They aren't trying to wake you up. Male birds - and it is almost always the guys - break into song at dawn because it is the best time to warn other males to stay off their turf and away from their mates.
That is what Canadian researchers have discovered after analyzing the acoustics of what they call the dawn chorus.
Most birds do their singing in the early morning, and in most species the male is the vocalist, says Paul Handford, an evolutionary biologist at the University of Western Ontario.
Researchers have known for some time that male birds don't sing for the joy of it, but to warn other males to stay off their real estate and away from their partner.
Each species has its own song or songs. Individual birds perform their own signature versions, with slight variations in tone or pitch that identify them to their neighbors. It is a little like calling out their name, over and over again.
"It is a very important thing; it is a daily advertisement of their continuing tenure, their presence," Handford said.
The theory about why birds sing in the morning has to do with the fact that there is generally less heat and wind, factors in atmospheric turbulence, at that time. Researchers thought that would mean that their songs would travel farther.
Handford and his colleague Timothy Brown decided to test that theory by playing recordings of a white-throated sparrow and a song sparrow at both dawn and midday, and then measuring their strength and quality at various distances. They conducted the experiment in open grassland and in the forest.
They found the songs didn't carry farther in the morning, but were much more consistent. Think about an afternoon at a lake, when the sound from a neighbor's radio drifts in and out as the winds shift.
"That is the kind of thing birds are facing," Handford said.
It is very important for the males to get their message out, and it takes a lot of energy, he said. Any variability in the signal quality could lead to doubt as to the identity of the singer. Singing in the morning leads to a more consistent signal.
"Instead of having to cup your ear and say "What the hell was that," it comes through in a repeatable fashion," Handford said.
For the experiment, which was published in a recent edition of the International Journal of Avian Science, he used the recordings of two very different bird songs.
The white-throated sparrow lives in the forest and, like cardinals and robins, its song is full of whistles and flute-like sounds.
The swamp sparrow has a song typical of birds that live in open country. It is not very musical, more a rapid repetition of nearly identical notes, a trill.
The researchers played recordings of both songs, re-recording them at four distances between 80 and 300 feet. Then they analyzed the quality using specialized computer programs.
Technology is essential to distinguishing bird songs. Even researchers such as Handford have trouble picking out the individual versions of the songs, but computers can quickly identify the differences.
The hummingbird song, for example, can sound like a buzzing insect to the human ear. But if you slow it down, Hanford said, it is actually a beautifully complicated series of flute-like notes.
(Distributed by Scripps Howard News Service)