From what I understand birds have a very weak sense of smell and taste yet they are extremely selective about what they eat. Many of their food sources are not highly visible and I can’t imagine that birds just go around hitting every plant until they find something. The only thing that makes sense to me is that they use vision as a substitute for smell. If this is in fact the case then it would depend on nearly everything putting out some kind of fumes that would be identifiable only through ultra violet lights that we cannot see but birds can. This would explain how vultures can locate dead things in a rain forest while they fly several hundred feet above the canopy. It would also explain how birds can tell when fruit is ripe or seeds are present etc.
What are current theories on how birds identify food?
Birds mostly identify food by sight. They search vegetation for insects or fruit by eye until they find something. Although they can see UV light, they mostly use visible wavelengths.
Of course, lots of birds eat prey that is in plain sight, like flying insects, other birds, or rodents.
Turkey vultures are one of the few birds that have a well developed sense of smell (although they also have keen vision). They find food in forest by skimming above the canopy and smelling dead animals that have started to decompose.
And these enigmatic ultra violet fumes that you have deduced a priori must exist have gone undetected to science?
Why do you imagine that ordinary good vision doesn’t work to find food? The obvious visually distinctive feature of live prey is movement - swimming fish, running rodents, flying insects, wiggling caterpillars or worms. Berries have evolved bright colors to be noticeable to animal vision. We can distinguish seeds scattered on the ground, so can birds.
You have to remember that bird see MUCH better than we do. Not just the matter of UV light - many species have higher resolution vision than we do, able to see more detail at a greater distance than we can. Many species are tetrachromats, meaning they see a different spectrum/rainbow than we do.
Some species are also taught about food sources by their parents, at least to some degree, and also by watching flock mates.
Also leaks in gas lines - which apparently smell like snack time to turkey vultures. Power companies use them to help spot leaks in pipelines.
Here’s my staff report on how vultures find food.
They aren’t as good at it as you might think. Humans don’t make it any easier.
Some of the more intelligent species also learn to spot visual patterns that could be food, often man-made. Where I live in the boreal forest we have a lot of ravens. You can’t leave garbage bags full of anything out; they’ll spot the black bags, know they usually have edible garbage in them, then swoop down and start tearing them open and scattering the items, whether there’s anything edible in them or not.
Also I’ve had a sealed bag of new dog food laying in the open box of my truck in the middle of town. Looks nothing like natural food. Within a couple hours half a dozen ravens had landed on my truck, tore the bag open, and started eating the food.
Highly variable. Sparrows will only eat off the ground, and hang around under winter feeders and eat what other birds drop. We feed sparrows a wide variety of baked products, and they eat it all, but seed-eating munias ignore it.
Hummingbirds and sunbirds are attracted to the bright colors of nectar-bearing flowers.
Black vultures cannot smell at all, and follow circling turkey vultures.
Woodpeckers can heat insects noshing under the bark. If woodpeckers hammer on your house, call the exterminators.
Now that’s something! Like having a soldering iron for a beak. 
I once saw a woodpecker eating insects in my pajamas. How he got into my pajamas I don’t know. [/Groucho]
Roadrunners sometimes find food by spotting a crudely lettered sign that says “Free Bird Seed”.
I thought I recalled reading that at least some birds have some tastebuds and sense of touch in their beaks.
What is an average (to the extent that is meaningful) bird’s field of vision? Can a pigeon/chicken see what is at the tip of their beak to be able to pick grain/seeds out of similarly sized dirt/gravel?
I think some birds like the woodcock might have more sensory ability in their beak than others. Their eyes are towards the back of their heads, and their long beak is used for probing under surfaces.
Birds do have some tastebuds, although not as many as humans, and some have sensitive beaks.
Here are a pigeon’s and an owl’s field of vision. Pigeons have eyes on the side of their heads, and an almost 360 degree field of vision, but do have a small area of binocular vision in front so they can pick up food.
Woodcocks actually have a more than 360 degree field of vision, and can see directly behind them. Woodcocks have lots of sensory cells in their bills which they use to detect earthworms in the soil. Kiwis do as well. Kiwis, which are the only birds with the nostrils at the tip of the bill, also have a good sense of smell.
Thanks Colibri.
So lets say you took some seeds or corn, and mixed it with identically shaped stones, and painted them all the same color. How does a chicken/pigeon identify the corn/seeds? Do they simply eat corn AND stones, and use the stone in their gizzards or regurgitate/pass them through?
Given some of the wacky avian testing I’ve read abut, I’m SURE someone has done something like this! 
Pigeons, chickens, and other seed eating birds normally pick up some small stones and grit by accident when feeding. This actually helps grind up food in the gizzard.
In the experiment you describe, though, I wouldn’t be surprised if the birds could tell the seeds from the stones by their hardness when they picked them up, and reject the stones.
Then there’s owls and some other nocturnal birds, who have some limits from the human perspective: poor or no color vision, don’t see as well in the daytime, coupled with some specific traits: they might not see detail but a small moving object like a mouse is rapidly detectable.
Birds have tongues, which can sense touch, hardness, texture, temperature, etc. As a tongue owner yourself, you should have some notion of how this would work.
I would think with their visual acuity they could tell the difference between corn kernels and painted rocks.
While this is true, the tongues of many birds are fairly hard and not as sensitive or mobile as that of humans. Bird tongues.
Parrots have particularly fleshy and mobile tongues that they use to manipulate food.
… also to lick the sweat off their human servants when they’re sweaty in summer, but I digress…
Clearly, different bird species have adapted in different ways to different diets, so there is not just one way “birds” as a whole identify their food. Probably about the best generalization to be made is that for the majority of birds vision plays a significant role, but it’s not always the only one.