The Chihuahuan Desert (CD) covers a large part of Mexico and the south-west of the United States and is one of the richest deserts in the world. Most species in arid North America are resident species (56%), but only a fraction (14%) is primarily restricted to arid environments. The highest number of species is found in shrublands, but a set of species are mainly found in grasslands. Within the CD, the Cuatro Cienegas Basin (CCB) has a high number of plant and animal taxa. When compared to other local areas, it is only surpassed by the Tehuacan-Cuicatlan Valley in southern Mexico. With the exception of the Tehuacan Valley, there is a negative relationship between precipitation and bird richness. This could be because the range of values between sites is so low that it is not enough to have a noticeable effect on diversity. In contrast, latitude could explain the differences in species richness, since southern sites have a markedly higher number of species. This pattern shows the importance of the neotropical component in increasing the number of species. In addition, the diversity found in the CCB can be explained by the presence of relatively large water bodies that must attract a number of birds. In particular, migratory and transient species use the area as a stopover site on their migratory routes. Furthermore, the CCB has a number of different vegetation types which share few species among them. This must also contribute to the surprisingly high richness found in the region.