Climate change is causing more mating season aggression among birds

Researchers at the University of Groningen (UG) have discovered that climate change is intensifying feuds between great tits and pied flycatchers during the breeding season. The study revealed that, in some years, great tits killed as many as 10 percent of the male pied flycatchers.

Great tits may just seem like funny, fluffy birds, but biologist Jelmer Samplonius explained that they can become very aggressive during the breeding season. Samplonius observed great tits and pied flycatchers for his Ph.D. thesis at the University of Groningen.

As he checked the nest boxes used in his field studies, Samplonius regularly found a dead flycatcher in a box occupied by great tits. He said that when a flycatcher enters a box with a great tit inside, it does not stand a chance.

“The great tit is heavier, as the flycatchers are built for a long migration from Europe to Western Africa and back. Also, great tits have very strong claws,” said Samplonius. In this case, the dead flycatchers usually had fatal head wounds, “and it appears that the great tits then eat the brains.”

The birds have always competed for nesting locations. “Pied flycatchers try to steal nesting facilities from the tits. They may be no match when fighting inside the nesting boxes, but they are more agile flyers.”

Samplonius collaborated with his Ph.D. supervisor Christiaan Both to investigate whether climate change has any effect on the clashes between the two species.

“Both species need to time the birth of their young with a peak in the availability of caterpillars,” said Samplonius. This peak coincides with the first appearance of leaves, and warming temperatures are causing the event to occur earlier in the year.

“Pied flycatchers now migrate to Europe earlier, but their adaptation is not as good as that of the great tits. Their earlier arrival is not linked to the actual temperature at their breeding grounds,” said Samplonius. The team discovered this after documenting the arrival of flycatchers over ten years in two national parks.

The milder winters that result from climate change increase the survival rate of great tits, so there are more breeding birds, leading to more competition and conflict.

A second reason for the increased competition is that great tits and pied flycatchers have adapted to climate change differently. The biggest clashes happen in colder springs, when tits start building their nests relatively late and the flycatchers arrive early.

“Interestingly, we didn’t see an effect on the overall flycatcher population of about 300 breeding pairs in our study,” said Samplonius. “We noted that the males killed were usually those who arrived late in the season. These late birds quite often don’t find a female to breed with, so that may explain why this behavior has no impact on the population.”

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer

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