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than in allopatry (Saetre et al., 1997). Results were mixed, however, in a recent analysis of vocal divergence in this system (Haavie et al., 2004). In sympatry, songs of pied flycatchers Ficedula hypoleuca have converged on those of collared flycatchers Ficedula albicollis because of heterospecific copying and singing of mixed songs. Although collared flycatcher songs in the zone of contact have diverged away from pied flycatcher songs typical of allopatric populations, the net effect of these changes is greater song similarity in sympatry.

Allopatric divergence of songs among suboscines and other birds in which differences in song are genetically determined may evolve more slowly but should also contribute to reproductive isolation. In a large comparative analysis of antbirds (Thamnophilidae), Seddon (2005) found evidence of vocal divergence both as a correlated effect of morphological evolution and as a response to habitat-dependent selection on signal transmission. In addition, among trios of closely related species, sympatric forms exhibited striking vocal divergence in comparison to allopatric taxa, providing strong evidence for reproductive character displacement and the role of song in reproductive isolation. Despite divergence of song in allopatry, individuals of different species may recognize each other as potential mates upon secondary contact, leading to hybridization. If song differences are genetically determined, hybrid offspring may have intermediate songs (de Kort et al., 2002).

Given divergence in vocalizations, the development of song preferences through sexual imprinting may contribute to reproductive isolation even without genetic evolution of female preferences (Irwin and Price, 1999; Price, 1998). In Darwin’s finches, for example, cultural inheritance clearly plays a greater role than bill morphology in determining songs and song preferences (Grant and Grant, 1996, 1997) and is critical in promoting reproductive isolation after secondary contact of populations that have diverged in feeding adaptations in allopatry (Grant and Grant, 2002). Ecological selection against hybrid individuals also helped maintain species boundaries, at least before changes associated with El Niño events in the 1980s (Grant and Grant, 1998). In the past 20 years, however, increased fitness of hybrids has resulted in substantial genetic introgression from Geospiza fortis to Geospiza scandens on Daphne Major (Grant et al., 2004). Although learned songs and song preferences are strong determinants of pair formation in these species, reproductive isolation is imperfect because of constraints on mate choice imposed by asymmetries in population size and operational sex ratios as well as infrequent cases of individuals misimprinting on heterospecific songs (Grant and Grant, 1996, 1997, 1998).

Song learning and sexual imprinting explain the recent diversification of brood parasitic indigobirds (genus Vidua), the best and perhaps

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