The "bird theory" is the one almost universally relied upon for the
explanation of these phenomena, where the seeds distributed, or supposed
to be distributed, are not winged. But we are satisfied that birds perform
no such important office, in the matter of seed-distribution, as is
generally attributed to them. We have examined, during the past two
seasons, a large number of bird-droppings, and find our previous
impressions respecting them fully verified. With all the more delicate
seeds--those of our common field grasses and weeds--the chances are a
thousand to one that none of them will ever pass the cloaca of the bird
eating them, in any condition to germinate. All seed-eating birds are also
gravel-eaters; and the pebbles and gravel they eat are mostly silex, or
the material from which our best buhrstones are made. These pass into the
gizzard, or pyloric division of the bird's stomach, where they are
utilized, the same as we utilize our buhrstones. The gizzard has sharply
corrugated interior walls, extremely thick and muscular, which
involuntarily contract and expand, giving the bird a tremendous grinding
power over his food, considering the size of his grinding apparatus. The
seeds--all the seeds, in fact, he eats--pass at once into his crop, or the
natural "hopper" to his "gristmill," where they undergo a moistening or
macerating process previous to being ground into the finest pulp in the
gizzard. As a general rule, all the seeds a bird eats are ground into this
pulpy state before they pass into the intestinal canal, extending from the
gizzard to the cloaca. The hard, semi-translucent, and highly elastic
outer coating of most small seeds, may be measurably preserved in its
passage through the gizzard, and, resuming its oval shape in the thinner
pulpy mass contained in the upper portion of the intestine, present the
appearance of seed in the cloacal discharges, and thus deceive the casual
observer. But the use of a spatula and a small piece of polished stone
slab will show that the entire discharge is excrementitious matter, with
the single exception of this silicious coating of the seeds.