56 WHY DO BIRDS MIGRATE? 
birds induced to travel by a more abundant supply of food. Indeed, 
the focusing of so large a number of individuals in a comparatively 
imited area doubtless increases the severity of the competition for 
subsistence. As I have said in discussing the return of the Brown 
Pelicans,* ‘The immediate cause of the journey is doubtless physio- 
logical and the prompting comes from within. With birds, the season 
of reproduction is periodic, and with migratory species, whether the 
journey be to a nearby islet or to another zone, the return to the breed- 
ing ground is only one phenomenon in a cycle of events which 
includes, in regular order, migration, courtship, nest-building, egg-laying, 
incubation, the care of the young, the molt, and the retreat to winter 
quarters’”—or, as might be better said of these tropical and subtrop- 
ical birds, the desertion of the nesting-ground. 
The yearly life-cycle in the vegetable world parallels, in a sense, 
that which exists in the world of birds. In orderly succession the 
plant develops leaf, blossom and fruit, sheds its foliage, and, after a 
period of rest, the phenomena are repeated. With birds it is the return 
of the season of physical fruition which arouses not only the sexual 
but also the homing instinct under the guidance of which these mobile 
creatures repair to the place of their birth. 
Migration, then, in its simplest form, is merely a journey to the 
‘nesting-ground, made without apparent relation to either food or 
temperature. , 
When, now, we turn from these birds which migrate only afew 
miles to others which travel thousands, there is no reason to doubt 
that with both the initial impulse to migrate is in the annual recurrence 
of the period of reproduction. The migration of fish to their spawning- 
ground, and of seals to their ‘rookeries,’ are further examples of jour- 
neys made solely to reach certain breeding-grounds. 
It is not difficult for us to understand why the Pelicans, Boobies, 
Terns and other birds return to certain isolated islets within the area 
of their winter wanderings, but the reasons why birds travel beyond 
subtropical zones to the shores of the Arctic or Antarctic seas are less 
evident. Temperature, as it affects environment and particularly 
as it controls the food-supply, is, with these, a powerful factor. Not 
only must we consider existing climates, but we must take into account 
those profound climatic changes incident to the development and 
passing of the Glacial Period, and which have apparently exerted so 
great an influence on the distribution of life in the northern parts of 
the world that Allen believes we have here the origin of bird migra- 
tion itself. He writes (’80, p. 151): ‘‘Nothing is doubtless more thor- 
oughly established than that a warm temperate or sub-tropical climate 
prevailed down to the close of the Tertiary Epoch nearly to the North- 
ern Pole, and that climate was previously everywhere so far equable 
that the necessity for migration can hardly be supposed to have existed. 
With the later refrigeration of the northern regions, bird life must 
‘Camps and Cruises of an Ornithologist,” p. 88. 
