There is a high degree of concordance among the patterns of geographic size variation in birds in the eastern and central United States. This is demonstrated for 12 species by assuming that wing length measurements are an indicator of body size on the intraspecific level, and by arranging the data in the form of a grid of means of wing lengths for sample areas. Maps giving isophenetic lines for wing length indicate gradually increasing size clines northward and westward from Florida in the Hairy Woodpecker (Dendrocopos villosus), Dowy Woodpecker (Dendrocopos pubescens), Blue Jay (Cyanocitta cristata), Carolina Chickadee (Parus carolinensis), White—breasted Nuthatch (Sitta carolinensis), and Eastern Meadowlark (Sturnella Magna). In each case there is a trend for larger (or longer—winged) birds to extend southward in the Appalachian Mountains and for smaller (or shorter—winged) birds to extend northward in the Mississippi River valley. Maps made by a computer and automatic plotter using contour intervals of 0.5 mm of mean wing length for the Downy Woodpecker, for male White—breasted Nuthatches, and for female Blue Jays show that, in addition to the patter just mentioned, relatively longer—winged birds extend southward in the interior highlands of Arkansas, and relatively shorter—winged birds extend northward up other river valleys. These subtle relationships between intraspecific size variation and topographic features suggest that the link between the two phenomena may be precise adaptations to even minor climatic gradients. The relationship between these findings and the subspecies concept is discussed. Correlation coefficients for the patter of variation in the Downy Woodpecker with seasonal and annual wet—bulb temperature, vapor pressure, and absolute humidity were all either equal to or higher than correlations with dry—bulb temperature. Since these variables reflect the combined effects of temperature and humidity, the obvious indication is that size variation is more closely related to this combination than to temperature alone. Additional correlations using the mean wing length data for seven other species confirmed that wet—bulb temperature patterms are more closely related to bird size than either dry—bulb temperature patters or latitude. These relationship can be expressed numerically as regressions of mean wing length on either annual wet—bulb temperature or mean annual total heat per pound of air. Since increased evaporation at high altitudes and in arid areas accentuates the depression of a wet—bulb thermometer, my hypothesis may partially account for several cases of size variation in birds cited by others as disturbing exceptions to Bergmann's ecogeographic rule. Sections of a translation of Bergmann's paper published in 1847 are given. The biological mechanisms by which these relationships are maintainted are unknown, and the wide range of tolerance by birds to diurnal and seasonal temperature variations tends to mask them. If the well—established inverse relationship between weight and metabolic rate per gram of homeotherms is operative on the intraspecific level, the relationships can be discussed in terms of avenues of heat loss and of energy budget equations.