Experimental analysis of mass change in female green-rumped parrotlets (Forpus passerinus): the role of male cooperation

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Abstract
Mass change was determined by weighing nine unmanipulated pairs of green-rumped parrodets during prospecting, egg laying, hatching, and fledging. Male and female mass were similar at the onset of prospecting. However, female mass had increased 25% by the start of egg laying, and females maintained the heavy mass through incubation. Females began losing mass at the time of hatching and reverted to weights that were similar to those of males by the end of hatching. Males neither gained nor lost mass during breeding. To test predictions from mass change hypotheses, 25 females were assigned manipulated broods of four or eight young. Females were weighed on the first day of hatching and 6, 10, and 27 days later, or until first fledging. Females with four and eight young lost the same amount of mass. Females lost less mass during brooding if their mates fed them more often. Females with four young tended to lose less mass during brooding if they spent less time away from the nest, whereas females with eight young tended to lose less mass if they spent more time away from the nest. Mass change after brooding was not related to provisioning rates of nestlings by females or males of either experimental group. Our results contradict the hypothesis that mass loss is due to stress, and correspond to some of the predictions of the adaptive, gonadal, and brooding starvation hypotheses.

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