Murder is the collective name for a flock of crows. The term probably originated in folktale. It was thought that crows gathered to judge the behavior of fellow crows, and the punishment was death. The concept is not without factual basis. Crows are opportunistic scavengers and aren’t above feeding on a dead flockmate. To add to the superstition, crows were often seen on European battlefields and in graveyards. Of course, the truth is that the birds were also there before the site became a battlefield or graveyard. Superstition feeds on coincidence.

Crows are members of the Corvidae family. Corvids include crows, ravens, jays, nutcrackers and magpies. All are noisy, lively and brainy. They are among the smartest animals on the planet — smarter than apes, dolphins and Congress. Not only have crows been known to use tools, they’ve been known to use tools to create other tools. They are highly social, and family units stay together for long periods.

Since I began writing this column, I’ve received a lot of questions. I am most often asked to identify a particular bird. The second most frequently asked question is: “why are thousands of crows coming together at night?” If a flock of crows is called murder, is a big flock called a mass murder?

One of the biggest gatherings is in Bangor, usually around Kenduskeag Stream. An even larger roost is in Waterville, near Elm Plaza on the opposite side of the highway. This has been going on for years in both locations. Last week, Norm Famous of Sidney reported a flock of crows in Waterville that was five miles long, containing over a thousand crows per mile. That sounds about right. Why?

The first potential reason is a theme that I’ve mentioned several times in previous columns: safety in numbers. I even Googled the Bangor Daily News website to refresh my memory on how many times I have written “safety in numbers” on these pages. (Five.) Crows fear hawks and eagles, and they dread great horned owls. Crows can defend against daytime predators, but there’s little they can do against a nocturnal threat. Roosting together in a large, open area increases the chances of detecting danger, and reduces the risk of being singled out as owl food. Note that all of these roosts are in spots that provide extensive visibility, aided by artificial city light.

Just as importantly, there needs to be enough chow nearby to feed thousands of birds. In late autumn, agricultural fields provide plenty of grain. It’s so much food that there is no need to fight over it. There are significant cornfields north and west of Bangor. The fields adjacent to the Kennebec River just north of Waterville provide plenty of forage. Many of the crows spend their days in the fields and then gather on the nearby roost at night.

Ornithologists believe that the crows learn from each other. Crows are not only sociable, but they are also talkative, with a great variety of vocalizations. As they gather in the evening, it’s likely that crows can tell who had a good day of foraging and who didn’t. The next morning, successful crows can expect to be followed to the fields by their less successful comrades.

So, if big flocks yield all these great advantages, why don’t they flock together in summer? For starters, more daylight equals less danger. The threat of owls is diminished by the length of day. The food supply is different. The abundance in farm fields is not yet available and each crow needs to forage more widely, which leads to more dispersal.

Furthermore, crows are nest raiders and I suspect they don’t trust each other during breeding season. They sure don’t trust their cousin the raven. It’s common to see crows driving off ravens each spring. And, of course, the crows are spending their nights on the nest and not in mass roosts.

Although crows have been roosting like this forever, they are becoming accustomed to roosting closer to cities. Perhaps urban areas have fewer owls and the artificial lighting makes it easier to see those that are present. Cities are warmer in winter. Maybe it’s just because we stopped shooting them. In 1972 it became illegal to shoot crows unless they were actively causing a problem. Even when it was legal to shoot any crow anytime, the discharge of firearms in cities was illegal and there was a margin of safety there. As I said, crows are smart.