Abstract
Environmental and behavioral factors have long been assumed to affect variation in avian field metabolic rate (FMR). However, due to the difficulties in measuring continuous behavior of birds over prolonged periods of time, complete time-activity budgets have rarely been examined in relation to FMR. Our objective was to determine the effect of activity (measured by detailed time-activity budgets) and a series of extrinsic and intrinsic factors on FMR of the omnivorous ring-billed gull (Larus delawarensis). The experiment was conducted during the incubation period when both members of the pair alternate between attending the nest-site and leaving the colony to forage in aquatic and anthropogenic environments (city, agricultural). FMR was determined using the doubly labeled water method. Time-activity budgets were extrapolated from spatio-temporal data (2-5 days) obtained from bird-borne GPS data loggers. Gulls had low FMRs compared to those predicted by allometric equations based on recorded FMRs from several seabird species. Gulls proportioned their time mainly to nest-site attendance (71% of total tracking time), which reduced FMR/g body mass, and was the best variable explaining energy expenditure. The next best variable was the duration of foraging trips, which increased FMR/g; FMR/g was also elevated by the proportion of time spent foraging or flying (17% and 8% of tracking time respectively). Most environmental variables measured did not impact FMR/g, however, the percent of time birds were subjected to temperatures below their lower critical temperature increased FMR. Time-activity budgets varied between the sexes, and with temperature and capture date suggesting that these variables indirectly affected FMR/g. The gulls foraged preferentially in anthropogenic-related habitats, which may have contributed to their low FMR/g due to the high availability of protein- and lipid-rich foods. This study demonstrates that activities were the best predictors of FMR/g in ring-billed gulls, thus providing strong support for this long-standing theory in bioenergetics.
Highlights
Several factors are known to modulate energy expenditure in animals including environmental variables such as, season, weather, and food availability [1,2] as well as intrinsic variables such as behavior, physiology, reproductive status and effort [1,3,4]
Because the effects of environmental variables are often masked by activity, we examined how these variables related to time-activity budget to determine if they may indirectly affect field metabolic rate (FMR) through behavior
Females were smaller than males and when this difference in body mass was taken into account, the metabolic rate was significantly lower by 28% in males compared to females (FMR/g; males: 0.68 ± 0.04 kJ/g/d; females: 0.87 ± 0.05 kJ/g/d; t-test t44 = -3.08, p = 0.004)
Summary
Several factors are known to modulate energy expenditure in animals including environmental variables such as, season, weather, and food availability [1,2] as well as intrinsic variables such as behavior, physiology, reproductive status and effort [1,3,4]. Considering the variability of these factors, it is not surprising that large inter-individual differences in daily energy expenditure, or field metabolic rate (FMR), have been reported in several avian species (reviewed in [5]). Determining how these variables influence energy expenditure in birds has received considerable attention in recent years. Most studies that have attempted to relate environmental variables to FMR have been unable to establish a clear relationship [6,7]. Few studies have examined the combined influence of intrinsic and extrinsic factors on FMR in free-living animals [8]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
