The Annual Stimulus for Migration

Abstract

Among the fascinating aspects of bird migration are its cyclic nature and its temporal precision.Twice annually, in a migratory species, there must be some type of stimulation which causes the development of the complex migratory behavior pattern.It is the purpose of this discussion to present an inventory of our knowledge concerning the nature and mechanism of this periodic stimulation.For present purposes, migration is defined as a regular, usually annual, movement of at least a portion of the population of a species from the breeding area to a non-breeding area, and a return of at least a portion of the individuals to the breeding area.It is definitely oriented and periodic but is not necessarily to be implied to "involve reproduction" as suggested by Gadow, Bullough, Rowan, and others.Care must be exercised in excluding, as non-migratory, many so-called vagrant and sporadic movements; some of these may represent initial phases in the development of more typical migratory movements or modifications of more typical migratory patterns.The traditional German designation (Weigold, 1924, and others) of "instinct" and "weather" migrants likewise should be applied with caution since the establishment of distinct differences is difficult in many instances.The latter could in some instances represent a developmental stage of the former.With their aerial mobility, high metabolic rate, and relatively highly developed instinctive behavior, birds are perhaps the best equipped, among all groups of animals, for the development of migratory behavior.Because of these "preadaptions" for migration and further because of the peculiarly scattered distribution of migratory habits among the families and orders of birds, it appears not unlikely that migratory behavior may have evolved several or many times in the course of the evolutionary history of modern birds and that, in studying migration, we may actually be confronted with many subtle cases of convergent evolution.Thisin turn may mean that the selective processes, the stimulatory mechanisms, the mechanisms of navigation and orientation, and other aspects of migration could be different in different species.With this suggestion in mind, it may be pertinent to present a few remarks concerning some of the pitfalls associated with the interpretation of experimental data and observations relating to migration in general and to the stimulation of migration in particular.(1) The tendency to regard all migration as having a common origin, or at least, as the result of the same factors.This assumption may be true but the available evidence, at present, generally indicates it to be improbable.There seems to be no common denominator in migration other than migration itself.(2) The rather frequent tendency to transfer conclusions concerning one species into explanations concerning other species and to construct general theories based on evidence from one OY a few species.This is a corollary of 1.In some instances transfer may be valid; likewise in many instances it is not.(3) Confusion of correlation zerith cause and effect.This is not an uncommon difficulty in many fields of biologic investigation.Usually this has involved the assumption that in a correlation between two phenomena one must be cause and the other effect, whereas in reality both may be the effects of an undetected cause.An example in migration may well be the correlation between migration and the reproductive cycle such as in the recrudescence of the gonads.(4) Excessive generalization.Few experiments and observations concerning bird migration have true statistical reliability.Most data and observations in t.his field are, from the standpoint of the entire field, suggestive rather than definitive for purposes of generalization.Experiments which will yield significant data are difficult to plan and execute.