When feather-pecking behaviour by hens becomes repetitive, plumage damage often results for the recipient of the pecks. The forceful removal of feathers and vigorous pecks directed at the skin may also cause pain, fear and even wounds. ‘Outbreaks’ of pecking behaviour have been reported in all housing systems in which poultry are managed. Pecking may progress to cannibalism and death, thus constituting significant hen welfare and farm economy problems. Farmers apply preventative management practices to minimise the risk of outbreaks. However, outbreaks are unpredictable and, once in progress, are difficult to control, especially in non-cage housing systems. For more than a century, research has been directed at trying to identify the causal factors underlying this problem, without success. The problem is multi-factorial and different studies often identify contradictory findings, such as, for example, in relation to the effects of adding forage to increase environmental enrichment, among others. The present review aims to provide background information about severe feather-pecking behaviour in laying hens, with mention of the resultant issues from repeated performance, such as, for example, on feather cover over the life of the laying hen. On-farm surveys, epidemiological studies and experimental trials have generated much information that has improved our general understanding of the significance of the problem, even though studies have typically been inconclusive due to its multi-factorial causes. While ‘Good Practice Guides’ are available and provide relevant advice for farmers to manage flocks to minimise the risk of outbreaks, we suggest significant progress towards identifying the root-cause(s) of the problem will more likely be achieved through controlled experimental trials using research models than through survey approaches. For example, using a stress-induction model, researchers should first focus on the impact of cumulative stressors in the flock that seem to predispose a hen to either become a feather pecker, or be the victim of pecking. Subsequent research should then investigate the affected hens for altered behavioural or (neuro-) physiological states, or physical stimuli on the skin and feathers, that may increase the motivation of hens to become feather peckers.

中文翻译：

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蛋鸡啄羽的原因和随后的福利问题：综述

当母鸡的啄羽行为变得重复时，啄羽的接受者通常会损坏羽毛。强行去除羽毛和剧烈啄皮肤也可能导致疼痛、恐惧甚至伤口。在管理家禽的所有鸡舍系统中都报告了啄食行为的“爆发”。啄食可能发展为自相残杀和死亡，从而构成严重的母鸡福利和农场经济问题。农民采用预防性管理措施，以尽量减少爆发的风险。然而，爆发是不可预测的，一旦发生就难以控制，尤其是在非笼养系统中。一个多世纪以来，研究一直致力于试图确定这个问题背后的因果因素，但没有成功。问题是多因素的，不同的研究经常发现相互矛盾的发现，例如，关于添加草料以增加环境丰富度的影响等。本综述旨在提供有关蛋鸡严重啄羽行为的背景信息，并提及重复性能所导致的问题，例如蛋鸡一生中的羽毛覆盖。农场调查、流行病学研究和实验试验产生了许多信息，提高了我们对问题重要性的总体理解，尽管由于其多因素原因，研究通常没有定论。虽然可以使用“良好实践指南”并为农民提供相关建议以管理鸡群以最大程度地降低爆发风险，我们建议，通过使用研究模型的受控实验试验，而不是通过调查方法，更有可能在确定问题根本原因方面取得重大进展。例如，使用压力诱导模型，研究人员应首先关注鸡群中累积压力源的影响，这些压力源似乎使母鸡容易成为啄羽者，或成为啄羽的受害者。随后的研究应该调查受影响的母鸡的行为或（神经）生理状态的改变，或皮肤和羽毛上的物理刺激，这可能会增加母鸡成为啄羽的动机。