I am going to try to introduce a different point of view here. Maybe it is relevant, maybe not.
Some things seem so obvious to us, that we do not question them. We buy a bag of bird seed. Birds eat bird seed, so of course a pigeon should eat anything in that bag. We know kids can get "spoiled" and so can birds, and both can be persnickety and waste food. I have some house mice in cages; they eat the seeds of the specially-formulated mouse food but generally leave the pellets. Mice have a good sense of smell, and should be able to tell what is edible. If it is questionable, they will nibble a bit, wait to feel any ill efects, then proceed. They cannot vomit, so once the food is in their system it has to be processed. That is why poisons work on mice. But pigeons do not smell their food, as far as we know, and eat so fast that I sometimes wonder if they appreciate the taste at all.
Some of my adult feral pigeon rescues ignored whole corn kernels and whole dried peas. Some did not. Some changed their feeding habits during re-hab. Same with some of my baby or squeaker rescues.
Three questions arose in my mind when large seeds were ignored:
Could this individual pigeon actually swallow the seed? If there were canker nodules, maybe not. (I have never recognized an instance of canker nodules, so my experience is limited). I have seen street pigeons attempt to swallow huge pieces of pizza and pita bread and wurst (sausage) in Cologne, and succeed, so a piece of corn is no problem for the normal adult feral.
If the size was not a problem, did this pigeon recognize the corn or whatever as food? I have seen pigeons ignore small black rapeseed, until they saw others eating it. Once I had handfed my squeaker rescues corn kernels, and in some instances taught them to peck, they ate whatever I put in front of them. I read that during WWII, in the Pacific theatre, American soldiers (pilots, air crew) would starve if stranded in the jungle. Japanese soldiers ate what they saw the monkeys eating, and did okay. Food might be in front of you, but if it's unfamiliar you may go hungry. If you sample everything you encounter, you will probably poison yourself. Some societies tried out unknown foods on their dogs, or on their children, first.
Maybe the pigeon has to see you pecking and swallowing the corn first. If you won't eat that funny-colored corn or that moldy-colored pea, what's wrong with it? LOL.
In summary, can the pigeon swallow the food? (He knows how to peck, can peck, can pick up the food, can manuever it into his throat, and can get it into his crop). If so, does he know it's edible? If so, is it what he needs, nutritionally? If so, does he have room for it in his crop, or is he full of some "junk food" (something not as beneficial to him, or possibly harmful, but temporarily satisfying him)? The last question, is he like us humans? Faced with plenty of food, both healthy and unhealthy, and plenty of choice, some of us eat wisely, some foolishly. Some of us binge and waddle away. An overloaded pigeon usually needs to be able to fly. We no longer have to be able to hunt and run down some of our food. We open our mouths, and food flows in. Occasionally we might need to employ a fork or a spoon, but we can do without those if we choose. Some of us get spoiled, and eventually depend on others to make rational, healthful decisions for us. It's a crazy world out there. LOL.