SpruceRoots Magazine - February 2000


Audible Landscapes
Dawn Chorus in the Tlell Watershed

by Erica Thompson

The first challenge of Dr. Tarmo Poldmaa's day is to rouse eight students, a few parents, and teacher Mark Ruzylo from sleep. Waking such a large group, coaxing them from their warm sleeping bags and out into the forest all before 4:30 in the morning strikes most of us as an unattractive challenge by any early morning standard. It is so early, PM has just slipped into the AM, but there is a trail of kids hiking out into the Tlell's forests bent on beating the first light.

This experience is part of the Living and Learning School's 'school in the bush' on the Tlell River and for most the scheduling of this lesson sets a new record in dedicated learning and its fortunate sunlight is not a necessary ingredient to songbird identification. Identifying birds in the Graham Island watershed by song recognition is birding with your ears.

Poldmaa is an evolutionary biologist with a penchant for birds and their songs and his role this early morning was, "to teach the kids about birdsong and to get them to experience what 99% of us miss every spring morning ­ a phenomenon known as Dawn Chorus."

For those who haven't had the opportunity to wander the woods with a bird loving biologist, 'dawn chorus' is the collective birdsong of a new morning. Poldmaa says, there are more songbirds singing in the Tlell watershed than visual landscapes suggest. You can be sure there are more birds in your forests than visual senses pick up as they are most often obscured in the canopy or tree cavities. When birding, you've got to bend an ear to the audible landscapes to hear the symphony of birds such as Juncos, thrushes', stellar jays, pine grosbeaks, Townsend's warbler, winter wrens, chickadees, and sparrows. Most birds make sounds, though they are not all are considered to be 'true songbirds' or what biologists call 'oscines' which are capable of producing elaborate and complex vocalizations, he says.

"It is hard not to notice the melodic song of an American Robin or the burst of notes and trills of a Winter Wren after the long winter. They are the first tantalizing hints of the longer warmer days of spring," Poldmaa says. "Slowly, as the days warm we begin to notice other songsters, the Varied Thrush, whose harmonic notes pierce the misty depths of cedar stands, the tiny Golden-Crowned Kinglet, whose incredibly high pitched call is almost inaudible flit amongst the crowns of spruce trees. The Hermit Thrush with its haunting flutish notes mix with the white noise of April showers. These are but a few that add into the cacophony of birds in the Tlell River watershed."

The elegant Varied Thrush resembles a robin with a black band across the breast and orange in the wings and over the eye and its song has been described as a hum with single whistles single held as long notes first at one pitch, then rising or dropping in tone. During the spring breeding season, the secretive thrush has been known to pose boldly in trees singing on display all day long; its song has been likened to jazz improvisation as the flutish notes spiral down from an initial clear whistle.

The small olive green Golden-crowned Kinglet ­ the male with his bright orange crown, the female's bright yellow ­ are smaller and plumper than warblers, and are generally heard before they are seen. Their high and thin 'see see see' calls ending in chatter is almost beyond the range of human hearing.

The particulars of bird songs are shaped to some degree by their habitat. Poldmaa likens the experience of a bird transmitting song through a rainforest like two people trying to talk to each other from opposite shores of a rushing river. To be heard, he says, we have to modify our voices and even then some of our message may be lost. Birds face similar challenges and it has been only recently that evolutionary biologists have begun to consider the role of the environment through which a vocalization must travel as a force in the evolution of song and call structure.

The distance a sound will travel will depend upon two things: the frequency it is transmitted at and the amount of degradation, which occurs to the signal as it travels through the environment. Low frequencies degrade less quickly than high frequencies and are therefore more useful in cluttered environments where there is a lot of noise or absorption in way of the song, such as running water, wind, rain, leaves, branches, trees and rocks. Thus, birds breeding deep in the cedar, spruce and hemlock forests of the Tlell watershed tend to sing deeper tones and longer notes such as whistles, which translate best throughout their environment. Whereas, birds living in open fields or the edges of woodlands like the song sparrow or those which spend most of their time high in the canopy, like the Golden-crowned Kinglet, tend to have a lot of trills and high-pitched songs, Poldmaa says.

