Egg recognition is a variable but common anti‐parasitism defense among different species of birds with brood parasites. In contrast, nestling recognition is rare. Very few studies have found nestling recognition in brood parasite hosts and determined the rejection mechanism behind this behavior. Hosts may use the number of hatchling down‐feathers to reject parasite nestlings. We tested whether hatchling down‐feathers is a visual cue for the red‐rumped swallow, a host that can recognize and reject parasite nestlings. Our results indicated that red‐rumped swallows do not recognize foreign nestlings based on hatchling down‐feathers. The closed nest structure and hatchling morph may explain the absence of such a mechanism. None of the rejection mechanisms found in previous studies could explain the nestling recognition in swallows. Olfactory cues, tactile cues, or other visual cues, except for single nestling or hatchling down‐feathers, may provide nestling recognition in red‐rumped swallows. More study is needed to evaluate these possibilities.

中文翻译：

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燕子（Hirundo daurica）是否使用孵化羽绒的视觉提示来区分寄生虫外来雏鸟？

卵的识别是不同种类的具有巢寄生物的鸟类之间可变但常见的抗寄生虫防御。相反，雏鸟识别很少见。很少有研究在亲代寄生虫宿主中发现雏鸟识别并确定这种行为背后的排斥机制。寄主可以使用孵化的羽绒数量来拒绝寄生的雏鸟。我们测试了孵化羽绒是否是红腰燕子（可以识别和拒绝寄生虫雏鸟的宿主）的视觉提示。我们的结果表明，红腰燕子不能识别出基于孵化羽绒的外来雏鸟。封闭的巢状结构和孵化变体可以解释这种机制的缺乏。先前研究中发现的排斥机制均不能解释燕子中的雏鸟识别。嗅觉提示，除了单个雏鸟或孵化的羽绒外，触觉线索或其他视觉线索可能会在红腰燕子中提供雏鸟识别。需要更多的研究来评估这些可能性。