Helping does not enhance reproductive success of cooperatively breeding rufous vanga in Madagascar

Abstract

SummaryIn many studies on cooperative breeding, helping by auxiliary individuals is considered to enhance the reproductive success of breeders. However, confounding factors other than helping could cause the differences.The endemic Madagascan rufous vangaSchetba rufa(Vangidae) is known as a cooperatively breeding species. In order to evaluate the effect of helping in this species, we compared the reproductive success of breeding pairs helped by auxiliary birds, and pairs breeding alone, based on a 6‐year study of an individually colour‐banded population in the Ampijoroa Forest Station, western Madagascar.This species is single‐brooded. Brood reduction was rare and most cases of nesting failure were due to total loss of clutches and broods, probably as a result of predation.Monogamous pairs with one to four auxiliary birds comprised 24–43% of all breeding groups. Most auxiliary birds were male offspring remaining in their natal territory. Auxiliary males provided a considerable contribution towards antipredator defence, territory defence and to the provisioning of nestlings. In about one‐third of the groups, however, the auxiliary males did not help at all. Male offspring may remain in their natal territory in order to avoid harassment by other territorial individuals, and to increase the probability of territory acquisition and of copulation with unrelated breeding females.The probability of breeding successfully was higher, and the number of fledglings produced was larger among pairs with auxiliary birds than among those breeding alone. However, provisioning by auxiliaries neither enhanced the growth rate of nestlings, nor reduced the number of days they required to fledge.A pair‐match comparison of the same pairs between years with and without auxiliaries showed no effect of group size on their reproductive success. Even provisioning by auxiliaries did not affect the reproductive success.Pairs accompanied by auxiliary birds for more than 1 year enjoyed higher reproductive success even in those years when they were without auxiliaries than did pairs always breeding alone.Neither the simple presence of auxiliaries nor their helping behaviour enhanced the reproductive success of breeding pairs. The quality of the breeding pair and/or their territory may have affected their reproductive success and, as a result, increased the number of auxiliaries.