Pied and Collared Flycatchers adjust their breeding time differently to advancing springs, possibly leading to temporal isolation.
When you study speciation, there is no escaping from reproductive isolation. In order to understand how new species arise, you need to know what prevents them from interbreeding. There are several reproductive isolation mechanisms (see here for an overview), but in this post I will focus on temporal isolation. The rationale behind this particular mechanism is straightforward: members of different species cannot interbreed, because they reproduce at different times.
Classic cases of temporal isolation are plant species that flower at different times. A nice example in birds concerns the Madeiran Storm Petrel (Oceanodroma castro) which breeds on the Azores, Portugal. On some islands there are distinct populations that breed four to five months apart. These populations are diverging morphologically, they are probably on their way to become different species.
A similar process might be unfolding on the Swedish Island of Öland where Collared Flycatcher (Ficedula albicollis) and Pied Flycatcher (F. hypoleuca) coexist. Päivi Sirkiä and her colleagues found that both species have advanced their timing of breeding in response to increasing spring temperatures. However, Pied Flycatchers showed a slower response compared to Collared Flycatchers. So, both species are now breeding at slightly different times. Is this the build-up of temporal isolation?
The change in breeding times is driven by increasing spring temperatures that cause earlier development of leaves on deciduous trees and a consequent peak in caterpillar larvae. Intriguingly, the difference in breeding times between the two Flycatcher species is only apparent in low quality habitats. The researchers explain this result by fact that Pied Flycatchers have a broader diet than Collared Flycatchers, “making them less obliged to match their onset of breeding with the climate-driven seasonal advancement of the peak of larval abundance.”
Even if temporal isolation only arises in low quality habitat, it might contribute to selection against hybridization (a process known as reinforcement). Because Flycatcher hybrids suffer from fertility problems, it makes sense to avoid hybridizing. In the end, climate-driven temporal isolation might lead to less hybrids.
Monteiro, L.R. & Furness, R.W. (1998) Speciation through temporal segregation of Madeiran storm petrel (Oceanodroma castro) populations in the Azores? Philosophical Transactions of the Royal Society B, 353(1371): 945-953.
Sirkiä, P.M., McFarlane, S.E., Jones, W., Wheatcroft, D., Ålund, M., Rybinski, J. & Qvarnström, A. (2018) Climate‐driven build‐up of temporal isolation within a recently formed avian hybrid zone. Evolution, 72(2): 363-374.
This paper has been added to the Muscicapidae page.