FORAGING EFFICIENCY OF PARENT JUNCOS AND THEIR YOUNG

From measurements of metabolizable energy (ME) intake (doubly labeled water technique), prey capture rates, and time spent foraging, we calculated both foraging efficiency (FE) and rate of energy gained while foraging (E(f)) of adult Yellow-eyed Juncos (Junco phaeonotus) and their offspring throughout the breeding season. E(f) (kJ ME acquired per hr spent foraging) of adult Yellow-eyed Juncos varied 1.7 fold over the breeding cycle, as adults adjusted both the amount of time they spent foraging and their FE to meet changing energy demands and time constraints. Males paired to incubating females, and males and females feeding fledglings acquired energy at substantially lower rates (ca. 15-17 kJ/hr) than incubating females (26 kJ/hr) or than males and females feeding nestlings (21 kJ/hr). From this, we infer that adult juncos may reduce their FE when energy demands and/or time constraints are relaxed, either because the costs of inefficient foraging are negligible during these periods or because there are hidden costs or risks associated with foraging more efficiently. E(f) of juvenile juncos increased 2.1 fold from a low of 5.3 kJ/hr during their first week of parental independence (a period of marked juvenile mortality due to starvation) to 10.9 kJ/hr (about 75% of the lowest adult level) by six-weeks post-independence. Juvenile FE increased linearly with age as the young gained proficiency at foraging. By extrapolation, juvenile FE would not have reached adult levels until the young were about four months old.