Climate warming reduced crossbreeding between two songbird species

15 December 2017

The European pied flycatcher.

Global climate warming is a major threat to many living organisms. However, a recent study shows that two bird species react differently to warmer spring temperatures, which has reduced the negative effects of competition and crossbreeding. Both species must rush to adapt to climate change, but they are under less pressure to compete with each other.

Collared flycatchers are small passerine birds that are expanding their European breeding range towards the north in response to global climate warming. Approximately 50 years ago this species started to colonise Öland, a Swedish island in the Baltic Sea. Since then, it has expelled members of the closely related pied flycatcher from preferred habitats through competition over nest sites and crossbreeding.

Harmfull crossbreeding

Crossbreeding between the two species is harmful since hybrid individuals seem to be infertile. However, a new study published in Evolution by researchers at Uppsala University and the Finnish Museum of Natural History shows that global warming could spell some hope for pied flycatchers within the breeding range of collared flycatchers.

Global climate change has advanced the time of bud break and therefore the period that insects are most abundant. This affects many breeding bird species in the Northern Hemisphere that are reliant on caterpillars to feed their offspring. Both flycatcher species migrate all the way from Africa every year to feed on the nutritious caterpillars while breeding. The birds need to time the onset of breeding to match the peak in food abundance.

Earlier breeding

As springs get increasingly warmer, the birds need to breed earlier to ensure that there are enough caterpillars to feed their quickly growing offspring. However, the two species have reacted differently to rising spring temperatures: over the last 14 years, collared flycatchers have advanced their breeding more than pied flycatchers. This means that late breeding pied flycatchers experience a reduced risk of crossbreeding.

“Time is one barrier that reduces interbreeding and facilitates co-existence,” says Päivi Sirkiä, researcher at the Finnish Museum of Natural History and lead author of the study.

Diets could explain differences

So why do these closely related and apparently ecologically similar birds respond differently to the changes in their shared local environment? A likely explanation is provided by minor differences in diets. Pied flycatchers have a broader niche use, including a slightly more variable diet than collared flycatchers. This makes pied flycatchers less obliged to match their breeding with the advancement of the peak abundance of caterpillars.

The build-up of reproductive isolation between species typically takes very long time but the recent contact zone on Öland means that causes and consequences of crossbreeding can be studied in real time. The study was conducted by careful monitoring of more than 3,200 instances of flycatcher breeding, including determining how many nestlings were hatched to each breeding pair, and how many of these nestlings successfully fledged. This gave the researchers a good idea of the importance of earlier breeding for collared flycatchers: when they bred earlier, they successfully fledged more nestlings.

Facilitated cohabitation

The results of this study suggest that changing environments may sometimes facilitate cohabitation and thus potentially prevent extinction of very closely related species through different speed of adaptation to new conditions. Even minor differences in the response to environmental change of closely related, co-occurring species can quickly affect the level of reproductive isolation and patterns of competitive exclusion between species. This indicates the complexity of understanding the ongoing effects of climate change on biodiversity.

Full article: Päivi M. Sirkiä , S. Eryn McFarlane, William Jones, David Wheatcroft, Murielle Ålund, Jakub Rybinski, and Anna Qvarnström: Climate-driven build-up of temporal isolation within a recently formed avian hybrid zone. Evolution. https://doi.org/10.1111/evo.13404