Climographic Studies of Certain Introduced and Migratory Birds

Abstract

Considering the relation of animals to their environments, one may conclude that where natural factors limit the range or distribution of a species, usually very few complicated factors are involved. The extent to which these factors actually limit in their effect ordinarily involves not the entire life-cycle of the species or the individual, but some particular or critical period of the life of the organism. Of all the periods in this life cycle, the breeding season followed by that in which the young are raised, is undoubtedly the most critical. At such periods the environmental conditions must be as nearly optimum as possible else catastrophe results. By optimum is meant that which is most suited to the physiological requirements of the individual. The toleiance of the adult is greater than that of the young, and if the conditions under which the young are raised are too far from this optimum, the young will perish though the adult may survive, but such a sequence of events quickly results in the extermination of the race. It is not the purpose to review here the many studies of physiological conditions and physical factors which control the movements of an organism. Many studies have been made involving temperature, humidity, raiiifall, and radiation, as individual factors influencing or limiting the distribution of animals, but there are few studies which take into consideration a possible combination of two or more of these factors as indicating the range preference or range limitations of a species. The author has been unable to find any work using two factors as a means of measuring or comparing certain phases of the migration problem of animals, or to explain either the obvious success or equally obvious failure of species introduced into a new environment. Irhe purpose of this study is to present two distinct problems of avian life history by means of duplex data involving both temperature and humidity. These two problems are: (1) the problem of the introduced species: why it is either a success or a failure; and (2) a comparison of the summer and winter ranges of certain migratory birds as shown in terms of these two factors, temperature and humidity. The first involves the question: Can we, in terms of temperature and humidity, show any cause or reason why certain species have been introduced successfully into certain regions and entirely unsuccessfully into others? The second involves the questions: