Essential amino acid requirements of granivorous and omnivorous songbirds and the provision of natural foods
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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.
野生鸟类在栖息环境中，可获取食物的蛋白质含量与质量往往存在差异，因此必须摄入足量蛋白质并维持氨基酸的适宜配比。目前家禽的氨基酸需求已有充分研究记录，但野生鸟类的相关数据仍极为匮乏，这使得我们无法可靠评估野生鸟类所摄食物的营养充足性。本研究针对两种鸣禽——杂食性赫氏鸫（Catharus guttatus）与食谷性白喉带鹀（Zonotrichia albicollis）——的三种必需氨基酸（赖氨酸、蛋氨酸、精氨酸）维持需求量进行了测定。结果显示，赫氏鸫与白喉带鹀对赖氨酸的需求量相近（分别为20.02 mg/天与19.95 mg/天），对蛋氨酸的需求量亦相仿（分别为12.3 mg/天与10.85 mg/天）；但赫氏鸫对精氨酸的需求量（18.07 mg/天）显著低于白喉带鹀（34.5 mg/天）。与既往研究结果一致，多数饲喂必需氨基酸浓度不足日粮的鸟类会出现摄食量与粪便排泄量减少、体重减轻，以及氮水平降低但未出现负氮平衡的现象。然而本研究首次发现，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，多数昆虫的各类必需氨基酸含量均相对较高；对于赫氏鸫与白喉带鹀而言，种子类食物可满足其赖氨酸与精氨酸的需求，但无法满足蛋氨酸需求；而果实类食物普遍缺乏各类必需氨基酸。因此，以果实为主要食物来源的鸟类虽可能摄入足量蛋白质，但通常需要辅以其他类型的食物，才能满足自身的必需氨基酸需求。
