Who Wins in a Fight Between Bluebirds and Swallows?

“Mi casa es su casa” doesn't quite fly in the bird world. Competition for the best territories can be fierce—to the point where some species avoid it all together. 

The Mountain Bluebird is one dazzling example. Part of the population breeds in the grasslands of British Columbia, arriving early in March when the nest boxes are still unoccupied. They tend to beat out their co-habitants, the Tree Swallows, by nearly two weeks.

But Mountain Bluebirds may soon be at a disadvantage, according to new research published today in The Auk: Ornithological Advances. As climate change tampers with the timing of the seasons, Tree Swallows are migrating sooner than expected, prompting clashes with their cerulean rivals.  

Karen Wiebe, an ornithologist at the University of Saskatchewan and lead author on the study, spent the last two years studying how bluebirds and swallows compete for breeding and nesting spaces. Wiebe set up an experiment where she placed identical plywood nesting boxes on fence posts near Riske Creek and Williams Lake in central British Columbia. By observing the interactions between neighbors, she learned that the species that gets first dibs generally wins.

“When one pair of bluebirds and a pair of swallows were living next to each other, I would randomly block the door on one of the boxes. I then checked the second and only available box periodically. Whichever species laid eggs first was considered the winner,” Wiebe explains. “Mountain Bluebirds were able to successfully defend [their nest boxes] 77 percent of the times against the swallows. They get a clear advantage from previous ownership.” For Mountain Bluebirds, laying eggs first is a priority. Wiebe’s observations show they beat out the swallows by at least seven days.

She then conducted another experiment, in which both pairs of birds were already in the process of building nests. Soon after, she removed both boxes and put a third empty box at a different pole. In this case, “Tree Swallows won between 65 and 70 percent of the times they were competing for a brand new box,” Wiebe says.

Both of these species have been classified as climate threatened by Audubon’s Birds and Climate Change Report. Meanwhile, a study published in 1999 documented the possibility of swallows shifting their migrations in response to global warming. But newer research and more up-to-date references are needed to understand if that trend is still relevant, says Chad Wilsey, the director of conservation science at Audubon.

“Unfortunately, our climate change models don’t measure the timing of specific ecological events (such as migration)," Wilsey says. "The paper has interesting observations on how much better defenders than competitors certain birds can be, but more data is needed to see if climate change could be playing a role here."