by Patricia Robertson
From late May to early July last summer, Tara Stehelin stood in the predawn hours on the hills overlooking Fish Lake, holding a digital recorder and shivering.
Stehelin was there to record the dawn singing of dusky flycatchers, members of a larger group of mostly tropical birds whose singing patterns have been very little studied.
Along with her field assistant, she spent four to five hours in the field each day, making notes on the birds’ behaviour and recording their daytime songs as well.
She marvels at the fact that, although these birds are spring migrants from Mexico, “they’re singing at three a.m. in the Yukon and it’s minus-two degrees. What are they doing here? Then in June you see this explosion of insects, which is a very reliable and plentiful food source, and it explains itself.”
What drives her to sacrifice a few hours of sleep for science? “I’ve always been interested in the function of bird song, and the dawn singing is one of the most intriguing aspects.” But while doing her master’s in behavioural ecology, Stehelin found many theories to explain dawn singing but very little actual data.
That’s where the dusky flycatcher comes in. It’s a nondescript, medium-sized songbird, smaller than a robin but larger than a warbler, and part of a larger group of birds called suboscines.
Oscine means songbird, and it used to be thought that suboscines were a lesser developed songbird group. That’s because, unlike the oscines, who must learn their songs from adults, the suboscines’ songs are inborn.
A dusky flycatcher, like other suboscines, can be raised in complete isolation and will still develop the normal song of its species. But what biologists have since found is that the simpler songs of the suboscine group don’t mean they’re less adept at singing. Instead, the reason is evolutionary.
“The two groups were separated geographically for over 60 million years and evolved in different ways,” says Stehelin. “That’s one of the reasons I’m studying dusky flycatchers - you can’t necessarily apply the same theories about song function to both groups.”
So far, in the research of Stehelin and others, the songs of the two groups appear to have similar functions even though their development has been so different.
Bird song is primarily performed by male birds and is used to attract mates. More importantly, it’s also used to repel other males and establish a territory.
But dawn singing, which can start about 15 minutes before sunrise and usually lasts for about 40 minutes, has a different function from daytime singing. The male uses it to protect the territory from other males and to defend the female, especially if she’s fertile. In other words, dawn singing is directed at other males, while daytime singing is used to attract the mate in the first place.
There’s a secondary function to the singing, too. “It’s called extra pair mating, which is a fancy word for cheating,” explains Stehelin. Genetic testing has shown that, in almost all songbird species, there’s some level of infidelity.
But it’s not only the males who’re doing the cheating. “We’re finding that early morning singing may be when females are visiting other males. Singing is one of those traits that females use to assess how strong a male may be - indirectly, how good his genes are.”
Do stronger birds have more elaborate songs, or do they sing longer, or louder? “That’s a very good question. It seems to depend on the species.” Other studies suggest that song rate and complexity, along with bright colour, are indeed linked indicators of male strength.
Dusky flycatchers sing fairly simple songs made up of three syllable types or sounds. When Stehelin was studying the birds in southwest Alberta, she found that they tended to mix up those syllables in more complex ways when the females were fertile.
Once males became paired, however, the daytime song rate decreased significantly - either because it’s no longer needed for mate selection, or because the male is so busy guarding the female that he doesn’t have time to sing.
Stehelin’s digital recorder, along with a computer program she’s designed, allows her to identify individual singers with up to 100 per cent certainty.
“In the same way we can recognize other humans by voice, birds can recognize each other by voice, too. The variations are very subtle with the dusky flycatcher since the songs are innate, not learned.”
It turns out that female dusky flycatchers sometimes sing, too, though Stehelin doesn’t yet know why. “There are other tropical species where the female is also involved in defending the territory. Singing could be communication between the pair except that they also have an elaborate array of call vocalizations, so it seems redundant to use songs to communicate.”
Singing, it turns out, is a high-stakes endeavour for the male. “If a male isn’t singing as fast as his neighbours or in the right manner, he may not enjoy any reproductive success, and these birds only live for two years,” Stehelin points out.
And with a short breeding season, as little as 55 days, it seems that every minute counts. Those males are literally singing as if their lives depend on it.
For more information on northern birdsong, see
http://www.taiga.net/yourYukon/col462.html
This column is co-ordinated by the Northern Research Institute at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/yourYukon