ASPECTS OF THE ECOLOGICAL ENERGETICS OF DEVELOPMENT IN RAINBOW BEE-EATERS

Prefledging developmental energetics of rainbow bee-eaters were invest igated 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 allomet ric predictions. On average,the egg contents contained 26% yolk, 80% w ater and had an energy density of 25.82 kJ g(-1) dry mass. The 3.3-g h atchling 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 except ionally 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 fo r the logistic growth model being 0.266 and t(10-90) 16.5 days. Nestli ngs attained asymptotic mass after about two-thirds of the nestling pe riod had elapsed, exceeded adult mass by up to 3.5-g and then underwen t a 15% prefledging mass recession. Fledglings contained 67% water, ha d 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 ma ss hierarchies and brood reduction through nestling starvation were co mmon and may be adaptations to short-term food shortages. The mean inc ubation constancy (54% of daytime) and the mean nestling feeding rate of 4 meals per nestling h of breeders were comparatively low; the form er probably reflects the insulation of the burrow from ambient tempera ture oscillations and the latter the slow nestling growth rat, Members of breeding pairs often contributed to feeding the brood quite dispar ately 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 thr ough its influence on breeders' food gathering capacity and by favouri ng a relatively high degree of physiological maturity and self-suffici ency in fledglings.