Polyandry has been studied in many species, especially birds [1]. Exclusively fraternal polyandry (several full or half-brothers with one unrelated female) is only known in human societies [2, 3], in which it is an important mechanism for limiting reproductive output [3] in association with scarce environmental resources [2]. However, the social organization of the Kagu Rhynochetos jubatus, a bird species endemic to New Caledonia, has the characteristics of this mating system. Kagu are cooperative breeders and evolved in the absence of predators [4, 5]. Breeding birds and their helpers contributes to the care and defense of the chick [6]. Kagu populations occur in both poor and rich habitats [7] and differ substantially in food supply and associated reproduction rates [8]. This enabled usto verify whether fraternal polyandry increased reproductive output in low-density situations but limited reproduction in high-density populations. Our 15-year study revealed that, regardless of resource availability, Kagu were organized in facultative fraternal polyandrous families grouped in clans. Within a clan, all breeding females were unrelated, whereas all males were related. There was no extra-clan paternity. An average family size of four to five adults was optimal for breeding success. Males that have long-established families in their own territory regularly visited their parents. We conclude that fraternal polyandry in Kagu increases reproductive output under poor environmental conditions but limits population growth when the population is near carrying capacity because the clannish spatial organization prevents new families from establishing territories.

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