The Migration Impulse of Our Avifauna 
By Paul B. Riis 
TWICE annually we witness the mysterious phe- 
nomenon of bird migration as they fly on to the 
North to their visiting grounds in the Spring 
and again in the Fall when they return to their Winter 
range. We have grown so used to these flights that we 
accept them as indifferently as the changing of the sea- 
son. But back of this movement, a historv is concealed 
reaching far down the ages and revealing itself to us in 
flashes only, permitting but a fleeting glimpse of the in- 
ception of the migrating impulse. 
Migrations among certain mammals, such as the seal to 
its breeding grounds, the fishes to their spawning grounds 
and some species of insects and butterflies to warmer 
climes, have long been recognized and bear a strong re- 
semblance to the movements of birds. Webster's defini- 
tion of migration is : "To remove from one country to 
another ; to pass periodically from one region or climate 
to another. Any bird, therefore, which leaves its feed- 
ing grounds to go to its breeding grounds, no matter how 
near or far away, is a migratory bird. We may well in- 
clude the blue jays, chicadees, downy and hairy wood- 
peckers and white breasted nuthatches among these, since 
their movements North and South to nesting sites are 
fairly evident. 
According to Frank M. Chapman in his "Handbook 
of Birds of Eastern North America" : migration is the 
most distinctive phase of bird life. The yearly life-cycle 
in the vegetable world parallels in a sense with 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 is re- 
peated. With birds it is the return of the season 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 grounds, made without 
apparent relation to either food or temperature. 
Migrations are taking place all the time though they 
are of a less marked character. Those birds termed 
"permanent residents" are so called because they are 
represented in a given locality the entire year, yet they 
also make short and gradual migrator}- excursions, those 
moving South from a given locality being supplanted by 
new arrivals from the North. The "summer residents" 
are distinguished as birds which come to us from the 
South, rear their young and depart again in the Fall. 
The "transient visitants'' are the countless birds which 
pass through here in the Spring to nest at points further 
North, passing through here again in the Fall on their 
way South. The "winter residents" include those species 
which visit us in the Fall and spend the Winter with us, 
moving to points North in the Spring to nest. Here, in 
Rockford (111.), we have had cardinals, tufted titmouse 
and red billed woodpeckers as Winter residents the past 
Winter. All of these birds come from southern locali- 
ties, so that the above statement cannot be applied to 
these birds. Their presence here merely denotes a 
spreading of the species to new ranges, and it has been 
observed that inexplicably this spreading of southern 
species has almost invariably taken place in the Winter 
with a possible retreat South in Spring. 
The birds to arrive earliest in Spring are also usually 
the last to depart in Fall. During January here we gen- 
erally find the birds stationary with occasional visits from 
snow buntings and red polls. During February a gradual 
movement among the Winter residents takes place, and 
the number of our chicadees, downy and hairy wood- 
peckers decreases and new arrivals such as the bluebirds 
and robins towards the end of the month may be seen. 
From this date on, especially during March, the migra- 
tion becomes more marked, although the dates of arrival 
and departure are dependent on weather conditions. The 
migrations continue all through April and reach their 
zenith during the first half of May. After this date, the 
birds decrease in numbers and are then either permanent 
residents or Summer residents. 
The month of June is given over to family cares, build- 
ing of nests and rearing of young. Towards the end 
of June, but more so during July, the one brooded birds 
may be seen flocking together and establishing roosts 
or wandering aimlessly over the country preparatory to 
the migration South. The post nuptial molt takes place 
in August and after the middle of this month bobolinks, 
swallows, martins and red winged blackbirds can be seen 
wending their way southward, followed in September by 
the majority of birds. The earliest Winter residents 
appear towards the end of this month. Migration at this 
season is the reverse of the Spring movement, the birds 
decrease gradually until the earliest Spring arrivals have 
also left us. A few stragglers may still be seen in De- 
cember if the season is sufficiently open, but generally the 
permanent residents only and Winter visitants of the 
locality can be found then. 
As one might expect, the insectiverous birds travel 
farther South than the granivorous birds. But here 
there are also exceptions, and the reason for this must 
be sought elsewhere from the mere question of food 
supply. It can be satisfactorily explained that such seed 
eaters as the bobolinks, wintering in Bolivia, are an 
American type of austral origin, while such insect eaters 
as the winter wren and the kinglet, which remain with 
us, are European types of boreal origin. The homing 
The adult 
and the 
last flocks may be composed entirely of young birds. The 
migrants also are distinctly diurnal or nocturnal travel- 
lers, and the respective times chosen are intimately in- 
fluenced by the feeding habits and dispositions of the 
birds. The height of flight varies from a few yards to 
a distance of four to five thousand yards, but usually 
an altitude of 1,600 yards (nearly a mile) is reached. 
Temperature apparently is no factor in the movement 
nor are the birds prompted to seek new feeding grounds. 
Allen expresses his beliefs thus: The immediate cause 
of the journey is doubtless physiological and the prompt- 
ing comes from within. With birds, the reason of re- 
production is periodic, and with migratory species, 
whether the journey be to a nearby islet or to another 
zone, the return to the breeding 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 young, the molt, the re- 
treat to Winter quarters, or as might better be said of 
these tropical and subtropical birds, the desertion of 
the nesting ground. Not only must one consider the 
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 ap- 
parently vested so great an influence on the distribution 
of life in the northern parts of the world, so that we 
believe to have here the origin of bird migrations. 
Nothing is doubtless more thoroughly established 
than that a warm temperature or subtropical climate 
instinct here again shows very markedly, 
birds usually migrate first, decreasing steadily. 
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