| Summary: | I studied the breeding biology and energetics of (Phaethon lepturus) at two colonies, Punta Cruz (PC) and South Peninsula (SP), located in coastal boulder talus on Cayo Luis Pena, Culebra, Puerto Rico During 1984-86. Thirty to thirty-five pairs made nesting attempts at each colony in each of the three seasons, although nests were distributed over very different coastline distances, 170 m at PC and up to 420 m at SP. Overall reproductive success, calculated by the Mayfield method, at the two colonies combined was 0.162, 0.250 and 0.262 for 1984-86.Most nesting failures occurred during the egg stage, the most important factor at PC being agonistic encounters between conspecifics, including overt and severe fighting. The most important cause of nesting failure at SP was predation on eggs, especially by black rats (Rattus rattus). At SP, a decrease in evidence of predation by rats and an increase in hatching success from 0.115 to 0.230 to 0.456 over the three seasons were concurrent with predator control efforts, especially the use of poisoned rat bait.Chicks grew to adult body mass in 30 days, and reached 140% of adult mass before declining back to adult body mass, to fledge at 71-73 days. Growth of bill and wing was continuous through the development period, and chicks left the nest before the bill and wing reached adult dimensions.Incubation shifts were approximately eight days, and males spent slightly more time incubating than females. The metabolic rate of incubating adults was approximately 2.6 W, about 1.4 times basal metabolic rate.The frequency of feedings received by chicks F$\sb{\rm c}$ was 1.49 day$\sp{-1}$, and the mean feed mass m was 0.052 kg (SD = 0.017). The power received by chicks from the combined efforts of their parents P$\sb{\rm c}$ was 3.6 W, during the stage at which chicks could receive full sized meals. The average total feeding interval experienced by chicks T$\sb{\rm c}$ was 5.8 $\times$ 10$\sp4$ s (16.2 hours), but the distribution of these intervals was bimodal, with few intervals near the average. The interval between feedings given by an individual adult T$\sb{\rm a}$ was more variable than feed mass m (higher coefficients of variation). (Abstract shortened with permission of author.)
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