The recent development of molecular methods for sex identification makes large-scale studies of sex allocation in birds possible. This review identifies three types of sex allocation studies of birds. These relate either to the population primary sex ratio, the facultative manipulation of the sex ratio, or to sex-skewed provisioning. Many recent studies have suggested that facultative sex ratio manipulation may be more widespread among birds than has previously been considered. In contrast, only a few studies have been made of the population primary sex ratio, and of sexskewed provisioning. Despite all these studies, the mechanism for adjusting the sex ratio remains a mystery. In particular, estimating the cost of making sex ratio adjustments is indispensable for quantitative predictions of sex allocation. The relationships between the three aspects outlined above have not yet been sufficiently clarified. In both Budgerigars Melopsittacus undulatus and Great Reed Warblers Acrocephalus arundinaceus, fathers provide extra-feeding for a certain sex of offspring, while mothers manipulate the primary sex ratio. I suggest that the distinction between the sexes in terms of providing extra-feeding or manipulating the sex ratio, may in fact affect the population primary sex ratio and cause a bias towards the sex for which fathers provide extra-feeding.