Pre-migratory Movements
Recent banding studies have demonstrated many migrants, especially young of the year, have a tendency to disperse after fledging. These premigratory movements have also been called "post-fledging dispersal," "reverse migration," and "postbreeding northward migration." Demonstration of this phenomenon is especially important as it relates to locality-faithfulness (Ortstreue), range extension, and gene mixture between populations. These movements cannot be considered as true migrations even though they are repeated annually by the species between breeding grounds and some other area. These movements are generally repeated by the same age class in the population but not the same individuals.
Nevertheless, these regular northward movements are quite striking, especially in herons. The young of some species commonly wander late in the summer and fall for several hundred miles north of the district in which they were hatched. Young little blue herons as well as great and snowy egrets are conspicuous in the East as far north as New England and in the Mississippi Valley to southeastern Kansas and Illinois. Black-crowned night herons banded in a large colony at Barnstable, Massachusetts, have been recaptured the same season northward to Maine and Quebec and westward to New York. In September most of them return to the south.
These movements have been noted in several other species as well. Broley (1947) nicely illustrated this northward movement of bald eagles along the Atlantic coast ([Fig. 29.]). Birds banded as nestlings in Florida have been retaken that summer 1,500 miles away in Canada. Van Tyne and Berger (1959) surmised the summer heat of Florida was too great for this eagle, a northern species that has only recently spread into Florida to take advantage of abundant food and nesting sites, which it exploits during the cooler season. Postbreeding northward movements are also shared by wood ducks, yellow-breasted chats, eastern bluebirds, and white pelicans.
A somewhat different type of postbreeding migration is the so-called "molt migration" exhibited by many species of waterfowl (Salomonsen 1968). These birds may travel considerable distances away from their nesting area to traditional molting sites where they spend the flightless period of the eclipse plumage. At such times they may move well into the breeding ranges of other geographic races of their species. These movements may be governed by the availability of food and are counteracted in fall by a directive migratory impulse that carries those birds that attained more northern latitudes after the nesting period, back to their normal wintering homes in the south.