Food intake, locomotor activity, and egg laying in zebra finches: contributions to reproductive energy demand?
We tested two alternative hypotheses: (1) that energy costs of egg production in a small, short-lived passerine, the zebra finch (Taeniopygia guttata), are additive to the animal's total energy budget and are met by an increase in food intake or (2) that birds compensate for the energy costs of egg production by reducing energy expended on locomotor activity, thus, reallocating, but not increasing, their total energy budget. Breeding pairs of zebra finches had 8% lower daily food intake than nonbreeding pairs. Among breeding pairs, food intake varied with stage of egg formation but was lowest during egg laying, when egg formation costs were predicted to be highest. Female feeding behaviour (number of feeding bouts, peck rate) did not vary significantly during the laying cycle. Clutch size, but not egg size, was negatively correlated with food intake; that is, daily food intake was lowest for pairs in which females laid the largest clutches. Breeding pairs had much lower locomotor activity levels (114 hops h-1) than nonbreeding pairs (214 hops h-1), and activity declined sharply from 177 hops h-1 during the prelaying period to 106 hops h-1 at the start of the yolk-formation period. Clutch size was negatively correlated with locomotor activity on the day of peak energy demand for laying; that is, daily locomotor activity level was lowest for pairs in which females laid the largest clutches. Therefore, females reduced activity but did not increase food intake during the laying cycle, despite the additional energy requirements of egg production. These data suggest that the prevailing view of small passerines as "classic" income breeders needs to be reevaluated.
Williams
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Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada. tdwillia@sfu.ca
Ternan
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SP
D000818:Animals; D004435:Eating; D004734:Energy Metabolism; D005260:Female; D008297:Male; D009043:Motor Activity; D010058:Oviposition; D012098:Reproduction / Q000502:physiology*; D020308:Songbirds / Q000502:physiology*