I will never look at a junco the same way again. Read down this column until you get to the part about testosterone and sex.

Recently, I was invited to review a movie called “The Ordinary Extraordinary Junco.” I now understand why the movie was made, and why the biologists at Indiana University are so enthusiastic about it. Apparently this little gray bird has a lot to teach us, about birds and about ourselves.

The dark-eyed junco is widely distributed across North America. The slate-colored junco is the subspecies common throughout most of Canada and the eastern United States. There are actually six subspecies in the U.S., and the color differences can be quite dramatic. There are even variations within the variations. This is one of the big reasons scientists became fascinated with the junco. Species with widespread distribution often evolve regional differences and nowhere is that more obvious than with juncos. Isolated populations with dissimilar appearances and behaviors help biologists gain valuable insights into evolutionary processes.

At some point, regional subspecies become so dissimilar that they won’t even breed with each other, whereupon scientists split them into separate species. However, where junco subspecies overlap, they readily interbreed despite dramatic differences in color. This suggests that junco diversification is a recent and fast-moving phenomenon, giving us a chance to watch evolution unfold almost in real time. Genetic studies show that our dark-eyed junco split off from an older, yellow-eyed Mexican species only about 10,000 years ago.

One reason juncos make good study subjects is that they are so observable. Juncos are an abundant member of the Emberizidae family, which includes sparrows and buntings. As a sociable bird, it is easy to lure into nets, sometimes in small flocks. As a ground-dwelling bird, it is relatively easy to observe on the nest. A century ago, it was a study of juncos that convinced scientists that migration and breeding behavior were triggered chiefly by changes in the length of daylight, not by seasonal temperatures.

There may be a lesson here for us. Like 90 percent of bird species, juncos form pair bonds, and both parents raise the family. But, like most birds, males and females are apt to stray a bit, and genetic studies show that junco siblings in a nest may have multiple fathers. Scientists at Indiana University wanted to understand the effect of testosterone on male behavior and breeding success. They gave half of their captured males a boost of testosterone and the other half a placebo, then released them all onto their breeding grounds and watched.

Biologists discovered that the overly hormonal males enjoyed more copulations with more females, significantly increasing the number of their potential offspring. This should give these oversexed males an evolutionary advantage, right?

Not so fast. They also discovered that the straying dads were much less attentive to their own mates and nestlings, decreasing the survival chances for their offspring and diminishing the odds of passing along their genes. Furthermore, the constant, tiring search for a quickie and an increased tendency toward aggression decreased the average lifespan of the testosterone-boosted males. It turns out that Mother Nature has gotten the balance just about right. Cancel my prescription.

All of this is revealed in just the first half hour of the 88-minute movie. Later chapters take viewers to Central America, where junco studies in tropical lowlands and volcanic highlands provide more insight into bird evolution.

This documentary even provides a glimpse into bird adaptation as it is currently happening. I’m sure you have witnessed big flocks of juncos around your house in spring and fall. The birds melt into the Maine forest in summer and reappear under bird feeders in migration. In California, a different subspecies breeds in the mountains along the Nevada border, then migrates westward to the lowland coast in winter. No juncos lingered in the lowlands into summer… until now. In the early 1980s, a small group began to adapt to city life and learned to breed on the campus of the University of California – San Diego. Under our very noses, these birds colonized and adapted to a new, completely different habitat, creating another isolated population. That’s what juncos do, and it’s why they are so fascinating to study.

The beauty of this video is that it presents the science of a common backyard bird in easily accessible simplicity. There are eight chapters, which can be viewed online at juncoproject.org. I’m going to pop some corn and go watch it again.

Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at www.mainebirdingtrail.com. Bob can be reached at duchesne@midmaine.com.