Data from: Climate-driven build-up of temporal isolation within a recently formed avian hybrid zone

Divergence in the onset of reproduction can act as an important source of
reproductive isolation (i.e. allochronic isolation) between co-occurring
young species, but evidence for the evolutionary processes leading to such
divergence is often indirect. While advancing spring seasons strongly
affect the onset of reproduction in many taxa, it remains largely
unexplored whether contemporary spring advancement directly affects
allochronic isolation between young species. We examined how increasing
spring temperatures affected onset of reproduction and thereby
hybridization between pied and collared flycatchers (Ficedula spp.) across
habitat types in a young secondary contact zone. We found that both
species have advanced their timing of breeding in 14 years. However,
selection on pied flycatchers to breed earlier was weaker, resulting in a
slower response to advancing springs compared to collared flycatchers and
thereby build-up of allochronic isolation between the species. We argue
that a pre-adaptation to a broader niche use (diet) of pied flycatchers
explains the slower response to raising spring temperature, but that
reduced risk to hybridize may contribute to further divergence in the
onset of breeding in the future. Our results show that minor differences
in the response to environmental change of co-occurring closely related
species can quickly cause allochronic isolation.