There are many reasons behind the type of songs we hear and time of day and season they are sung. To figure out why songbirds sing at dawn we must first wonder why they sing at all.

It seems male birds are most often the singers of the forests; their calls are an expression of the testosterone their bodies house in springtime. Although there are a few species where females which sing it is most commonly the males filling the forest air with song.

"The simple reason birds sing in the morning is they receive an initial pulse of testosterone in the early morning and their response is to sing." If you elevate the levels of testosterone in the blood in some female bird species, scientists have found they will sing too," Poldmaa says. It is springtime and after the winter the birds are ready to find a mate and breed and they are singing loudly about it.

"Most of us notice that bird song is particularly evident in the spring and then tapers off as the summer moves on. Most male birds are territorial and through song they are establishing and defending their territory and trying to attract a mate. High levels of testosterone equal high levels of aggression, which in turn trigger birds to sing, he says. Later in the season, birds are busy feeding their young and levels of testosterone drop dramatically and are replaced by a 'lutinizing' hormone, which stimulates parenting behavior and a reduction in song.

"Interestingly if a bird looses its offspring to a predator and there is time to re-nest the hormone levels will flip-flop and the male begins to sing again." Some sophisticated birds have two songs: one, which serves as a mate attractor and one to defend territory. For others the same song serves both functions. Females may use the vigor of a song to judge a potential mates qualities.

"Female birds of a number of different species have been shown to settle based on the males' dawn chorus performance."

So why do birds choose dawn as their preferred performance time? It is known that most songbirds migrate at night and that females arrive at their breeding grounds in the hours before sunrise ­ the same time the males are singing loud and proud as early as possible. Poldmaa says, one theory suggests males are announcing to each other that they have survived the night and are still ready to defend their territory. Spring nights are cold, predators abound, and it is the most dangerous of times.

"Most birds that die do so during the night, so the morning is when territory vacancies will be most noticeable. If their neighbour isn't' on his favorite perch at the crack of dawn there is a good chance he didn't survive the night and his absence is a signal to other males this 'territory is vacant and available' and the signal to the females that her mate is dead."

If he did survive the night but fails to muster up a good vocal performance it can send the message to other males that he is too weak to defend his territory or his mate, Poldmaa says.

When listening to 'dawn chorus' it seems a song isn't just a song; it is a language and just like human language there are local dialects. In fact, Poldmaa says, much of the scientific work done on the transmission of human dialects has come from the study of birds. It has been found birds of a particular species in one area will sound slightly different in another. For example, American Robins found on Haida Gwaii will have a different song type than those of Vancouver. Their songs will sound structurally the same but the Haida Gwaii Robin will be able to distinguish city Robin just as we can distinguish someone speaking English with an Australian accent.

Most interestingly, these differences in dialect can occur on a local scale. Island populations found in the Yakoun valley may have a slightly different dialect than those in the Tlell watershed. These dialects may act as population markers for arriving females who don't like to breed with males they may be closely related to.

With the dawn upon them, the group made their way back to their school by the river. The students busy calling out the names of unseen birds whose songs they had heard on the rising morning. Townsend's Warbler, Hermit Thrush, Swainson's Thrush, Winter Wren and Golden-crowned Kinglet.

Recalling the dawn chorus spent with the Living and Learning School, Poldmaa says, " what I had hoped to teach these kids was that song is a complex form of communication in birds used for attracting a mate, defending one's territory and that by learning to listen to birds we can learn a lot about how they live. I also wanted them to realize the diversity of birds that really existed within this watershed by using their ears instead of their eyes. Most birds are secretive and rarely seen but if you sit and listen they will eventually make a sound and by that alone we know they are there."

Graphics: top - Kungst'aasl Guujaaw, middle - Anne Wilson, bottom - William Broadhead

SpruceRoots Magazine - February 2000