Extraordinary Clutch Size and Hatching Asynchrony of a Neotropical Parrot

A13STRACT.--We studied the nesting biology of the Green-rumped Parrotlet (Forpus passerinus) in the llanos of Venezuela. Clutch size averaged 7 eggs (range: 5-10), and eggs were typically laid daily or every other day over 7-16 days (• = 10). Incubation began with the first egg. Clutches hatched over 2-14 days (œ = 8.6) and fledged over 4-17 days (œ = 9.7). An average of 5.7 chicks hatched (range: 2-10) and 4.7 chicks fledged (range: 1-8) per successful nest. The number of young fledging increased slightly with clutch size, but parents that hatched more eggs were able to fledge significantly more young. Chicks fledged at 28-35 days of age, and chicks that hatched later and in nests with larger broods took significantly longer to fledge. Hatching success was significantly lower for last eggs in large clutches than for eggs laid earlier or in smaller clutches. However, fertility and volume of last eggs were similar to other eggs. Fledging success was uniformly high for nestlings in small broods, declined slightly for the last two chicks in medium sized broods, and declined sharply for the last two nestlings in large broods. Last-hatched and penultimately hatched chicks were found dead with empty crops significantly more often than earlier-hatched chicks, which implies death by starvation. We evaluated these results in relation to current hypotheses for the evolution of clutch size in tropical birds and the causes of hatching asynchrony. We propose the limited breedingopportunities hypothesis to explain both the large clutch size and hatching asynchrony in F. passerinus. When opportunities to nest are limited through intraor interspecific competition, females might be expected to invest heavily in reproductive ffort and guard their clutches from nest-site takeovers. This could lead to large clutches, incubation beginning with the first egg, and a large hatching interval. Received 30 August 1990, accepted 23 March 1991.