Climate change is having devastating effects on our environment, from rising sea levels to severe weather. Here's one effect that scientists didn't anticipate, however: murderous, brain-eating birds.

Great tits (Parus major) are adorable, beautifully-adorned birds common across Europe, North Africa and Central Asia. They typically build their nests in tree cavities in the spring, and once their young have fledged, they abandon their nests and go about their merry way.

This is a convenient pattern for another bird, the pied flycatcher (Ficedula hypoleuca), which migrates from Africa for the summer. Flycatchers are keen on taking over the abandoned nests of great tits; it certainly beats building their own nests, and after a long migration it's nice to have a home ready-made and waiting for you.

Unfortunately, though, climate change is causing the nesting times of these two species to overlap. So when flycatchers come knocking, they find that many of the nests are still occupied, and that doesn't sit well with the territorial, and much larger, great tits.

And apparently, the one thing you don't want to mess with is a nesting great tit mama.

"When a flycatcher enters a box with a great tit inside, it doesn't stand a chance," said biologist Jelmer Samplonius from the University of Groningen, in a press release. "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."

Samplonius is one of the first to recognize this intensifying "war of the birds," and has conducted the first study detailing some of the gruesome behavior that ensues. Not only do the great tits make short work of the unsuspecting flycatchers that wander into their nests, but they're also developing a taste for their brains.

"The dead flycatchers were all found in active tit nests and had severe head wounds, and often their brains had been eaten by the tits," wrote researchers in a paper recently published in Current Biology.

Often, the mutilated bodies of the flycatchers are found inside the nests of great tits while the birds are still roosting. It's got to be an odd sight for the chicks to see after they hatch, to be faced with the grisly reality of what their parents had been up to.

The truth is, though, that great tits and flycatchers have a long history of confrontation that climate change has only recently escalated. When not caught by surprise inside an occupied great tit nest, the more agile flycatchers are known to pester and annoy great tits by swooping and picking at them in the air. This often causes great tits to abandon territories in frustration.

So when flycatchers wander unsuspecting into an occupied nest, it's perhaps understandable that the great tits have been relishing the opportunity to fight back.

The good news is that this war doesn't seem to be having a negative impact on the populations of either bird β€” not yet anyway. Researchers haven't noticed a drop-off, but they're worried that climate change is only going to make the situation worse over time.

"We expect that [as the situation worsens], population consequences of interspecific competition may become apparent," wrote the researchers.

Climate change is turning these cute birds into crazed murderers
Great tits may look harmless, but climate change is changing their nesting patterns and flycatchers are running into them more often.