What determines levels of genetic diversity in mitochondrial DNA remains unresolved. We have investigated the factors that are correlated to the level of synonymous diversity of mitochondrial DNA in more than 300 bird species. We find that diversity is significantly correlated to clutch and range size, but not significantly correlated to many other variables including body mass, latitude, and longevity. The correlation between diversity and range appears to be a consequence of a correlation between range and effective population size since a measure of the effectiveness of natural selection, which is expected to be correlated to the effective population size, is also correlated to range. The slope of the relationship between diversity and range is shallow, consistent with Lewontin's paradox, and very similar to the relationship found in mammals.