The genetic diversity-area relationship (GAR), compared to the extensively explored species-area relationship (SAR), remains poorly recognized despite the importance of understanding it for the development and application of biodiversity theory. It has been hypothesized that maintaining genetic diversity within a population is mechanistically similar to maintaining species diversity within a community, implying that GAR trajectories should mathematically behave as SAR ones. Here we test this prediction by fitting microsatellite heterozygosity and allelic richness in relation to distribution range size across bird species against eight well-known SAR models. The Monod model best described the data of resident and migratory species combined, and especially the data of resident species only, showing that with increasing range size, genetic diversity across species rapidly increased up to a certain level and then tended towards an asymptote. None of the candidate models provided an adequate fit for the data of migratory species, likely because their breeding range size mostly is large in that a GAR curve has become flat. Our work takes the first step towards formulating GARs and applying them to predicting the effect of habitat fragmentation on genetic diversity.