Cooperative breeding and delayed dispersal in the Pale chanting goshawk, Melierax canorus

Doctoral Thesis


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University of Cape Town

A population of Pale Chanting Goshawks Melierax canorus, some of which live in families, was studied during 1988-1992 for a total of 117 group-years near Calitzdorp, South Africa. The aims of the study were to identify ecological and social factors that might predispose individuals in the population to delay dispersal and become non-breeding or co-breeding members of Pale Chanting Goshawk families, and to determine why cobreeders breed cooperatively in polyandrous trios. In all vegetation types within the study area, non-breeders, as juvenile and adult offspring, delayed dispersal from their natal territories. However, co-breeding males occurred only in one vegetation type, Karroid Broken Veld. Co-breeding males participated in all reproductive activities, including copulation. Karroid Broken Veld also supported the largest known groups of Pale Chanting Goshawks and the highest frequency of groups with non-breeders, which resulted in some of the highest recorded single species raptor densities in the Afrotropics. Pale Chanting Goshawks in Karroid Broken Veld preyed primarily on two otomyinid rodents, Otomys unisulcatus (42-48 % of prey biomass) and Parotomys brantsii (18-32 % ). The habitat quality of Karroid Broken Veld for Pale Chanting Goshawks was high since, compared with other vegetation types, it incorporated: (1) optimal habitat for otomyinid prey, (2) a very high estimated biomass of otomyinids, almost twice that of other vegetation types, and (3) a hunting habitat with an optimum combination of prey visibility and perch availability that facilitated hunting efficiency. Territorial space was limited throughout the study area, constraining the number of nonbreeders per group to two and inhibiting new breeders from establishing territories. Juvenile non-breeders probably delayed dispersal to increase their probability of survival, and dispersed later as sexually mature adults, since they could not increase their fitness further by becoming helpers at the nest. During the nestling period, co-breeding beta males provided prey at an equal rate to dominant males, that enabled polyandrous trios to undertake more frequent and successful breeding attempts in years of high prey abundance. The help provided by co-breeders contributed more to this success than did density of dominant prey and territory size. In Karroid Broken Veld co-breeders delayed dispersal since their fitness as subordinate sibling males was probably higher than fitness achieved due from dispersing to a breeding vacancy in habitat of lower quality. I suggest that those ecological factors which contribute to habitat quality provided the proximate impetus, and the resulting saturation of the habitat the ultimate impetus, in promoting the establishment of Pale Chanting Goshawk family units. Once Pale Chanting Goshawk formed families, a range of secondary benefits evolved as birds adjusted their behaviour to benefit from the presence of other group members. For example, although breeders in high-quality habitat produced the highest number of offspring, the lack of territorial space probably forced more offspring to disperse. To increase offspring survival, breeders may have adjusted their reproductive strategy and allow non-breeders to partake and share in returns of cooperative hunts. Other secondary benefits included the possibility of inheriting a natal territory, budding-off onto territorial borders or helping close relatives as an experienced co-breeder.

Bibliography: pages 251-265.