There are two very large and common black birds found throughout North America -- the Northern Raven and the American Crow. They are fairly easy to tell apart. Ravens are much larger, their call is deeper and hoarser and they have heavier beaks. When ravens fly overhead their tail is wedge or V-shaped. The mnemonic device is "V" like the "V" in Raven. Ravens also tend to be in monogamous pairs though not always, occasionally there are flocks of ravens.
Crows on the other hand are two thirds smaller than ravens, their calls are slightly higher pitched, the tail is squared off and crows tend to be in large flocks. Crows are much more common in urban, suburban and rural areas. Ravens on the other hand are often found in wilder and more remote habitations like the mountains and deserts. In the San Francisco Bay area, both species have been increasing in number, adapting readily to the changes humans have wrought to the land. In addition ravens have been expanding their range and showing up in places like the city of San Francisco where they were rarely seen 20 years ago.
For an unknown reason and much to my chagrin, a huge flock of crows has been assembling in the two large redwood trees in my yard in Santa Rosa. Beginning shortly before the sun sets, there are hundreds of crows flying in from every direction. It takes them an hour or so of squawking, flying around, flapping loudly before they finally settle down for the night. And at 5:15 every morning for the last three weeks, they all wake up and announce loudly that it is another fine day to be a crow. This in turn causes our dog to start barking loudly as well.
I have always wondered about the collective noun for a group of these raucous birds: a murder of crows. Now I finally know. It's that you want to murder those crows. Of course I never would do that, just dream about it, if I could sleep.
This is Michael Ellis with a Perspective.