Surely hundreds of thousands, perhaps millions, of birds die every day - but hardly a single carcass in view. Where have all the dead birds gone?
Eaten by scavengers often within minutes of falling to the ground. (or more often, picked off by predators before dying of their own accord). A lot of hungry little mouths out there.
Cecil did a column about this many years ago. The column was about dead pigeons in cities, but the answer applies pretty well to birds in general.
Cecil basically says that scavengers will devour the carcasses pretty quickly, and also says that when a bird gets old or sick, it will often hide somewhere so that predators can’t easily get to it. The bird then dies somewhere out of sight.
Furthermore, a sick bird will often show very few outward signs of illness (in part, because showing weakness makes the individual a target for predation). Once the disease is serious enough that the bird can no longer hide the symptoms, death is usually inevitable and often occurs within hours - enough time to hide away into some corner that is inaccessible to larger predators (although rats go pretty much everywhere, and will clean up after the bird is dead)
So rats have picked them, every one? They must not be a long time coming, rat eradication programs notwithstanding, or other animals must be equally eager to chow down on pigeon carcass.
Which implies that if we were ever truly efficient at vermin eradication, we’d end up with a lot of pigeon carcasses. When will we ever learn?
I occasionally find a few feathers near our birdfeeders. Everything else gets eaten. Walking in the woods I see dead turkey/deer/raccoons/foxes/shrews/etc. The next time I pass the area there is less or no carcass. Efficiency in action.
I think an additional factor is that people are not very observant. I walk a couple of miles a day and I see bird carcasses every day. I don’t often see plump, three-dimensional, just-died, carcasses. Most are flat, dusty mats of feathers.
Came in here to say just this. I run almost every day and it is not unusual to see a dead bird on the ground. Usually, they are in a state of picked-overness, but I have seen some fairly fresh samples. I know I shouldn’t, but I often end up looking at the ground, and hence get a chance to observe them moreso than I would if I was looking at a distance ahead, like I should be.
I’m not a runner, but why is it bad to look at the ground once in a while? How else do you avoid stepping into an open manhole or tripping over a coke bottle? I tend to think people who have such incidents are klutzes or idiots, but maybe they are simply following advice from good ol’ Coach with his famous IQ.
What others have said. You would be surprised by what goes on under cover of darkness in the animal world. I knew we had raccoons regularly come thru our yard. But I never imagined what really happens at night until a feral cat colony adopted me and I feed them every day. The colony takes care of itself and woe be any raccoon that tries to change that fact.
Not at all bad to look at the ground “once in a while”, and certainly not bad to keep an eye on where you’re going.
But as D18 notes, advice on good running posture generally recommends that you don’t run with your head bent down, which is the way most of us look at the ground. Here’s a writer on running physiology explaining how to look around you while maintaining good running posture:
Well, when I say looking down, I mean looking down. After you get fatigued it’s easy to fall into a poor posture and literally be looking down at your feet. The recommendation is to look a moderate distance in front of you which encourages a better posture.
But of course, it means you see way more dead birds than you might otherwise, so there’s that! 
Or what Kimstu said!
Rats, raccoons, crows, cats – my dog has been known to pounce on a dead bird and shred it before we can even get him to drop it.
But the big thing I see is maggots. Leave a bird on the ground 24 hours and it will be infested. After that, decay happens pretty quickly.
Since the Q’s been answered, does anyone else hear Van Halen’s “Where Have All The Good Times Gone?” when reading that thread title?
Bird sspecies with huge populations are mostly migrants, and migrants mostly fly at night. I don’[t know, but I presume that most migratory birds die during the rigorous migratory process, which means they would fall out of the sky in darkness. Most scavengers are, I think, nocturnal.
So, most birds die at night, and are picked clean by morning. Imperfectly knowledgeable speculation.
To summarize the reasons:
-
Most birds die because they are killed by predators such as hawks or cats and are eaten almost entirely.
-
Birds that die of disease or starvation usually seek out a hidden shelter to take refuge in before they die where their remains won’t be seen.
-
If a bird dies in the open, it will quickly be eaten by scavengers or consumed by maggots or other insects. Most birds weigh no more than a few ounces and will be consumed within hours or at most days. The bones are thin and they and the feathers will be dispersed.
-
Roadkilled birds will soon be flattened and unrecognizable.
-
There are more bird carcasses around than most people notice. I see lots of them.
My earworm from the title is “Where Have All the Flowers Gone?”
And 6) when hummingbirds die, it’s really, really sad! 
Answered at about the 40 second mark of this documentary.