Juvenile Foraging Proficiency, Parental Effort, and Avian Reproductive Success

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 D ), the proportion of H D allocated to physical activity vs. maintenance metabolism (basal + thermostatic costs), and foraging efficiency. We examined diet selectivity among parents and their young, and we calculated prey capture rates based on the measured energy content of insect prey. We found that adult juncos feeding young are neither food limited nor working maximally. Adults fulfilled their own energy demands, and those of their four dependent young, while foraging for 75% or less of the daylight hours. In contrast, recently independent young, 4—7 wk after fledging, must forage for >90% of the daylight hours to meet their own energy demands. Juncos younger than this cannot attain energy balance in a 15—h day without supplemental feeding by their parents, even if they forage continuously. The H D of adult juncos (mean mass 19.5 g) remained fairly constant throughout the breeding season, averaging 73.8 kJ/d ( n =51), which is 2.1 times their measured nighttime basal metabolic rate. In contrast, H D of fledging juncos increased steadily with age, from 59.7 kJ/d during the 1 st wk out of the nest to 73.8 kJ/d during week 3. This increase resulted partly from an increase in the proportion of time spent foraging. Once their parents quit supplying them with food, the young juncos' H D increased dramatically, peaking at 100 kJ/d in 10—12 wk—old juveniles. The increase in H D of independent young resulted initially from a dramatic increase in time spent foraging, which gradually declined only to be supplanted by an increase in aggressive interactions, especially flights within the juvenile flocks, and a marked increase in energy demand associated with the postjuvenile molt. For adult and young juncos, thermostatic costs represented respectively 20 and 23% of total daily energy expenditure, an amount of energy approaching that devoted to all physical activities. Thermostatic costs would have been even greater than this except that juncos are much better insulated than typical 19.5—g passerine birds (their coefficient of heat transfer is only 76% of that predicted from their mass). For these small birds, adaptations that affect body insulation can have more effect on daily energy budgets than changes in the time allocated to various physical activities. Our data reveal that energy constraints are a major selective force in Yellow—eyed Juncos, operating not through food limitation among adults but rather through the inefficient foraging of young juncos. The critical stage when selection operates most strongly on the efficient use of time and energy occurs during the first 3 mo of a junco's life. The lack of proficient foraging by young juncos explains observed patterns of parental time and energy expenditure and breeding season phenology.