The uropygial gland is a holocrine complex exclusive to birds that produces an oleaginous secretion (preen oil) whose function is still debated. Herein, I examine critically the evidence for the many hypotheses of potential functions of this gland. The main conclusion is that our understanding of this gland is still in its infancy. Even for functions that are considered valid by most researchers, real evidence is scarce. Although it seems clear that preen oil contributes to plumage maintenance, we do not know whether this is due to a role in reducing mechanical abrasion or in reducing feather degradation by keratinophilic organisms. Evidence for a function against pathogenic bacteria is mixed, as preen oil has been demonstrated to act against bacteria in vitro, but not in vivo. Nor is it clear whether preen oil can combat pathogenic bacteria on eggshells to improve hatching success. Studies on the effect of preen oil against dermatophytes are very scarce and there is no evidence of a function against chewing lice. It seems clear, however, that preen oil improves waterproofing, but it is unclear whether this acts by creating a hydrophobic layer or simply by improving plumage structure. Several hypotheses proposed for the function of preen oil have been poorly studied, such as reduction of drag in flight. Similarly, we do not know whether preen oil functions as repellent against predators or parasites, makes birds unpalatable, or functions to camouflage birds with ambient odours. On the other hand, a growing body of work shows the important implications of volatiles in preen oil with regard to social communication in birds. Moreover, preen oil clearly alters plumage colouration. Finally, studies examining the impact of preen oil on fitness are lacking, and the costs or limitations of preen‐oil production also remain poorly known. The uropygial gland appears to have several non‐mutually exclusive functions in birds, and thus is likely to be subject to several selective pressures. Therefore, future studies should consider how the inevitable trade‐offs among different functions drive the evolution of uropygial gland secretions.

中文翻译：

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Preen 油和鸟类健康：对证据的批判性审查

尿尾腺是鸟类独有的全分泌复合物，可产生含油的分泌物（preen oil），其功能仍有争议。在此，我批判性地检查了有关该腺体潜在功能的许多假设的证据。主要结论是我们对这个腺体的了解仍处于起步阶段。即使对于大多数研究人员认为有效的函数，真正的证据也很少。尽管似乎很明显，栲油有助于羽毛维护，但我们不知道这是否是由于减少机械磨损或减少角蛋白生物对羽毛降解的作用。对抗病原菌的功能的证据是混合的，因为已经证明芫荽油在体外对细菌起作用，但在体内无效。也不清楚荠菜油是否可以对抗蛋壳上的病原菌以提高孵化成功率。关于 preen 油对皮肤癣菌的影响的研究非常稀少，并且没有证据表明它具有抗咀嚼虱子的功能。然而，似乎很清楚，美化油可以提高防水性，但尚不清楚这是通过形成疏水层还是仅仅通过改善羽毛结构来起作用。一些关于美化油功能的假设没有得到很好的研究，例如减少飞行阻力。同样，我们不知道昙花油是否具有抵御捕食者或寄生虫的作用，是否使鸟类变得不好吃，或者是否可以用环境气味来伪装鸟类。另一方面，越来越多的研究表明，油中挥发物对鸟类社会交往的重要影响。此外，美化油明显改变了羽毛的颜色。最后，缺乏检验油对健康影响的研究，而且油生产的成本或局限性也知之甚少。尾骨在鸟类中似乎具有多种非相互排斥的功能，因此可能会受到多种选择压力的影响。因此，未来的研究应该考虑不同功能之间不可避免的权衡如何驱动泌尿系分泌物的进化。尾骨在鸟类中似乎具有多种非相互排斥的功能，因此可能会受到多种选择压力的影响。因此，未来的研究应该考虑不同功能之间不可避免的权衡如何驱动泌尿系分泌物的进化。尾骨在鸟类中似乎具有多种非相互排斥的功能，因此可能会受到多种选择压力的影响。因此，未来的研究应该考虑不同功能之间不可避免的权衡如何驱动泌尿系分泌物的进化。