Bird Migration at Different Latitudes in Eastern North America

We attempt to quantify the relationship between migration and latitude in the avifauna of eastern North America. Progressing northward up the east side of the continent, the proportion of breeding species that moves south for the winter increases steadily, from 12% at 25°N (southern Florida) to 87% at 80°N (Ellesmere Island), a mean increase of 1.4% for every degree of latitude. Conversely, the proportion of wintering species that moves north for the summer decreases with latitude, from 52% at 25°N to none at 70°N, a mean decrease of 1.1% for every degree of latitude. These relationships hold despite the fact that 24% of all species breeding in eastern North America leave the area completely in fall to winter south of 25°N, mostly in Central and South America (including the Caribbean Islands). These trends are similar to those in western Europe, but at any given latitude an average of 17% more breeding species leave for the winter in eastern North America, and 10% more wintering species leave for the summer. This difference is attributed to climate, in that at any given latitude temperatures are cooler in eastern North America than in Europe. We argue that relationships between migration and latitude exist because latitude is a good surrogate measure of factors likely to more directly influence migration, such as climate and daylength, which in turn control the amplitude of seasonal changes in food supplies.