Parental care and clutch sizes in North and South American birds.

The evolutionary causes of small clutch sizes in tropical and Southern Hemisphere regions are poorly understood. Alexander Skutch proposed 50 years ago that higher nest predation in the south constrains the rate at which parent birds can deliver food to young and thereby constrains clutch size by limiting the number of young that parents can feed. This hypothesis for explaining differences in clutch size and parental behaviors between latitudes has remained untested. Here, a detailed study of bird species in Arizona and Argentina shows that Skutch's hypothesis explains clutch size variation within North and South America. However, neither Skutch's hypothesis nor two major alternatives explain differences between latitudes.

Occurrence of myeloproliferative disorder in patients with Noonan syndrome.

We report four cases of Noonan syndrome associated with chronic myelomonocytic leukemia in childhood. These children shared some hematologic features: thrombocytopenia, splenomegaly in the first months of life, occurrence of chronic myelomonocytic leukemia without abnormalities of the initial bone marrow karyotype, and, in three cases, improvement of the hematologic disease. A common pathophysiologic process in such patients is suggested.