Kimberly A. Sullivan

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 (t,) of adult Yellow-eyed Juncos (Junco phaeonotus) and their offspring throughout the breeding season. , (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. 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.

Foraging proficiency and body condition of juvenile American Dippers

We quantified foraging proficiency in nine free-ranging American Dippers (Cinclus mexicanus) when they were 39-42 days post-hatching, and correlated foraging proficiency with body condition. Although all nine individuals foraged extensively on the smallest size class of prey, individuals with a relatively low rate of caloric intake (RCI) from the larger prey classes consumed a greater proportion of the smallest size prey. Body mass and hematocrit were significantly correlated with RCI. Mass and hematocrit are relative measures of body condition and the body condition of newly independent juveniles may predict survivorship through the first year. Thus, these data are consistent with the assumption that greater foraging proficiency confers fitness benefits for free-living birds. Alternatively, these data also support the interpretation that juvenile dippers in better physical condition forage more proficiently than birds in poorer condition.

BROOD SIZE AND THERMAL ENVIRONMENT INFLUENCE FIELD METABOLISM OF NESTLING YELLOW-EYED JUNCOS

We used the doubly-labeled-water method to examine the effect of thermal environment (mean shade air temperature) and brood size (two to six young) on field met- abolic rate of nestling Yellow-eyed Juncos (Junco phaeonotus). The relationship between brood size and nestling field metabolic rate was curvilinear, such that nestling energy expenditure was lowest in broods of four (the most common clutch size) and highest in broods of two (the smallest brood size). Mean air temperature accounted for 21.0% of the variation in nestling field metabolic rate, whereas brood size accounted for 17.5% of the variation. Although brood reduction would decrease the broods field metabolic rate, the reduction is not proportional to the decrease in brood size. Received 15 November 1990, accepted 27 June 1991. LACK (1947) proposed that clutch size in al- tricial birds is determined by the maximum number of young that parents can feed ade- quately. Whereas some studies have supported this contention by documenting increased nest- ling mortality or decreased nestling mass in en- larged broods, in many species the most pro-

Time and heat allocations to balance conflicting demands during intermittent incubation by Yellow-eyed Juncos

We use a new method to analyze time allocations influencing average egg temperatures during intermittent incubation. It measures relative investment in parental self-maintenance versus embryo development rate, or the extent to which performance for either is compromised from integration of both functions. Unlike attentiveness (min/hour on the nest), we compare time when eggs cool and heat with the subsequent time when egg temperatures are kept relatively constant (tequil). When these times are equal eggs increase in mean temperature at a maximum rate/bout. Corresponding average egg temperatures are halfway between average egg temperatures achieved with high frequency incubation (tequil = 0) and those achieved with continuous incubation. We reanalyzed previously published data for Yellow-eyed Juncos Junco phaeonotus (Weathers and Sullivan 1989). Results showed variable rates of increase in mean egg temperatures, but over 4 of 8 days at 3 of 6 nests average egg temperatures represented a balance between the conflicting demands. Remaining days and nests showed allocations favoring embryo development rate. Thus females on average either balanced conflicting demands during incubation or were able to shift allocations toward development rate. This occurred because several bouts with short tequi favoring self-maintenance early and late in a day were balanced or exceeded by a few bouts with long tequil favoring development rate at midday.

Facultative migration in yellow-eyed juncos

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