Cooperative breeding in the skuas of the Chatham Islands

Abstract

Cooperative breeding, widely reported in birds, is found in <1–5% of territories in some populations of the Brown skua, Catharacta lonnbergi. In the New Zealand region, up to 30-50% of skua territories may be occupied by trios or larger groups. This study examines its occurrence at the Chatham Islands, east of New Zealand. Here 16% of territories are occupied by trios and 2% by groups. All members of skua trios and groups participate in sexual and other breeding activity, and the associations arc thus communal. Sexual discrimination of breeding birds by morphometric measurements shows that all communal groups known since 1978-79 have been polyandrous. These groups are long-lived associations, some of which are known to have persisted for at least 14 years. Trios are as long-lived and stable as pairs, and birds on communal territories do not move from them even when an appropriate-sex space becomes available on an adjacent pair territory. The members of trios are not close kin. All members of communal associations participate in territorial defence and chick rearing. In trios, the males appear to be equals, although in any one year the actual paternity of offspring may reside with only one of them. Overall reproductive success for Chatham Island skuas is high, for both pairs and communal groups, compared with other populations. However, communal trios and groups have lower reproductive success than pairs even over a l0 year period, particularly when considered on a per adult basis. Furthermore, no improvement in chick ‘quality’ is discernible. Unusually for skuas, the breeding population at the Chatham Islands is non-migratory. Skuas are present on their breeding territories during the winter, and exhibit characteristic territorial and agonistic behaviours, albeit at lower intensity than during the breeding season. It is suggested that communal breeding in this skua population is not adaptive per se, but a secondary consequence of year-round residence. This is a departure from the conventional resolution of communal breeding. Residence is facilitated by benign climatic conditions and year-round prey availability. When territory space becomes available outside the breeding season, in a small number of cases more than a pair of skuas are able to establish themselves. Thereafter, trios and larger groups persist and behave in the same manner as pairs. The flux between trios and pairs when birds are lost is determined, in part, by the sex of that bird. Thus a trio which loses its one female will ‘acquire’ a replacement female and persist as a trio, whereas a trio which loses one of its two males will thereafter continue as a pair. Keywords: Cooperative breeding, communal breeding, polyandry, Stercorariidae, skuas, skua trios, skua behaviour, skua breeding, Chatham Islands