anner.

I now propose to consider the question from a more general point of
view, and to discuss its application to some important problems in the
natural history of birds.

_Changed Conditions and persistent Habits as influencing Nidification._

Besides the causes above alluded to, there are two other factors whose
effect in any particular case we can only vaguely guess at, but which
must have had an important influence in determining the existing details
of nidification. These are--changed conditions of existence, whether
internal or external, and the influence of hereditary or imitative
habit; the first inducing alterations in accordance with changes of
organic structure, of climate, or of the surrounding fauna and flora;
the other preserving the peculiarities so produced, even when changed
conditions render them no longer necessary. Many facts have been already
given which show that birds do adapt their nests to the situations in
which they place them, and the adoption of eaves, chimneys, and boxes,
by swallows, wrens, and many other birds, shows that they are always
ready to take advantage of changed conditions. It is probable,
therefore, that a permanent change of climate would cause many birds to
modify the form or materials of their abodes, so as better to protect
their young. The introduction of new enemies to eggs or young birds,
might introduce many alterations tending to their better concealment. A
change in the vegetation of a country, would often necessitate the use
of new materials. So, also, we may be sure, that as a species slowly
became modified in any external or internal characters, it would
necessarily change in some degree its mode of building. This effect
would be produced by modifications of the most varied nature; such as
the power and rapidity of flight, which must often determine the
distance to which a bird will go to obtain materials for its nest; the
capacity of sustaining itself almost motionless in the air, which must
sometimes determine the position in which a nest can be built; the
strength and grasping power of the foot in relation to the weight of the
bird, a power absolutely essential to the constructor of a
delicately-woven and well-finished nest; the length and fineness of the
beak, which has to be used like a needle in building the best textile
nests; the length and mobility of the neck, which is needful for the
same purpose; the possession of a salivary secretion like that us