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Full text of "Man In The Modern World"

THE INTELLIGENCE OF BIRDS
Some very interesting experiments by Professor Rowan of Alberta
have thrown a good deal of light on this mysterious question of the
impulse to migrate. In autumn, he caught a number of birds which
usually leave the regions of an Alberta winter for the south (crows and
the little finches called juncos were the kind he used), and kept them
in unheated aviaries. So long as they were supplied with plenty of
food, they remained perfectly healthy and happy, even with the
temperature many degrees below zero. One lot were simply kept
thus, as "controls" for the experiment: but another lot, in place of
being exposed to the natural shortening of the days in early winter,
had their days artificially lengthened by electric light, a little more
every evening. In midwinter, Rowan liberated a number of birds.
The controls made no attempt to migrate southwards, but just hung
about the place. The birds whose day had been lengthened, however,
for the most part did move away—but apparently most of them
moved north and not south!
Other birds were killed and examined: all the controls, as was
expected, had their reproductive organs shrunken to the tiny size
characteristic of birds in winter; but the long-day birds showed re-
productive organs which were enlarging like those of ordinary wild
birds in early spring about the time of northward migration.
The view held by Rowan—and though it cannot yet be regarded as
completely proved, it certainly seems probable—is as follows. The
extra length of day caused the birds to spend more of their time in
activity, less in sleep; this, by some mechanism we do not yet under-
stand, caused the reproductive organs to begin to grow instead of
shrinking; and the secretions of the reproductive organs control the
migratory urge. When they are shrinking in early autumn, the
changed secretion in the bioocl impels the birds to move south. When
they arc tiny and inactive, as normally in the dead of winter, there is
no impulse to migrate at all; and when they are growing again, the
secretion impels to northward movement, even if the bird be already
in the most wintry and inhospitable conditions.
Whatever the precise interpretation, it is at least clear that the
impulse to migrate is a strange blind urge, controlled and set in
motion by the chemical agency of the reproductive secretions, and
wholly unrelated to reason, or to any consciously-envisaged destina-
tion.
Then again there is the well-known "broken-wing trick" practised
by so many birds when their young are threatened. Most writers of
natural-history books set this down as a remarkable example of in-
telligence :— the bird, seeing its offspring in danger, deliberately
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