You are not looking at black plastic bags caught in the trees.
You are looking at crows.
Lots and lots of crows. Crows in trees, crows in bushes, crows on the ground and crows in the air.
There is not very much printed material on crows but I have found the web site, Crows.net Project where people from all over may submit observations about crows. The site's creators hope that with more information, we can learn some crow truths. It is a great place to learn more about crows.
One thing I learned from the site is that it is common for crows to gather in roosts during the winter months. A roost is a place where many crows gather to sleep and/or merely "hang out" and may include as many as 200,000 birds. Often there is a "staging area" associated with a roost. A staging area is where roosting crows may gather during the day to eat and socialize. The staging area usually is close to a wide open area with trees available. I have found a staging area, if not the actual roost. Those of us in my area who notice such things have noticed many, many crows gathering near the river in a field where corn is planted during the summer.
Yesterday I drove the narrow, country road above this field and noticed that the woods were full of crows. As there was no convenient place to pull off, I stopped in the middle of the lane, rolled down my window and grabbed the camera that, as usual, lay beside me in the seat. The moment I stopped, crows began to move.
Many quickly flew skyward and far away. Others skillfully flew through the trees just far enough away to no longer feel threatened by me. Still others merely hopped up a bit higher in the tree and watched me as I watched them.
At the above link on Crows, Michael Westerfield describes his own theory about a roost. While he is quick to point out that his theory is undocumented and not backed by strong scientific data, what he says is reasonable and is a good place to start. Michael thinks that the roosts are mostly utilized by young, unmated birds without their own territory. His theory is very close to what I have described to my husband each time we pass the large group.
I think that the gathering is similar to the large clan or tribal gatherings that took place in human hunter/gatherer societies.
Clans, or small tribal groups, that were mostly related by blood or marriage kept to themselves through much of the winter, spring and part of summer, but would gather with many other similar groups in late summer. During this time the humans learned dances, shared stories stories, visited with relatives and found mates from among groups distant enough to not be considered incestuous.
In this way, the tribes kept a close link with other tribes through shared culture and genetics while still keeping the gene pool large enough to keep the tribe healthy.
My theory is that this may be what is happening with the crows. They come together to make new alliances, find mates, and perhaps even share information. As Michael Westerfield postulates, young crows probably test each other and themselves as they determine where they each fit in the crow pecking order.
I think, though, that older crows are also part of the roost, though keeping a connection with their home territory. I believe this without any evidence other than that I see my own crows (Yes, I know, they aren't "mine.") fly to and from that direction every day. The staging area isn't too far (as the crow flies) from my back yard, probably a little over a mile, maybe less.
There are usually a few crows that check out my compost pile each morning and late afternoon. When there is something good there, more crows soon appear. Crows seem to be intelligent enough that it would make sense for there to be mature, more experienced crows around a roost to keep an eye on the younger birds. My idea may stem from an active imagination, but like Michael Westerfield's theory, it is a place to start.
The DVD below, Nature: A Murder of Crows, is a PBS documentary describing many details about crows, including their use of tools, putting in them a class with chimpanzees, elephants and, oh yes, humans. It tells much about their language. The DVD is about 60 minutes.