(The secretion corresponding in hen-birds
to catamenia, and the semen of the cock, are residues.) Since then
both the wings and the semen are made from residual matter, nature
cannot afford to spend much upon both. And for this same reason the
birds of prey are neither given to treading much nor to laying many
eggs, as are the heavy birds and those flying birds whose bodies are
bulky, as the pigeon and so forth. For such residual matter is
secreted largely in the heavy birds not given to flying, such as
fowls, partridges, and so on, wherefore their males tread often and
their females produce much material. Of such birds some lay many
eggs at a time and some lay often; for instance, the fowl, the
partridge, and the Libyan ostrich lay many eggs, while the pigeon
family do not lay many but lay often. For these are between the
birds of prey and the heavy ones; they are flyers like the former, but
have bulky bodies like the latter; hence, because they are flyers
and the residue is diverted that. way, they lay few eggs, but they lay
often because of their having bulky bodies and their stomachs being
hot and very active in concoction, and because moreover they can
easily procure their food, whereas the birds of prey do so with
difficulty.
Small birds also tread often and are very fertile, as are
sometimes small plants, for what causes bodily growth in others turn
in them to a seminal residuum.
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