Nature did not supply your Canary with teeth to help grind his food. The
food goes directly to the crop where it is mixed with digestive juices
and softened. Thence it passes to the gizzard, where with the aid of
gravel previously swallowed, it is "ground up" for assimilation. The
gizzard is a small thick pouch with very tough, muscular walls which rub
against each other at great pressure. When gravel is present, this
action grinds the food into a soft, easily digested mass. Therefore it
is essential that your canary have free access to a plentiful supply of
gravel of the right kind and size. Gravel composed of limestone or
magnesia should not be used, because it is readily dissolved by the
bird's digestive juices. Beach sand may have harmful impurities and is
usually worn and rounded.
Gravel should be hard, sharp rather than round, and not affected by
digestive juices. French's Bird Gravel is crushed silica sandstone
quartz and meets these requirements perfectly. It is thoroughly washed,
carefully screened to a uniform size, and then sterilized by heat. When
sprinkled on the floor of the cage, it helps keep the bird's feet in
condition by wearing off the scales and dirt, and it is a distinct aid
in keeping the cage floor clean and dry.