders varied annually in the same manner, and the effect of sex varied annually but influenced the survival rate of each group in the same manner. In both males and females overall survival estimates of auxiliaries were similar to those of breeders, whereas survival estimates of adult males were higher than those of females, although the effect of sex varied annually. The probability of transition between categories of social status varied in a manner expected for a cooperatively breeding species: nestling males were more likely than nestling females to become auxiliaries, whereas females were more likely to become breeders in the subsequent year. Similarly, among auxiliaries, females were more likely than males to become breeders. Survival of males being higher that of females likely contributes to the male-biased sex ratio observed in adults of this species and, indirectly, the propensity of younger males to delay dispersal.