As you walk around the city or countryside, you might occasionally spot a dead mouse, rat, or squirrel. Driving down the highway, you regularly see road-killed raccoons, opossums, or some other mammal. But how many dead birds have you seen? Maybe an occasional road kill, but that’s all. Why is that? Birds are light in weight, thin skinned, have little fat, are covered with feathers, and have some hollow bones, so they are less dense and weigh a lot less than comparable sized mammals. When a dead bird hits the ground, it is almost immediately invaded by small decomposers in the form of bacteria and insects. Vultures and coyotes might also take part in the feast, but the tiny organisms are usually the quickest and most efficient. Being thin and light, a small bird decomposes into an unrecognizable blob in about a day and will disappear in three. A larger bird will take a bit longer, but a small mammal, say a rat, might continue to exist in recognizable form for a week or more. A road-killed hawk or owl will be reduced to nothingness in a few days to a week while a fat, thick-skinned, heavy boned raccoon might lie as a blob on the shoulder of the road for two weeks or more.
Birds also float. If they land in a pond, puddle, or lake, they are exposed to the air and the scavengers and decomposers and rarely fossilize, compared to mammals that tend to sink and get covered by sediment. This explains why the fossil record of birds is so poor compared to other vertebrate groups.
There is a scientific paper published in 1903 by M. Cline. Now, this is before we knew much about flight, or even feathers. Ms. Cline stated, in all seriousness, that birds are able to fly because they take a deep breath of air, making themselves lighter than air (ok, already the argument loses credibility) and fly off. Her proof is that the piles of feathers you occasionally observe in the field or forest are the remnants of the birds that took too deep a breath and exploded! These feather piles are, of course, the aftermath of a poor bird meeting up with a predator. Raccoons, foxes, or opossums will consume the muscle, but leave the inedible parts – the bones and feathers. The bones are rapidly consumed by rodents who want the calcium and beetles ingest the feathers quickly as they are mostly protein.
The only time I have seen a number of dead birds is around bodies of water, after an epidemic of avian cholera hit the waterfowl populations. One of the weirdest things I ever experienced, though, is standing under a juniper tree talking to a colleague, discussing some subject of global importance, no doubt, when a swallow fell from the tree and landed in my outstretched hand, dead.
By the way, you can’t get West Nile Virus from handling a dead bird, but it’s still not a good idea to pick one up.