Pied flycatchers are turning up dead in the nests of great tits, and new research suggests global warming is to blame for the interspecies violence.

Pied flycatchers and great tits are natural rivals. The two bird species compete for similar nesting sites, and both rely on caterpillars, an abundant but short-lived food source, to feed their young.

Having spent years studying the two species, researcher Jelmer Samplonius began to notice some years hosted dozens of flycatcher deaths, while other years were nearly free of fatalities.

In an attempt to better understand the fluctuating violence, researchers surveyed the number of pied flycatcher fatalities observed in great tit nest boxes in the Netherlands between 2007 and 2016. The data showed that during the deadliest years, great tits kill 10 percent of pied flycatchers.

The new analysis showed climate change affected the two species differently, resulting in a greater synchronicity of the two birds' behavioral patterns, which explains an increase in conflicts.

As Europe's winters have warmed, great tits have begun breeding earlier. Pied flycatchers haven't adjusted, as they spend much of the year in Africa.

During mild winters, great tits proliferate, occupying more territory and nest boxes. When pied flycatchers arrive after their migration from Africa, they're faced with less space and fewer resources.

"When pied flycatchers and great tits are more synchronous in their timing, this leads to a higher level of conflict over nesting sites," researcher Jelmer Samplonius said in a news release.

So far, the violence doesn't seem to be affecting the two species population dynamics, which suggest the fatalities are mostly impacting surplus males.

But surplus males aren't surplus every year. When populations shrink, extra males can help numbers rebound. While pied flycatchers are currently buffered by healthy population numbers, they may not always be.

"If buffers are diminished, population consequences of interspecific competition may become [more] apparent, especially after warm winters that are benign to resident species," researchers wrote in their new paper on the subject -- published this week in the journal Current Biology.

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