A Preliminary Report on Some Experiments on Bird Migration

discovered that it was possible to cause recrudescence of the gonads of birds after the breeding season by artificially lengthening the periods of daylight, he performed several experiments on bird migration which involved the releasing of birds with their gonads at various stages of development.From his results Rowan concluded that birds released with their gonads at winter minimum or spring maximum showed no inclination to migrate, whereas those released with their gonads either in a state of recrudescence or regression departed upon gaining their freedom.From histological studies, he concluded that interstitial tissue occurs abundantly in gonads that are regressing or recrudescing, but is "wanting in the resting state of the winter testis and if present at all in the summer is but sparsely represented" (op.cit., p. 203).These conclusions are the basis for Rowan' s theory of the mechanism of annual migration.This theory states that the secretion of the interstitial cells of the gonads is responsible for arousing migratory behavior, and that with the disappearance of this tissue the stimulus to migrate lapses.From the results of later work with crows (Proc.Nat.Acad.Sci., vol.18, 1932, pp.639-654), however, Rowan concluded that the southward passage "appears to be independent of the influence of the gonads."The problem of how the increasing day length induces gonadial development has received the most emphasis since Rowan' s initial work, but little or no experimentation has been done on migration per se.To relate the recent discoveries in endocrinology to migration, Bissonnette (Wilson Bull., vol.49, 1937, pp.262-264) has proposed a theory involving the relationship of the gonads and pituitary.He believes that there is an inherent rhythm of activity of the pituitary and that the gonads, therefore, are controlled by the cyclic activity of this gland.Birds migrate in the fall due to the regression of pituitary activity and do not breed on the wintering grounds because this gland, and hence the gonads, remain in the "refractory phase."On recovery of the pituitary the birds are stimulated to migrate before, or while, their sex glands recrudesce.Eventually the recrudescence of the gonads reaches such a point as to induce migration to stop and mating to occur.Detention in the winter range, therefore, should be followed by a. breeding cycle there.According to Rowan and Bissonnette, migratory birds which are retained on their wintering grounds and released when their gonads attain breeding condition should mot migrate.The purpose of this report is to describe two experiments undertaken to test this hypothesis.Method.-Thespecies used was the Oregon Junco (Junco oreganus), a type found commonly on the Pacific coast and in the interior of western North America.It was chosen for two reasons.First, the birds of this species could be obtained in adequate numbers and kept in captivity.Second, and most important, the species contains both migratory and resident races.The migratory races used were J. o.