Sex allocation theory, and its success in predicting sex ratios in such taxa as parasitoid wasps, is often cited as one of the crowning achievements of theoretical evolutionary biology. Its success in some vertebrate taxa, particularly birds, has been more modest. I discuss two reasons for this. First, it is difficult to obtain avian sex ratio data before substantial offspring mortality has occurred. Second, the theory and data required to predict sex allocation patterns (let alone sex ratio patterns) in vertebrates are complex and hard to obtain. Recently developed molecular genetic techniques allowing sex identification from DNA samples have largely solved the first problem and there have been several striking empirical demonstrations of sex ratio biases consistent with sex allocation theory in wild bird populations. Solution of the second problem may come with the incorporation of realistic life history data into models and the use of experimental manipulations to reveal the fitness consequences of allocation strategies. Further data concerning sex ratio variation in taxa such as birds, with chromosomal sex determination, are valuable because they allow the investigation of the role of constraint vs. adaptation in evolution.