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Fatal Shift.

Imagine making the long journey to Northern Europe all the way from Africa, hunting around for a place to build a home in hopes of raising young ones, finding a nice cozy spot, and settling in, only to have the irate original occupant of the place return and slash you to death and proceed to eat your brains. That's the grim story playing out for many male pied flycatchers as our warming world shifts the onset and intensity of seasons across the world.

The pied flycatcher, a small migratory bird that spends winters in Africa and returns to Europe in spring to breed, is known to build its nest on top of the previously constructed nest of a resident woodland bird in Europe the great tit. The behavior, which has been dubbed "nest take-over," can sometimes lead to fatal conflicts when improperly executed. Pied flycatchers are often able to drive great tits away from their nesting sites by flying around them as they are attempting to settle into a nest. But, if a flycatcher manages to enter a nest that hasn't yet been abandoned, the bigger, stronger great tit will kill it and eat its brains.

While great tits typically breed two weeks earlier than pied flycatchers, there are some years when, due to seasonal variations, the breeding periods of both species overlap, and when that happens, the birds are in direct competition for resources, which includes not just nests, but also their key food source when they are raising their young--caterpillars.

Now, according a new study published in Current Biology in January, climate change is causing an uptick in the frequency of this overlap and leading to an increasing number of nest-seeking male flycatchers being killed by male great tits guarding their nests.

The study, by University of Groningen biologists, analyzed ten years' worth of data on flycatcher fatalities in great tit nest boxes in the Netherlands and found that flycatcher fatalities were higher in mild winter years.

Milder winters, one result of climate change, increase the survival rates of great tits, so the number of breeding great tits is higher in those years, explains Jelmer Samplonius, lead author of the study. More great tits mean fewer empty nests and, therefore, more competition for the flycatchers. "Both species need to time the birth of their young with a peak in the availability of caterpillars," Samplonius says. This peak is linked to the appearance of the first leaves on trees, and higher average temperatures mean that this period has shifted to earlier in the year.

While both birds are responding to this shift in seasonal patterns--greater tits by laying their eggs earlier during warmer winters or building their nests later during colder springs, and pied flycatchers by migrating to Europe earlier--the flycatcher's adaptation is not as good as that of the great tits.

The researchers found the biggest problems occur in colder springs, when tits start building their nests relatively late but the flycatchers still arrive early. "In this situation, the overlap in breeding time is greatest, and so is the number of conflicts," Samplonius says, noting that in some years great tits killed up to 10 percent of male flycatchers inside a nesting box in just two weeks of competition.

Interestingly, the researchers found that these murders didn't seem to have an impact on pied flycatcher populations, yet. That's because the males killed were usually those that arrived late in the season (and tried to make their way into tit-occupied nests) and often wouldn't have found females to breed with anyway. But, of course, that could change.

Caption: Migratory pied flycatchers are known to make use of the nests of great tits (pictured). Warming climates have led to greater overlap in the nesting seasons of the two birds, and as a result, to more frequent killing of flycatchers by great tits.
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Title Annotation:TEMPERATURE GAUGE; role of climate change increasing number of great tits killing migratory pied flycatchers
Publication:Earth Island Journal
Date:Mar 22, 2019
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