Field metabolic rate and activity of ring-billed gulls

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 not yet been examined in relation to FMR. Our objective was to determine the effect of 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.

摘要：长期以来，学界普遍认为环境与行为因素会影响鸟类野外代谢率（field metabolic rate, FMR）的变异。然而，由于难以长期持续监测鸟类的行为活动，迄今尚未有研究将完整的时间-活动预算与野外代谢率相结合开展关联分析。本研究旨在探究精细化时间-活动预算，以及一系列外源与内源因素对杂食性环嘴鸥（Larus delawarensis）野外代谢率的影响。实验于繁殖期孵化阶段开展，此时配对的环嘴鸥双亲会交替执行两项活动：一是值守巢址，二是离开繁殖群前往水生或人为活动相关生境（城市、农业区域）觅食。本研究采用双标水法（doubly labeled water method）测定环嘴鸥的野外代谢率，时间-活动预算则通过鸟类搭载的GPS数据记录仪获取的2~5天时空数据推演得到。相较于基于多种海鸟野外代谢率数据构建的异速生长方程预测值，环嘴鸥的野外代谢率水平偏低。环嘴鸥的主要活动为巢址值守，占总监测时长的71%，该行为降低了单位体重的野外代谢率，是解释其能量消耗的最优变量。次优预测变量为觅食旅程时长，该因素会提升单位体重的野外代谢率；此外，用于觅食或飞行的时间占比（分别占总监测时长的17%与8%）同样会升高单位体重的野外代谢率。多数测定的环境变量未对单位体重野外代谢率产生显著影响，但当环境温度低于环嘴鸥下限临界温度的持续时长占比升高时，会提升其单位体重野外代谢率。环嘴鸥的时间-活动预算存在性别差异，并随环境温度与捕获日期发生变化，提示这些变量会间接影响单位体重野外代谢率。环嘴鸥优先选择人为活动相关生境觅食，由于此类生境中富含蛋白质与脂质的食物资源充足，这可能是其单位体重野外代谢率偏低的原因之一。本研究表明，活动行为是环嘴鸥单位体重野外代谢率的最佳预测因子，从而为生物能量学中的这一经典理论提供了强有力的实证支撑。