Harding Ann M A, Piatt John F, Schmutz Joel A, Shultz Michael T, Van Pelt Thomas I, Kettle Arthur B, Speckman Suzann G
Alaska Pacific University, Environmental Science Department, 4101 University Drive, Anchorage, Alaska 99508, USA.
Ecology. 2007 Aug;88(8):2024-33. doi: 10.1890/06-1695.1.
Flexible time budgets allow individual animals to buffer the effects of variable food availability by allocating more time to foraging when food density decreases. This trait should be especially important for marine predators that forage on patchy and ephemeral food resources. We examined flexible time allocation by a long-lived marine predator, the Common Murre (Uria aalge), using data collected in a five-year study at three colonies in Alaska (USA) with contrasting environmental conditions. Annual hydroacoustic surveys revealed an order-of-magnitude variation in food density among the 15 colony-years of study. We used data on parental time budgets and local prey density to test predictions from two hypotheses: Hypothesis A, the colony attendance of seabirds varies nonlinearly with food density; and Hypothesis B, flexible time allocation of parent murres buffers chicks against variable food availability. Hypothesis A was supported; colony attendance by murres was positively correlated with food over a limited range of poor-to-moderate food densities, but independent of food over a broader range of higher densities. This is the first empirical evidence for a nonlinear response of a marine predator's time budget to changes in prey density. Predictions from Hypothesis B were largely supported: (1) chick-feeding rates were fairly constant over a wide range of densities and only dropped below 3.5 meals per day at the low end of prey density, and (2) there was a nonlinear relationship between chick-feeding rates and time spent at the colony, with chick-feeding rates only declining after time at the colony by the nonbrooding parent was reduced to a minimum. The ability of parents to adjust their foraging time by more than 2 h/d explains why they were able to maintain chick-feeding rates of more than 3.5 meals/d across a 10-fold range in local food density.
灵活的时间预算使个体动物能够通过在食物密度降低时分配更多时间觅食来缓冲食物供应变化的影响。这一特性对于以零散且短暂的食物资源为食的海洋捕食者来说应该尤为重要。我们利用在美国阿拉斯加三个具有不同环境条件的繁殖地进行的为期五年的研究中收集的数据，研究了一种长寿海洋捕食者——普通海鸦（厚嘴海鸦）的灵活时间分配情况。年度水声调查显示，在15个繁殖地年的研究中，食物密度存在一个数量级的差异。我们利用亲鸟时间预算和当地猎物密度的数据来检验两个假设的预测结果：假设A，海鸟在繁殖地的停留时间随食物密度呈非线性变化；假设B，亲厚嘴海鸦的灵活时间分配可缓冲雏鸟面临的食物供应变化。假设A得到了支持；在食物密度从差到中等的有限范围内，海鸦在繁殖地的停留时间与食物呈正相关，但在更高密度的更广泛范围内则与食物无关。这是海洋捕食者的时间预算对猎物密度变化的非线性反应的首个实证证据。假设B的预测结果在很大程度上得到了支持：（1）在广泛的密度范围内，雏鸟的喂食率相当恒定，只有在猎物密度较低时才会降至每天3.5次以下；（2）雏鸟喂食率与在繁殖地停留的时间之间存在非线性关系，只有在非育雏亲鸟在繁殖地停留的时间减少到最低限度后，雏鸟喂食率才会下降。亲鸟每天能够将觅食时间调整超过2小时，这解释了为什么它们能够在当地食物密度相差10倍的情况下，将雏鸟的喂食率维持在每天3.5次以上。
