Feather corticosterone reveals stress associated with dietary changes in a breeding seabird

Changes in climate and anthropogenic pressures might affect the composition and abundance of forage fish in the world's oceans. The junk-food hypothesis posits that dietary shifts that affect the quality (e.g., energy content) of food available to marine predators may impact their physiological state and consequently affect their fitness. Previously, we experimentally validated that deposition of the adrenocortical hormone, corticosterone, in feathers is a sensitive measure of nutritional stress in seabirds. Here, we use this method to examine how changes in diet composition and prey quality affect the nutritional status of free-living rhinoceros auklets (Cerorhinca monocerata). Our study sites included the following: Teuri Is. Japan, Middleton Is. central Gulf of Alaska, and St. Lazaria Is. Southeast Alaska. In 2012 and 2013, we collected “bill loads” delivered by parents to feed their chicks (n = 758) to document dietary changes. We deployed time–depth–temperature recorders on breeding adults (n = 47) to evaluate whether changes in prey coincided with changes in foraging behavior. We measured concentrations of corticosterone in fledgling (n = 71) and adult breeders' (n = 82) feathers to determine how birds were affected by foraging conditions. We found that seasonal changes in diet composition occurred on each colony, adults dove deeper and engaged in longer foraging bouts when capturing larger prey and that chicks had higher concentrations of corticosterone in their feathers when adults brought back smaller and/or lower energy prey. Corticosterone levels in feathers of fledglings (grown during the breeding season) and those in feathers of adult breeders (grown during the postbreeding season) were positively correlated, indicating possible carryover effects. These results suggest that seabirds might experience increased levels of nutritional stress associated with moderate dietary changes and that physiological responses to changes in prey composition should be considered when evaluating the effect of prey quality on marine predators.

气候变化与人为活动压力可能改变全球海洋饵料鱼类的群落组成与种群丰度。“垃圾食物假说（Junk-Food Hypothesis）”提出，海洋捕食者可获取食物的品质（如能量含量）因饮食结构改变发生变化，可能影响其生理状态，进而改变自身适合度。此前我们已通过实验证实：肾上腺皮质激素皮质酮（corticosterone）在羽毛中的沉积量，可作为海鸟营养应激的灵敏检测指标。本研究采用该方法，探究饮食组成与猎物品质的变化如何影响野生角海雀（Cerorhinca monocerata）的营养状态。本研究的采样位点包括：日本天卖岛（Teuri Is.）、阿拉斯加湾中部的米德尔顿岛（Middleton Is.），以及阿拉斯加东南部的圣拉扎里亚岛（St. Lazaria Is.）。2012年与2013年，我们收集了亲鸟投喂雏鸟的喙载猎物（bill loads）共758份，以记录饮食结构的变化。我们在47只繁殖期成鸟身上部署了时间-深度-温度记录仪，以判断猎物的变化是否与觅食行为的改变相契合。我们分别检测了71只雏鸟与82只繁殖成鸟的羽毛皮质酮浓度，以明确觅食环境对海鸟的影响。研究结果显示：各繁殖群落均存在饮食组成的季节性变化；成鸟在捕获体型更大的猎物时，会下潜更深且觅食时长更长；当亲鸟带回体型更小或能量更低的猎物时，雏鸟羽毛中的皮质酮浓度更高。繁殖期雏鸟羽毛与繁殖后换羽的成鸟羽毛中的皮质酮水平呈显著正相关，表明存在潜在的遗留效应（carryover effects）。上述结果表明：海鸟可能因饮食结构的适度改变面临更高水平的营养应激；在评估猎物品质对海洋捕食者的影响时，应考虑其对猎物组成变化的生理响应。