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A male hybrid flycatcher with a typical intermediate plumage phenotype (note the broken collar). Female hybrids are sterile (they lay eggs, which never hatch) while male hybrids only suffer partial reduction in fitness owing to a disadvantage in competition over mating. Since females constitute the heterogametic sex in birds, Haldane's rule is followed, i.e. faster evolution of hybrid dysfunction in the heterogametic sex. Photo credit Johan Träff.
