ertain of the cavities or holes in a hay-rick where sparrows roosted
lined with feathers, while others were not lined. Such departures from
a level line of habit as this are common enough among all creatures.
Instinct is not something as rigid as cast iron; it does not
invariably act like a machine, always the same. The animal is
something alive, and is subject to the law of variation. Instinct may
act more strongly in one kind than in another, just as reason may act
more strongly in one man than in another, or as one animal may have
greater speed or courage than another of the same species. It would be
hard to find two live creatures, very far up in the scale, exactly
alike. A thrush may use much mud in the construction of its nest, or
it may use little or none at all; the oriole may weave strings into
its nest, or it may use only dry grasses and horse-hairs; such cases
only show variations in the action of instinct. But if an oriole
should build a nest like a robin, or a robin build like a cliff
swallow, that would be a departure from instinct to take note of.

Some birds show a much higher degree of variability than others; some
species vary much in song, others in nesting and in feeding habits. I
have never noticed much variation in the songs of robins, but in their
nesting-habits they vary constantly. Thus one nest will be almost
destitute of mud, while another will be composed almost mainly of mud;
one will have a large mass of dry grass and weeds as its foundation,
while the next one will have little or no foundation of the kind. The
sites chosen vary still more, ranging from the ground all the way to
the tops of trees. I have seen a robin's nest built in the centre of a
small box that held a clump of ferns, which stood by the roadside on
the top of a low post near a house, and without cover or shield of
any sort. The robin had welded her nest so completely to the soil in
the box that the whole could be lifted by the rim of the nest. She had
given a very pretty and unique effect to the nest by a border of fine
dark rootlets skillfully woven together. The song sparrow shows a high
degree of variability both in its song and in its nesting-habits, each
bird having several songs of its own, while one may nest upon the
ground and another in a low bush, or in the vines on the side of your
house. The vesper sparrow, on the other hand, shows a much lower
degree of variability, the individuals rarely differing in their
so