A female swallow needs a husband who will help look after her young, but by the time she arrives at the breeding site, she might find all the best husbands taken. Her best tactic is therefore to mate with a mediocre husband or a husband with a good territory and have an affair with a genetically superior neighbor. This theory is supported by the facts: Females always choose more dominant, older, or more “attractive” (that is, ornamented) lovers than their husbands; they do not have affairs with bachelors (presumably rejects) but with other females’ husbands; and they sometimes incite
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