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THE NATURALIST
by Dave Evans

Sociable Hermits
July 14, 2004

I like to let visitors to Fort Abercrombie in on one of my favorite things about the place this time of year. I'll ask them just to stand on Miller Point and be silent for a little bit. Chances are, the most conspicuous sounds we hear will be the calls of the hermit thrushes right along the edge of the Sitka spruce forest. I think of it as the best early summer sound Miller Point has going.

From different trees we'll hear a long first note followed by a cascade of short notes. Songs will follow each other; overlapping as more male thrushes take up the chorus. Song is a most effective way of communicating in our forest. With the dense vegetation there, vision may not be the best sense to use to locate either rivals or mates.

If we're lucky enough, we'll see the birds on the spruce branches or flying down onto the dirt road. They're brown with a tinge of red, have a white breast speckled with brown spots, and are a bit smaller than a robin. On the ground they tend to run with rapid upward movements of the tail and frequent wing flicking.

Hermit thrushes have an extensive range throughout the United States and Canada, and it extends well into Central Alaska. They're birds of forests and forest edges which migrate south in late fall, usually being one of the last species to leave. The migration is fairly short-distance one as such trips go; our thrushes will migrate to the southwestern states and just into Mexico. Hermit thrushes of Fort Abercrombie feed on insects and spiders, with fruits and berries making up the rest of the diet.

After pairs form, a cup-shaped nest of grass, bark strips, and moss is build close to or on the ground. Four or five eggs are laid, which are incubated by female for a couple of weeks. Both parents care for the young who leave the nest in about two more weeks.

The reason we're hearing the males singing from up in the spruces at this time of season is that the song is used to set up territories. Females use the song to assess the quality of males as potential mates. Territory remains a strong aspect of the communication, and females entering a held territory are at first threatened and treated as intruders. They stay nearby (Oh, that song!) and this proximity results in a fortunate attitude change by the males.

In hermit thrushes and other bird species with wide distribution, there are regional differences in songs. There may be as much difference between a Kodiak hermit thrush song and Boston hermit thrush song as there is between human accents from the two regions. This existence of bird dialects is being studied, and turns out to give insight into mechanisms of human learning.

Apparently there is a critical periods--usually two or three weeks after hatching--during which a male has to hear another male singing. The young male's brain seems primed at that time and is particularly receptive to the information on what a proper song sounds like. Males reared in isolation end up with a twittering subsong that's not good for much. This critical period phenomenon goes far toward explaining the bird dialects. The most likely songs a young male will hear will be from males in its own regional population.

Another important aspect of song learning, though, is social interaction. If there is this kind of relationship with other birds, even outside of the critical period, the song of these "social tutors" will be acquired. This social aspect of birdsong learning is so potent that birds will even learn the songs of other species if they serve as the tutors. This cross-species tutoring is probably going on when mimicking birds like parrots and mynahs are learning the sounds of human language.

Critical periods and social tutors are not restricted to birds. In humans, there appear to be critical periods-times when the brain is especially receptive-to acquiring language skills, recognizing potential mates, developing food preferences, or even incorporating belief systems. Our critical periods are more flexible than those of other animals, but they do seem to be a component of human learning. Watch a 6-year-old start to work with language in a foreign country and you'll see a good demonstration of a brain that seems primed to receive and manipulate information of a certain kind.

And social tutors? Social interaction is tremendously important to the human organism. Social behavior is what we, as humans, do best. It's the specialization of our species; and parents, peers, the educational system, books, TV, and capital-S Society have all taken on the social tutor roles.

Next time you're out at Miller Point, stand quietly, and listen.

David A. Evans, Summer Naturalist
Fort Abercrombie State Historical Park.


http://www.kadiak.org/naturalist/sociablehermits.html Updated 2004 July 27

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