Female reproductive strategies in bar-headed geese (Anser indicus): Why are geese monogamous?

Summary In a semi-captive flock of Anser indicus with a surplus of females, permanent harem groups consisting of one male with one to five females, lasting for up to several years, were regularly observed. Polygynous groups contain one paired female to which the male is most attentive and secondary females which follow the paired male and are tolerated by the pair. Average annual reproductive success was lowest in lone females (0.02 young fledged per year), higher in secondary females (0.23 young) and highest in paired females (0.56 young per year). Differences seemed due to different degrees of male assistance. Secondary females could not be shown to be competitors of paired females in annual reproductive success. Lone females became secondary females mainly after an age of 3 years, i.e. when their chances to pair had dropped significantly. Females were more likely to become secondary instead of paired females in years when the adult sex ratio was more heavily female biased. As sex ratios in wild geese are usually around 1:1 or even biased towards males, females will not usually need to resort to the suboptimal secondary-female strategy. Hence, geese usually live in monogamous pairs instead of harem groups.