The standard metabolic rate (SMR) of birds correlates broadly with climate of origin. SMR tends to be higher in birds from cold climates and lower in tropical forms than would be expected from the bird's mass. SMR changes, on the average, 1% per degree change in latitude. The influence of climate on SMR is, however, subject to modification by other aspects of the bird's life history. For example, in tropical species adaptive modifications in SMR correlate with thermal microhabitat. Tropical birds which forage in the sun have SMR's averaging 25% lower than expected, while SMR of species which forage in the shade is normal. Species of penguins which undergo prolonged fasts during the breeding season do not show elevated SMR's typical of high latitude birds.

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We measured the allocation of time and energy in a population of adult Yellow-eyed Juncos (Junco phaeonotus) and their young (nestlings, fledglings, independent juveniles) throughout the breeding season using concurrent time-activity budgets and doubly labeled water. We constructed energy budgets by extrapolating laboratory measurements of metabolic heat production to field conditions using a linear heat-transfer model and the operative temperature and wind speed experienced by the free-living bird. From our data we calculated daily energy expenditure (H 0 ), the proportion of H 0 allocated to physical activity vs. maintenance metabolism (basal + thermostatic costs), and foraging efficiency.

The food requirements of dependent sons and daughters have important implications for evolution of the sex ratio, according to current sex allocation theory. We studied food requirements of nestling American kestrels (Falco sparverius), a moderately size-dimorphic falcon, by hand-feeding 61 birds from hatching to fledging. Daughters, the larger gender, consumed 6.99% more food than did sons. Sons did not have higher energy expenditure from higher effort during sibling competition than daughters did, so parents must supply more food to satisfy daughters' needs than to satisfy sons'. A review of all related studies shows a strong positive association between the degree of sexual size dimorphism and gender difference in food requirements.

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