Essential amino acid requirements of granivorous and omnivorous songbirds and the provision of natural foods

Wild birds must consume certain amounts of protein and an appropriate balance of amino acids while inhabiting environments where foods often differ in the quantity and quality of available protein. The requirements for amino acids are well documented for domestic bird species but are largely unknown for wild birds, which makes it impossible to reliably assess the nutritional adequacy of foods eaten by wild birds. We measured the maintenance requirements for three essential amino acids (lysine, methionine, and arginine) in two species of songbird, the omnivorous Hermit Thrush (Catharus guttatus) and granivorous White-throated Sparrow (Zonotrichia albicollis). Hermit Thrushes and White-throated Sparrows had similar requirements for lysine (20.02 and 19.95 mg/day, respectively) and methionine (12.3 and 10.85 mg/day, respectively), whereas thrushes had lower requirements for arginine (18.07 mg/day) compared to sparrows (34.5 mg/day). Consistent with previous studies, most birds fed diets with inadequate essential amino acid concentrations reduced food intake and fecal output, lost body mass, and had lower, but not negative nitrogen balance. However, we provide the first evidence that songbirds overcompensate when they consume diets very deficient in lysine. Available data on amino acid concentrations in natural foods suggests that most insects contain relatively high concentrations of all essential amino acids, seeds likely satisfy requirements of lysine and arginine but not methionine for Hermit Thrushes and White-throated Sparrows, whereas fruits generally contain inadequate amounts of all essential amino acids. Therefore, birds that eat mostly fruit may consume enough protein but likely must eat other types of foods to satisfy their essential amino acid requirements.

野生鸟类在栖息过程中需摄入定量蛋白质与适当比例的氨基酸，而其所处环境中的可食用蛋白质的含量与品质往往存在差异。家养鸟类的氨基酸需求量已有充分研究记录，但野生鸟类的相关数据却极为匮乏，这使得学界无法可靠评估野生鸟类所摄食物的营养充足性。本研究测定了两种鸣禽的三种必需氨基酸（essential amino acids）——赖氨酸（lysine）、蛋氨酸（methionine）与精氨酸（arginine）的维持需要量，受试对象分别为杂食性的隐夜鸫（Catharus guttatus）与食谷性的白喉带鹀（Zonotrichia albicollis）。隐夜鸫与白喉带鹀对赖氨酸的需求量相近（分别为20.02 mg/日与19.95 mg/日），对蛋氨酸的需求量亦相近（分别为12.3 mg/日与10.85 mg/日）；但隐夜鸫的精氨酸需求量（18.07 mg/日）显著低于白喉带鹀（34.5 mg/日）。与既往研究结果一致，饲喂必需氨基酸浓度不足日粮的多数鸟类均出现了摄食量与粪便排出量减少、体重下降，且氮平衡降低但未出现负氮平衡的状况。但本研究首次发现，鸣禽在摄入赖氨酸极度缺乏的日粮时会出现过度代偿行为。现有天然食物氨基酸浓度数据显示，多数昆虫的所有必需氨基酸含量均相对较高；对于隐夜鸫与白喉带鹀而言，籽实可满足其赖氨酸与精氨酸的需求量，但无法提供足够的蛋氨酸；而多数水果的各类必需氨基酸含量普遍不足。因此，以果实为主要食物的鸟类虽可摄入足够的蛋白质，但往往需要通过进食其他类型的食物来满足自身的必需氨基酸需求。