The case is different, however, with the fruit-eating birds. The fruits
they consume are retained but a comparatively short time in the crop, pass
hurriedly through the gizzard, and no doubt carry along with them some of
the smaller seeds of berries, and now and then the pit of a cherry or
small plum. The gizzard, in these cases, is simply gorged with the pulp
and juices of the fruit, its muscular action more or less relaxed, and
some of the seeds consequently escape the grinding process they would
otherwise undergo. And yet we are satisfied that a majority of these seeds
even, are more or less thoroughly triturated by a healthy gravel-eating
bird. This would certainly be the case if they were retained for any
length of time in the pyloric division of the bird's stomach. All birds
have gizzards, but their grinding capacity depends very much on the
character of the food they eat. Birds of prey, and others subsisting
mostly or entirely on animal food, have thin, membranous, and
comparatively flabby gizzards; while those living on hard grains and seeds
have extremely thick, powerful, and muscular ones,--those capable of
crushing up and thoroughly triturating all the food they take into their
crops. These gizzards are nature's gristmills, and they grind exceedingly
fine. If any seed escapes, it is because the mill has been flooded by the
bird, and not because of any defect in the grinding apparatus.

These birds are not, therefore "natural sowers of seeds," as Professor
Marsh and some others claim; but are, at most, only accidental or
chance-sowers. Nature never designed that they should do anything more
than consume the food they eat, or submit it to the proper action of their
digestive organs. It might as well be claimed that the secretary bird is a
"natural sower of serpents," as that many of the grain-eating birds are
"the natural sowers of seeds." The theory is too foraminated--too full of
loopholes and unsatisfactory conditions--to be accepted as an explanation