The expression "Birds of a feather flock together," or some variant thereof, goes back to the 16th century, predating telephone and electrical wires by at least three centuries. Still, the average non-birder is most likely to spot birds gathered in species-specific groups on overhead lines. That led one Seven Days reader to ask us recently: "Why do birds line up on telephone wires?"

The snarky response? "Because it's easier than sitting in a circle." Or perhaps "Because they can."

But what really attracts birds to human-strung wires, even when there are plenty of tree branches and other natural perches to be had? I remember one of my childhood camp counselors saying that birds land on power lines to warm their feet in cold weather. Even then, I suspected he was just winging it, especially given that birds land on electrical wires in warmer weather, too. So, WTF?

For a more scientific explanation, I consult local avian expert Mark LaBarr, a wildlife biologist and conservation program manager with Audubon Vermont, a program of the National Audubon Society. LaBarr, who's been with the nonprofit for 19 years, runs its bird-banding station at the Green Mountain Audubon Center in Huntington. He points out that about 350 different bird species can be seen in Vermont, including more than 200 that breed here.

Those species have various reasons for perching on utility lines, LaBarr continues. For some falcons, such as the American kestrel, the lines offer a bird's-eye view of grasshoppers, lizards, field mice and other prey. Insectivorous birds, such as swallows, catch their food on the wing, so they often hang out on power lines to spot juicy meals as they fly by. Those wires can be more desirable perches than trees, having no foliage to block the view or conceal predators.

During certain times of the year, especially late summer and early fall, some bird species cluster on wires in what LaBarr describes as a "pre-migratory get-together." In other words, the wire serves as a staging area before the flock takes flight for warmer climes, the avian equivalent of an airport preboarding area.

Birds on a wire aren't a year-round phenomenon, LaBarr notes, even for species that stick around for the winter. During the breeding season, he says, we won't see birds lined up, because it's not advantageous for them to have competitors crowding their space. Once the breeding season ends and their territoriality declines, birds are more prone to flocking together, often in long, jabbering rows.

Do birds actually step aside to make room for a newly arriving bird, as it often appears?

"Oh, I'm sure there's some jostling there," LaBarr says. "But whether a bird actually says to itself, 'Oh, I'll move over and allow some other bird to join us here on the line,' I don't think they consciously do that. One thing about flocking behavior is that there's safety in numbers. Somewhere in their genetics, they know having one more bird sitting next to them means that their chance of being hit by a predator is reduced by 50 percent."

We asked another pressing question: Why do birds on a wire always face in the same direction?

"I guess they do, come to think of it," LaBarr muses, "but I'm sure there's the odd bird that faces the opposite way. They're not unlike us humans."

The expert offered a couple of possible explanations for the common alignment. First, as every airplane pilot knows, it's easier to take off and land facing into the wind. Second, when birds face the weather, their feathers don't get ruffled.

Lest anyone wonder why birds don't get electrocuted whenever they cling to high-voltage wires, the reason is basic physics: As long as their bodies aren't grounded or completing a circuit, the electrical current doesn't pass through them.

Which doesn't mean that birds never get electrocuted on power lines, LaBarr says. That happens pretty frequently, especially to birds with larger wingspans, such as eagles and osprey, which often build nests on utility poles. When their wings or nesting materials touch two wires simultaneously, they get fried. To prevent such fatalities, as well as disruptions to the electrical grid, utility companies often build nesting platforms, such as those erected near Sand Bar State Park in Milton.

But, as avian causes of mortality go, LaBarr says, electrocution pales in comparison to more pernicious, human-created threats, including habitat destruction, windows and even domestic cats. One study, conducted by the University of Georgia in 2011, found that free-ranging felines kill an estimated 4 billion animals annually in the U.S., including more than 500 million birds — a staggering number.

One more good reason to hang out on wires.