A modern view, based on studies of living behavior, favors the theory
of photoperiodism, propounded by recent investigators as the cause
of the annually induced movements of the birds. This theory holds as
its major premise that quantity of light and length of day are the
stimulating causes of migration. Its proponents urge that migration
is a phenomenon far too regular to be created anew each season merely
under stress of circumstances, such as need for food; and that it
begins before the necessity for a change in latitude becomes at all
pressing. Swallows, nighthawks (_Chordeiles minor_), shore birds,
and others may start their southward movement while the summer food
supply in the North is at peak abundance; while robins (_Turdus
migratorius_), bluebirds (_Sialia sialis_), and others may leave
an abundant food in the South in spring and press toward northern
points when the food supplies there are almost entirely lacking
and when severe cold and storms are likely to play havoc with the
advance migrants. The regularity of arrival and departure is one of
the most impressive features of migration, and since birds travel in
almost strict accordance with the calendar, the proponents of the
theory ask: What phenomenon to which we may attribute the stimulating
impulse occurs with such precise regularity as the constantly
increasing light in spring?
Experimental work has abundantly demonstrated the effect of increased
light upon the growth, flowering, and fruiting of plants. Similarly,
experiments with the common junco, or snowbird (_Junco hyemalis_),
reported by Rowan (_42, p. 121_), resulted in increased development
of the sexual organs by the end of December, although the birds
were confined in outdoor aviaries in Canada and had been exposed
to temperatures as low as -44° F. From the first of November until
early in January, the juncos were subjected to ever-increasing
light, supplied in the aviaries by electric bulbs. As regards
illumination, they were thus artificially provided with conditions
approximating those of spring. At the close of this period, it was
found that the sexual organs of the birds had attained the maximum
development normally associated with spring. With gradual reduction
of the lighting over a period of little more than 1 month, the organs
returned to their normal winter condition.
After a consideration of all evidence, including the fact that no
ultraviolet rays were used, it was concluded that the explanation
lay in the increased exercise taken during the periods of increased
light. A simple test whereby certain birds were forced by mechanical
means to take more exercise, the light being so reduced that there
was merely sufficient glow for them to see the advancing mechanism
that forced them into movement, showed that the rate of development
of the sexual organs exactly paralleled that in the birds that were
exposed to extended periods of illumination in the outdoor aviaries.
Other features in this experiment--such as the behavior of the birds
themselves--also indicated that more activity due to increased light
is the governing cause of the spring development of the sexual
organs. If this development be accepted as a controlling cause of
migration, then this experiment must be recognized as of great
importance.
Upon closer analysis, however, it is found that this theory, like
those before discussed, is open to serious objections. First, some of
our summer residents that migrate south for the winter do not stop
in equatorial regions, where they might find the periods of day and
night about equally divided, but push on beyond, some penetrating as
far south as Patagonia. Also it might be asked: If the lengthening
day is the stimulating factor, why should our summer birds wintering
in the Tropics ever start northward, as in their winter quarters
the variation in the length of day from winter to summer is
imperceptible. Like all the other theories advanced, this also, as at
present understood, is subject to unanswered criticism.
=WHEN BIRDS MIGRATE=
It is known that at any given point many species leave in fall and
return in spring. Since bird banding has had such wide application as
a method of study, it is known also that in some species one of the
parent birds (rarely both) frequently returns and nests in the same
tree, bush, or box that held its nest in the previous season (fig.
1). One ordinarily thinks of the world of birds as quiescent during
two seasons each year, at nesting time and in winter. For individual
species this is obviously the case, but when the entire avifauna of
the continent is considered it is found that there are at almost all
periods some latitudinal movements.
[Illustration: B12M
Figure 1.--The bluebird may return regularly year
after year to nest in the same hole or box that was occupied in
previous seasons.]