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Behavioral Ecology Vol. 15 No. 4: 607-613
Behavioral Ecology vol. 15 no. 4 © International Society for Behavioral Ecology 2004; all rights reserved

Against the odds? Nestling sex ratio variation in green-rumped parrotlets

Amber E. Budden and Steven R. Beissinger

Ecosystem Sciences Division, Department of Environmental Science, Policy & Management, 151 Hilgard Hall No. 3110, University of California, Berkeley, CA 94720-3110, USA

Address correspondence to A.E. Budden. E-mail: aebudden{at}nature.berkeley.edu

We investigated nestling sex ratio variation in the green-rumped parrotlet (Forpus passerinus), a small neotropical parrot breeding in central Venezuela. There are strong theoretical reasons to predict a female-biased sex ratio in this system according to the local resource hypothesis; juvenile males are philopatric and there are high levels of competition between male siblings for access to breeding females. Data were collected from two breeding sites over a 14-year period incorporating 564 broods with a total of 2728 nestlings. The mean percentage of male nestlings across years was 51%. Despite extreme hatching asynchrony in this system and increased survival of earlier hatched offspring, there was no bias in sex allocation associated with egg sequence. Patterns in sex allocation were not associated with clutch size, age, or size of the breeding female or breeding site. The potential for selective resorption of eggs was considered; however, no significant relationship was found between extended laying intervals and the sex of subsequent eggs. Together, these results suggest that female parrotlets are unable to regulate the sex ratio of their clutch at laying or that facultative manipulation of nestling sex ratio may not confer a fitness benefit to breeders in these populations.

Key words: Forpus passerinus, green-rumped parrotlet, laying interval, local resource competition, sex ratio.


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