Interspecific competition promotes habitat and morphological divergence in a secondary contact zone between two hybridizing songbirds

Abstract

Interspecific competition is assumed to play an important role in the ecological differentiation of species and speciation. However, empirical evidence for competition's role in speciation remains surprisingly scarce. Here, we studied the role of interspecific competition in the ecological differentiation and speciation of two closely related songbird species, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (Luscinia luscinia). Both species are insectivorous and ecologically very similar. They hybridize in a secondary contact zone, which is a mosaic of sites where both species co-occur (syntopy) and sites where only one species is present (allotopy). We analysed fine-scale habitat data for both species in both syntopic and allotopic sites and looked for associations between habitat use and bill morphology, which have been previously shown to be more divergent in sympatry than in allopatry. We found that the two nightingale species differ in habitat use in allotopic sites, where L. megarhynchos occurred in drier habitats and at slightly higher elevations, but not in syntopic sites. Birds from allotopic sites also showed higher interspecific divergence in relative bill size compared to birds from syntopic sites. Finally, we found an association between bill morphology and elevation. Our results are consistent with the view that interspecific competition in nightingales has resulted in partial habitat segregation in sympatry and that the habitat-specific food supply has in turn very likely led to bill size divergence. Such ecological divergence may enhance prezygotic as well as extrinsic postzygotic isolation and thus accelerate the completion of the speciation process.

Sperm divergence in a passerine contact zone: Indication of reinforcement at the gametic level

“…; Sottas et al. ). It is thus possible that postcopulatory isolation associated with sperm divergence represented an important barrier to gene flow in early phases of secondary contact when precopulatory barriers were relatively weak.…”

“…In nightingales, precopulatory isolation is currently relatively strong, which may reduce the importance of possible postcopulatory prezygotic isolation caused by sperm divergence. However, precopulatory isolation is to a large degree caused by segregation of species habitats, which has evolved only after secondary contact in response to interspecific competition Sottas et al 2018). It is thus possible that postcopulatory isolation associated with sperm divergence represented an important barrier to gene flow in early phases of secondary contact when precopulatory barriers were relatively weak.…”

“…; Sottas et al. ). Despite relatively strong assortative mating, the species occasionally hybridize in sympatry and produce viable hybrid progeny.…”

“…The sister species diverged 1.8 million years ago (Storchová et al 2010) and currently hybridize in a zone of secondary contact spanning Central and Eastern Europe (Becker 2007;Reifová et al 2011a). The species are very similar morphologically as well as ecologically (Reif et al 2015), although partial habitat segregation associated with bill size divergence has occurred after secondary contact in response to interspecific competition in sympatric populations Sottas et al 2018). Despite relatively strong assortative mating, the species occasionally hybridize in sympatry and produce viable hybrid progeny.…”

Multiple routes to interspecific territoriality in sister species of North American perching birds

“…2017; Kyogoku and Sota 2017; Sottas et al. 2018). Recent empirical and theoretical work has documented influences of interspecific territoriality on species coexistence and evolution in diverse taxa (reviewed in Grether et al.…”