POSSIBLE EFFECT OF COMPETITION WITH MIGRANTS ON CLUTCH SIZE IN DESERTAND NONDESERT PASSERINES

Several life history parameters of desert and nondesert (mesic) passer ines 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, len gth of incubation and fledging periods, egg size, length of breeding s easons, and number of clutches laid per breeding season. There are cer tain differences in clutch size between desert and mesic passerines, a nd these differences are significantly related to the proportion of mi grants in each region: the larger the percentage of nonbreeding specie s,the larger the difference in clutch size between the two forms. Thes e results are interpreted as confirming Ashmole's hypothesis: migrants tend to spend more time in nondesert areas, where they compete with r esident 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 nondes ert forms.