Weights and Wing Areas in North American Birds

Abstract

SEVERAL years ago the writer started to collect data on the relationship between weight and wing area of such species of our native birds as came into his hands. While considerable data on this subject have been collected in Europe, North Africa, and the Malay States, the aim has been in most cases to relate these data to aviation. The method employed has been to compute the entire 'bearing' area of the bird, as its shadow would be cast under a vertical ray of light, wings, tail, and body alike, being considered. On the other hand, my idea was chiefly to note the relationship of the factors of weight and wing area in different groups of birds, the variations within the species, the adjustability of the ratio to different types of flight in such species as habitually vary their method of flight, and to note what effect, if any, the ratio has on the flight characteristics of such species. By measuring only the area of the wing stretched to its full extent it seemed that any relative difference would be expressed more definitely than by considering the entire supporting area, body, tail and wings together. In other words, since the more usual method of flight among birds is by means of flapping, in which the motions of the wings form the supporting and propelling mechanism, it has seemed more logical to consider the wing areas only, rather than include the entire supporting area which functions in soaring flight. I have therefore included in the accompanying table, first, the weight of the bird in grams, taken as soon after death as possible; second, the area of both wings, fully outstretched, in square centimeters, which were closely traced for this purpose, and the resultant outlines measured accurately with a polar planimeter; finally, the ratio obtained by dividing the weight in grams into the area of both wings, as previously determined. After weighing and measuring some hundreds of such species as came into my hands during the past few years, and arranging the results in a graph, it became evident that some rather interesting and unexpected conclusions could be drawn. That there is a normal diminution in relative wing area with the heavier species, will be readily seen. It seems almost incredible that a Loon should have a wing ratio of only 0.56 square centimeters per gram weight, while a Black-chinned Hummingbird shows a ratio of 5.0 square centimeters per gram, almost ten times that of the Loon, and the Golden-crowned Kinglet has a ratio of 8.87 square centimeters per gram, well over fifteen times that of the Loon. One possible explanation for this disparity in ratio may be that there is a constant ineffective marginal area, which may be actually or nearly the