Behavioral activiti es and predatory behavior of 18 American Kestrels (Falco sparverius, 9 males and 9 females) were observed for 350+ hours during 3 seasons (nonbreeding = Jan-Feb , breeding = mid-~lar-Apr, and postbreeding = late-Aug-Sept) in northern Utah. Daily energy expenditure (DEE) of male and female kestrels was estimated with a model that incorporated flight activity data from free-living birds and laboratory measurements on daytime and nighttime metabolic rates and energy costs of tissue production derived from captive kestrels. Production costs were included in the DEE for breeding and postbreeding kestrels.
The energy cost of gonadal growth for males (0.02 kcal/day) and females (0.20 kcal/day) was added to the DEE of breeding kestrels. Breeding females expended an estimated 10.13 kcal/day for producing an average clutch of 4.5 eggs. The energy costs of fat deposition (2.27 and 4.39 kcal / day for males and females, respectively) and molt (2.38 and 2.72 kcal/day for males and females , respectively) were added to the DEE of postbreeding kestrels. In addition to the DEE , the model predicted nonflight energy expenditure (NFEE) and flight energy expenditure (FEE) during the day, and energy expenditure during the night (NEE).
DEE of nonbreeding birds is generally higher (47.71 kcal/day) than those from the breeding (44.89 kcal / day) and postbreeding (42.42 kcal / day) seasons. DEE of females (48.69 kcal/day) is higher than males (41.31 kcal/day) primarily because females averaged 15.5% heavier than males during all 3 seasons, and females have higher costs of production. Kestrels are heaviest during the nonbreeding season and the amount of metabolizable energy available is highest. DEE is lower during the breeding and postbreeding seasons because thermoregulatory demands have decreased which may allow energy to be metabolized for production.
NFEE accounts for most (48.5%) of the DEE. Flight costs are relatively small because kestrels allocate an average 3% of the photoperiod (25.6 min/day) to flight activities. Egg production accounts for 20% of the DEE of breeding females. The energy cost of fat deposition and molt accounts for 11.6 and 15.9% of the DEE for postbreeding males and females, respectively. These reproductive and tissue production costs may also elevate the DEE of breeding and postbreeding females to that of nonbreeding females
