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Seed-eating birds utilize a unique process in order to digest their hard-shelled diets. Digestive enzymes cannot penetrate the seed shells (for doves and other species that swallow the shells) nor, in some cases, the inner seed covering (for species that crack seeds before eating). To get around this, birds have evolved a muscular organ known as the gizzard, or ventriculus, to help grind their food into smaller pieces.
Seed-eating in certain other birds increase the gizzard’s effectiveness by swallowing stones and gravel, which are stored and act as grinding surfaces. These stones are periodically regurgitated or passed in the feces, possibly to prevent their becoming smooth and, consequently, less effective. Be sure to always have grit available to your seed-eating birds or they will not be able to derive adequate nutrition from even a well-planned diet. Bits of cuttlebone also help to grind seeds, but only temporarily. A number of fishes and crocodilians have gizzards and utilize stones as well.
Pigeons swallow huge amounts of gravel, as they consume their seeds shell and all. While working with reptiles years ago at the Bronx Zoo, it was standard practice to trap pigeons for use as crocodile food (sorry, pigeon fanciers – I like pigeons too, but it was impossible to keep them out of certain exhibits, and they were implicated in the spread of diseases to the collection and staff). However, tests showed that the pigeons’ lead levels were incredibly high, due in part to ingesting the heavily-polluted Bronx gravel, and we ceased the practice (the pigeons were and remain fat and healthy none-the-less).
In the Bronx community where I grew up, “city” pigeons featured in the diets of people from several European countries. Elderly but quick-handed women tossed wet towels over pigeons as they came to feed on fire escapes (on bread put out by the same women, of course!) and knocked the squabs from nests with long bamboo canes. I never protested, despite my interest in all things avian, as their quick reflexes were just as likely to be used against annoying children as tasty pigeons! Well, that neighborhood is still home to some quite elderly people, so perhaps the lead-laced pigeons have not had their revenge!