Parental care and clutch sizes in North and South American birds

The evolutionary causes of small clutch sizes in tropical and Southern Hemi sphere regions are poorly understood. Alexander Skutch proposed 50 years ag o that higher nest predation in the south constrains the rate at which pare nt birds can deliver food to young and thereby constrains clutch size by Li miting the number of young that parents can feed. This hypothesis for expla ining differences in clutch size and parental behaviors between latitudes h as remained untested. Here, a detailed study of bird species in Arizona and Argentina shows that Skutch's hypothesis explains clutch size variation wi thin North and South America. However, neither Skutch's hypothesis nor two major alternatives explain differences between latitudes.