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ABSTRACT
Several life history parameters of desert and nondesert (mesic) passerines were compared for five large regions: South Africa, North Africa, southwest North America, Pakistan and northern India, and Australia. Desert and mesic passerines do not differ in their mean body mass, length of incubation and fledging periods, egg size, length of breeding seasons, and number of clutches laid per breeding season. There are certain differences in clutch size between desert and mesic passerines, and these differences are significantly related to the proportion of migrants in each region: the larger the percentage of nonbreeding species, the larger the difference in clutch size between the two forms. These results are interpreted as confirming Ashmole's hypothesis: migrants tend to spend more time in nondesert areas, where they compete with resident passerines, and cause higher mortality there in comparison to desert areas. As a result, during breeding, desert forms face stronger competition from other survivors and lay smaller clutches than nondesert forms.