Abstract

Prefledging developmental energetics of rainbow bee-eaters were investigated by comparing the mass, water fraction and energy density of the egg, hatchling and fledgling, determining the growth rate and pattern of the nestling and documenting the reproductive time investments of breeders. The incubation and nestling periods greatly exceeded allometric predictions. On average, the egg contents contained 26% yolk, 80% water and had an energy density of 25·82 kJ g ^-1 dry mass. The 3·3-g hatchling contained 83% water and had an energy density of 21·28 kJ g^-1 dry mass; its size and composition suggested that it was not exceptionally mature and thus that embryonic growth was inherently slow, but no extra energy loading was evident in the egg to meet the predicted high maintenance costs. Nestling growth was also inherently slow, K for the logistic growth model being 0·266 and t_10-90 16·5 days. Nestlings attained asymptotic mass after about two- thirds of the nestling period had elapsed, exceeded adult mass by up to 3·5-g and then underwent a 15% prefledging mass recession. Fledglings contained 67% water, had a mean energy density of 23·33 kJ g ^-1 dry mass and were capable of efficient flight. Nestling growth was highly labile and intrabrood mass hierarchies and brood reduction through nestling starvation were common and may be adaptations to short-term food shortages. The mean incubation constancy (54% of daytime) and the mean nestling feeding rate of 4 meals per nestling h of breeders were comparatively low; the former probably reflects the insulation of the burrow from ambient temperature oscillations and the latter the slow nestling growth rate. Members of breeding pairs often contributed to feeding the brood quite disparately and auxiliaries’ contributions in the 24% of breeding units that contained them were relatively small. The unpredictable nature of the species’ aeroplankton diet may have favoured slow nestling growth through its influence on breeders’ food gathering capacity and by favouring a relatively high degree of physiological maturity and self-sufficiency in fledglings.