How the widowbird got its absurdly long tail

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One glaring difficulty for early evolutionary biologists was the evolution of exaggerated male secondary sexual traits that would hinder the survival of males. Darwin was the first to suggest that sexual selection favoring an exaggerated trait could 'override' natural selection opposing the same trait, as long as the reproductive benefits outweighed the survival costs associated with it. During the modern synthesis, Ronald Fisher proposed a mechanism of ornament evolution wherein female preferences for an initially advantageous male trait drove that trait to an extreme where it no longer conferred a survival advantage. But little empirical evidence existed even into the early 1980s.  Andersson's work on long-tailed widowbirds provided the first experimental evidence of female preference for an extreme male ornament. This beautifully designed manipulation study was able to control for the confounding influence of variation in male territory quality, the experimental manipulation, and male behavior.

• The original paper

And a later paper summarizing subsequent work on widowbirds