Variation in the Onset of Incubation in a Neotropical Parrot

We studied the onset of incubation in the Green-rumped Parrotlet (Forpus passerinus), a cavity-nesting species with female-only incubation starting on the first egg and asynchronous hatching. We quantified how the onset of incubation varied among individual females, with stage of egg-laying and by ultimate clutch size. We then examined whether this variation affected the incubation period and hatching success of individual eggs. Female parrotlets initiated incubation in three characteristic patterns: slowly rising, rapidly rising, and pulsed. The diurnal incubation rate of the first egg was 80.0% and increased as the laying cycle progressed, but was not affected by ultimate clutch size. Females that were fed more often by their mates during laying had lower incubation rates. First-laid eggs had longer incubation periods than later-laid eggs, but nearly all eggs hatched in the order they were laid. Hatching success was not affected by laying order. Despite the strong hatching asynchrony and consistent first-egg incubation exhibited by this species, we found significant variation in the onset of incubation, although no serious fitness consequences of this variation were detected.