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Adulterous flycatchers on the decline

Tara Stehelin spent the past decade watching birds cheat. The Yukon behavioural ecologist started off studying the function of bird song. She knew birds sing to attract mates.

by Genesee Keevil

Tara Stehelin spent the past decade watching birds cheat.

The Yukon behavioural ecologist started off studying the function of bird song. She knew birds sing to attract mates. Males also sing to show off their strength, letting other males know they better not come near their territory.

But the ruckus at dawn remained an enigma. Why were Yukon birds putting so much effort into singing at 2:45 a.m.? Stehelin spent a whole summer getting up in the wee hours of the morning to find out.

That’s how she discovered the world of avian infidelity. Birds are socially monogamous, but females often flit around finding more than one sire for all the eggs in their nest. And most of these affairs happen near sunrise.

“It’s the only time males have a chance to sing to attract extra mates,” she said. “Because once his female is up and active, he spends the whole day following her around, guarding her to make sure she doesn’t cheat.”

It took years to figure this out, said Stehelin. The research also revealed something much more serious than the odd olive-sided flycatcher fling.

Stehelin noticed her philandering subjects were disappearing - fast. She was studying flycatchers and western wood-peewees, and both species were in decline.

Over the past four decades, 70 percent of olive-sided flycatchers disappeared. The wood-peewees are not far behind. “It’s inexcusable that these species are disappearing right under our nose,” said Stehelin. “And it’s abhorrent that it took so long to get onto our radar.”

Flycatchers aren’t flashy. A medium-size bird decked out in beige, grey and brown, they didn’t attract a lot of scientific attention. Wood-peewees are a bit smaller, with equally mundane colour schemes.

But flycatchers have one thing going for them. They can really belt out a song.

They overwinter in the jungles of South America. For their voice to carry through all that lush tropical vegetation, it has to be loud.

In a boreal environment, where sound carries relatively easily, flycatchers can be heard more than half a kilometre away. But Stehelin is hearing less of them.

When she first started studying these oft-overlooked flyers, flycatchers were disappearing at a rate of one to two per cent a year. Now, they’re dropping at roughly four per cent a year.

No one knows why, though Stehelin suspects it has to do with disruptions in South America. Deforestation, mining processes involving mercury, and pesticides are likely all taking a toll. To figure out exactly what’s plaguing the flycatchers, Stehelin needs to know exactly where in South America they overwinter.

Tracking these tiny flyers with bulky radio transmitters, even satellite collars, like those used on caribou, is not feasible. But geo-locators come in all shapes and sizes now, including flat ones that fit on flycatchers like tiny backpacks.

Made in the U.K., these wee, electronic light-sensors can pinpoint a bird’s location to within 100 metres using only sunrise and sunset data.

Trick is, Stehelin needs to get the sensor back to collect the data.

First, she has to find flycatchers, which is becoming tougher and tougher. They usually return to the Yukon mid-May, and settle into a territory by the end of the month. That’s when she starts getting up extraordinarily early, wandering the mountains and lakes around Whitehorse listening for that loud telltale song. “Flycatchers tend to come back to the same territory every year,” she said. “So I have an idea of where to find some pairs.”

Once she’s found her samples, Stehelin will set up low mist nets, and hit play on a recording of a male flycatcher to trick her male, hoping he unwittingly flies into the net in an attempt to fend off what he thinks is another male trying to attract his mate. “Sometimes they fly over the net, sometimes under it,” she said.

Sometimes Stehelin gets lucky.

She hopes to get light sensors on eight to 10 flycatchers this season, working the little backpack straps under the feathers, so the birds won’t preen them off. She’s picked a small sample in part to avoid too many invasive studies, and because the sensors are $250 a pop.

Next summer, to get the data off the sensors, Stehelin has to find the flycatchers wearing the backpacks, then trick them into flying into a mist net again. “It’s not quite as hard as it sounds,” she said with a laugh. “Because they are site faithful.”

Stehelin wants to sample wood-peewees too, but they’re a little smaller, and she’s worried the harnesses are too big. “Technology is changing so fast,” she said. “By next year, they’re already promising to have a smaller sensor, so I’ll likely wait until then.”

The sensors will give Stehelin a better idea of what hazards flycatchers are facing in South America, but she’s worried deforestation, mining, and pesticides are only part of the picture.

“I think climate change might also be involved,” said Stehelin. Earlier, warmer springs mean bugs arrive earlier. “The flycatchers used to take advantage of this huge influx of insects,” she said. Now, with insects peaking sooner, she suspects the flycatchers don’t arrive in time for this bug buffet.

Already late breeders, this loss of protein could have a huge impact on the success rates of fledgling flycatchers, said Stehelin.

This summer, she’s setting up insect traps to get a sense of when the burst of bugs occurs. “We don’t have a lot of historical data on this,” she said.

The mysterious disappearance of bees and wasps, a staple of many birds’ diets, is also likely taking its toll on flycatchers and peewees.

Wood peewees aren’t declining as dramatically as flycatchers, but if they continue to disappear at their current rate, 60 per cent of them will be gone in the next few decades.

“It’s inexcusable that we are allowing this to happen,” said Stehelin.

It took 95 per cent of rusty blackbirds disappearing before huge conservation efforts were launched. “We tend to manage species in crisis mode,” she said. “We wait until they’re critically endangered.”

Stehelin doesn’t want to wait until flycatchers and peewees reach these crucial proportions.

“This flycatcher project will go on forever,” she said. “Because I don’t plan on stopping.”

If anyone hears an olive-sided flycatcher singing after the end of May, please contact Stehelin at

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