Why do incidences of cooperative breeding vary annually in a Tibetan bird? Climate via demography as the driver

Annual fluctuations of the percentage of helped pairs are common to cooperatively breeding vertebrate populations inhabiting climatically variable environments, but we do not know why. For species with male-only helping, the temporal variation can be driven by climate, which may influence sex-specific vital rates, population growth, adult sex ratios, and finally how many males obtain no mates and become helpers. We clarified the climate−demography pathway using 15 years of data from a Tibetan bird where sexual maturity is reached at age one. A positive correlation existed between incidences of cooperative breeding and male-biased adult sex ratios across years. Adult annual survival was independent of seasonal climate, but juvenile females survived less well and immigrated more frequently in warmer autumns. This indicated a temperature-induced natal dispersal and the resultant high mortality, given increasingly male-biased juvenile sex ratios across the autumn when juvenile females dispersed extensively. Spring rainfall, likely as a carry-over effect, positively predicted the survival of juvenile males during the non-breeding period. Therefore, local climate via demography associated with juvenile survival drove the annual fluctuations of cooperative breeding. This research provides a framework to elucidate temporal variation in sociality for populations exposed to stochastic environments.