The time-activity budgets of breeding Blue Swallows Hirundo atrocaerulea and the effects of weather on nestling growth

Abstract

The Blue Swallow is an intra-African migrant that breeds in its fragmented range from South Africa to southern Tanzania, from October to March annually. The birds migrate north and are on their non-breeding range in NE DRC, Uganda, NW Tanzania and W Kenya, from April to September annually. The global Blue Swallow population, estimated at <1 500 pairs, is classified as ‘Vulnerable’, because of its small and declining population. Only the female Blue Swallow incubates the eggs and broods the nestlings for the first nine days after they have hatched from the eggs. The growth rate of Blue Swallow nestlings is reduced by adverse weather conditions characterised by low temperatures (<14 °C), 3–4 consecutive days of fog and intermittent rainfall. In the Blue Swallow Natural Heritage Site (25°36′ S, 30°45′ E), food shortages, because of three or more consecutive days of adverse weather conditions resulted in Blue Swallows shifting entirely to self-maintenance by abandoning eggs and nestlings and attempting to breed again when weather conditions improved. The post-fledging survival of the nestlings affected by adverse weather conditions is reduced. The reason is that at the time of fledging the mass of these nestlings was below the mean mass of the adults and because the mass of nestlings developing under good weather conditions is equal to or greater than the mean mass of the adult birds. Circumstantial evidence suggests that Common Fiscal Lanius collaris may be the predator of Blue Swallow eggs and nestlings. Adult longevity and the breeding success of Blue Swallows may have been reduced by adverse weather affecting their entire meta-populations in the small and isolated patches of suitable habitat remaining for the birds. These characteristics of the Blue Swallow meta-populations may be contributing to their continued decline.