Balancing Energy Budget in a Central-Place Forager: Which Habitat to Select in a Heterogeneous Environment?

Foraging animals are influenced by the distribution of food resources and predation risk that both vary in space and time. These constraints likely shape trade-offs involving time, energy, nutrition, and predator avoidance leading to a sequence of locations visited by individuals. According to the marginal-value theorem (MVT), a central-place forager must either increase load size or energy content when foraging farther from their central place. Although such a decision rule has the potential to shape movement and habitat selection patterns, few studies have addressed the mechanisms underlying habitat use at the landscape scale. Our objective was therefore to determine how Ring-billed gulls (Larus delawarensis) select their foraging habitats while nesting in a colony located in a heterogeneous landscape. Based on locations obtained by fine-scale GPS tracking, we used resource selection functions (RSFs) and residence time analyses to identify habitats selected by gulls for foraging during the incubation and brood rearing periods. We then combined this information to gull survey data, feeding rates, stomach contents, and calorimetric analyses to assess potential trade-offs. Throughout the breeding season, gulls selected landfills and transhipment sites that provided higher mean energy intake than agricultural lands or riparian habitats. They used landfills located farther from the colony where no deterrence program had been implemented but avoided those located closer where deterrence measures took place. On the other hand, gulls selected intensively cultured lands located relatively close to the colony during incubation. The number of gulls was then greater in fields covered by bare soil and peaked during soil preparation and seed sowing, which greatly increase food availability. Breeding Ring-billed gulls thus select habitats according to both their foraging profitability and distance from their nest while accounting for predation risk. This supports the predictions of the MVT for central-place foraging over large spatial scales.

觅食动物的行为会受到食物资源分布与捕食风险的共同影响，而这两类因素均随空间与时间动态变化。这些制约因素很可能塑造了动物在时间分配、能量投入、营养获取与避敌行为之间的权衡策略，进而决定了个体的活动位点序列。根据边际值定理（marginal-value theorem, MVT），中心地觅食者（central-place forager）在距离其核心据点更远的区域觅食时，要么需要增加携带的食物载荷量，要么需提升食物的能量含量。尽管这类决策规则有望影响动物的移动模式与生境选择特征，但目前鲜有研究阐明景观尺度下生境利用背后的内在机制。因此本研究旨在明确：在异质景观中筑巢的环嘴鸥（Ring-billed gull, Larus delawarensis）是如何选择觅食生境的。本研究基于高精度GPS追踪获取的位点数据，采用资源选择函数（resource selection functions, RSF）与停留时间分析方法，识别出环嘴鸥在孵卵期与育雏期选择的觅食生境。随后结合环嘴鸥调查数据、取食速率、胃容物分析与热量测定分析，对其潜在的权衡策略进行评估。在整个繁殖季，环嘴鸥优先选择垃圾填埋场与转运站点——相较于农田或河岸生境，这类生境的平均能量摄入水平更高。它们会选择距离繁殖群更远、未实施驱离措施的垃圾填埋场，而避开距离较近且已开展驱离行动的填埋场。而在孵卵期，环嘴鸥则优先选择距离繁殖群相对较近的集约化耕作农田。在裸露土壤覆盖的农田中，环嘴鸥的种群数量更多，且在翻耕备土与播种阶段达到峰值——该时段的食物可获得性会大幅提升。综上，繁殖期环嘴鸥会综合考量觅食收益、与巢址的距离，并结合捕食风险来选择生境。这一结果验证了大空间尺度下中心地觅食模式的边际值定理预测。