The paradox of the Birds-of-Paradise: persistent hybridization as a signature of historical reinforcement

The birds-of-paradise (Paradisaeidae) exhibit some of the most diverse color patterns and courtship displays among species. Paradoxically, birds-of-paradise hybridize more frequently than other birds, even hybridizing across species and genera with remarkably divergent color patterns. Hybridization among such distinctly colored species might suggest that reinforcement was unimportant for color pattern divergence because reinforcement favors trait divergence that reduces the likelihood of hybridization over time, and is expected to eliminate hybridization between species. Here I present an alternative view: that persistent but infrequent hybridization among species that differ markedly in prezygotic isolating traits, such as color pattern in birds, represents the signature of historical reinforcement, and occurs when (i) divergence in single traits can reduce, but not prevent, hybridization, (ii) trade-offs constrain the divergence of prezygotic isolating traits, and (iii) selection against hybrids is weak when hybrids are rare. Considering these factors, the paradox of the birds-of-paradise—where species with distinct prezygotic isolating traits are more likely to hybridize at low frequencies—is the expected outcome of reinforcement. Sexual selection by female choice could further intensify the effects of reinforcement, particularly if reinforcement directs sexual selection to different traits in hybridizing populations. This latter process could potentially explain the exceptional diversity of extravagant ornaments in the birds-of-paradise.