Positive Feedback Between Ecological and Reproductive Character Displacement in a Young Avian Hybrid Zone

Abstract

Character displacement can reduce costly interspecific interactions between young species. We investigated the mechanisms behind divergence in three key traits-breeding habitat choice, timing of breeding, and plumage coloration-in Ficedula flycatchers. We found that male pied flycatchers became expelled from the preferred deciduous habitat into mixed forest as the superior competitor, collared flycatchers, increased in numbers. The peak in food abundance differs between habitats, and the spatial segregation was paralleled by an increased divergence in timing of breeding between the two species. Male pied flycatchers vary from brown to black with brown coloration being more frequent in sympatry with collared flycatchers, a pattern often proposed to result from selection against hybridization, that is, reinforcement. In contrast to this view, we show that brown male pied flycatchers more often hybridize than black males. Male pied flycatcher plumage coloration influenced the territory obtained in areas of co-occurrence with collared flycatchers, and brown male pied flycatchers experienced higher relative fitness than black males when faced with heterospecific competition. We suggest that allopatric divergence in resource defense ability causes a feedback loop at secondary contact where male pied flycatchers with the most divergent strategy compared to collared flycatchers are favored by selection.

Learning the Hard Way: Imprinting Can Enhance Enforced Shifts in Habitat Choice

“…Coniferous habitats are therefore unlikely to be the originally preferred habitat of pied flycatchers and in the young hybrid zone onÖland competition with collared flycatchers appears to be responsible for a rapid shift in habitat occupancy [36]. Our present study provides a mechanism by which a competition-mediated switch into a suboptimal habitat could result in a changed habitat choice through learning.…”

“…Pied flycatchers are currently being rapidly excluded from their preferred deciduous habitats onÖland and are instead increasingly found breeding in mixed or coniferous forests [32,36]. We tested if the breeding habitat of male pied flycatchers influenced the risk of hybridizing and found that male pied flycatchers were more likely to pair with a heterospecific female in deciduous habitats, that is, the observed shift in habitat occupancy increases premating isolation between the species.…”

“…Pied flycatchers prefer to breed in deciduous forests [29] but are often found in coniferous forests onÖland [35]. The proportion of deciduous trees in pied flycatcher territories has declined in recent years as the number of collared flycatchers has increased [36] suggesting that competitionmediated habitat segregation is currently taking place. Here, we investigate potential mechanisms enhancing divergent habitat choice in these two ecologically similar Ficedula flycatcher species.…”

Asymmetric dominance and asymmetric mate choice oppose premating isolation after allopatric divergence

“…2009). In contrast to the above examples, asymmetries in competitive abilities can also launch processes which promote assortative mating, for example when asymmetric dominance relationships between taxa underlie their segregation into different habitats (Vallin and Qvarnström 2011; Vallin et al., 2012b; Winkelmann et al. 2014).…”

Climate adaptation and speciation: particular focus on reproductive barriers inFicedulaflycatchers

“…The still ongoing ecological character displacements in the form of timing of breeding and habitat use (Vallin et al. ) have direct effects on prezygotic reproductive isolation between the two species (Fig. , manuscript in prep ).…”