The occasional great invasions beyond the limits of their normal range of certain birds associated with the far North are quite different from migration patterns discussed previously. Classic examples of such invasions in the eastern part of the country are the periodic flights of crossbills. Sometimes these migrations will extend well south into the southern States.
Figure 29. Northern recoveries of young bald eagles banded as nestlings in Florida. The birds sometimes "migrate" over 1,500 miles up the coast during their first summer before returning south (From Broley 1947).
Snowy owls are noted for occasional invasions that have been correlated with periodic declines in lemmings, a primary food resource of northern predators. According to Gross (1947), 24 major invasions occurred between 1833 and 1945. The interval between these varied from 2 to 14 years, but nearly half (11) were at intervals of 4 years. A great flight occurred in the winter of 1926-27 when more than 1,000 records were received from New England alone, but the largest on record was in 1945-46 when the "Snowy Owl Committee" of the American Ornithologists' Union received reports of 13,502 birds, of which 4,443 were reported killed. It extended over the entire width of the continent from Washington and British Columbia to the Atlantic coast and south to Nebraska, Illinois, Indiana, Pennsylvania, and Maryland. One was taken as far south as South Carolina.
In the Rocky Mountain region, great flights of the beautiful Bohemian waxwing are occasionally recorded. The greatest invasion in the history of Colorado ornithology occurred in February 1917, when it was estimated that at least 10,000 were within the corporate limits of the city of Denver. The last previous occurrence of the species in large numbers in that section was in 1908.
Evening grosbeaks likewise are given to more or less wandering journeys, and, curiously enough, in addition to occasional trips south of their regular range, they travel east and west for considerable distances. For example, grosbeaks banded at Sault Ste. Marie, Michigan, have been recaptured on Cape Cod, Massachusetts, and in the following season were back at the banding station. Banding records and museum specimen identifications demonstrate that this east-and-west trip across the northeastern part of the country is sometimes made also by purple finches, red crossbills, and mourning doves.
ORIGIN AND EVOLUTION OF MIGRATION
The origin and evolution of bird migration has been discussed in ornithological literature for centuries. As we have seen from the foregoing discussion, migration exists in many forms throughout the world and probably arose to satisfy many different needs in different orders of birds at the same time. New pattens, traditions, and routes are arising today as well as disappearing. Currently, the migration patterns we see are a composite result of historic influences mixed with present day influences. Even though the migration of several different species may be very similar, the patterns exhibited today can be the result of quite different evolutionary processes. Because it cannot be substantiated by experimental facts, any explanation of how a particular pattern or route originates is pure conjecture.
The general anatomical and physiological attributes of birds enable them to develop more diverse and spectacular migratory behavior than any other group of animals. Their potential for long sustained flights is of primary importance in pre-adapting birds to successful migrations. Migration has long since become a definite hereditary habit of many species of birds that recurs in annual cycles, evidently because of physiological changes which prompt a search for an environment suitable for reproduction and survival. Like the bird's other habits its migratory behavior is just as characteristic as the color of its plumage and, like it, evolved through natural selection because it was advantageous for the survival of the population. Its origin has been thought by some to be a mystery locked in past ages, but by study of the history of how birds came to occupy their present ranges, information becomes available which suggests theories that may be developed and explored. Two that are commonly mentioned are termed the "Northern Ancestral Home Theory" and the "Southern Ancestral Home Theory."
According to the former of these hypotheses, in earlier ages when conditions of climate, food, and habitat were favorable for existence of birds throughout the year much further north than is the case today, many species remained in these northern latitudes as permanent residents. Today, such conditions are found only in more southern regions where migrations are much shorter or nonexistent. Gradually, however, in the Northern Hemisphere the glacial ice fields advanced southward, causing a southward movement of conditions favorable to northern birds, until finally all bird life was confined to southern latitudes. As the ages passed, the ice cap gradually retreated, and each spring the birds whose ancestral home had been in the North moved in again to fill newly opened breeding habitat only to be driven south again at the approach of winter. As the size of the ice-covered area diminished, the journeys to the summer breeding areas became even longer until eventually the climatic conditions of the present age became established, and with them, present patterns of the annual advance and retreat we call migration.