Relationships between elevation, reproduction and the hematocrit level of brown-headed cowbirds.

Hematocrit, furcular fat level and oviducal egg presence were determined for 267 brown-headed cowbirds (Molothrus ater) at eight sites ranging from 15 to 2390 m above sea level in California. Near the end of the breeding season males at a mid-elevation site (1250 m) possessed higher hematocrits than females, but there was no difference between yearling and adult males. Birds were bled during three 11-day periods beginning 1 May 1984 at one montane site (2075 m). The percentage of females with oviducal eggs increased significantly from 0 to 10 to 67% and the mean female hematocrit increased from 0.54 to 0.56 and then decreased significantly to 0.52. The hematocrit of males increased significantly from 0.54 to 0.57 and then remained level during the final period. Correlations were calculated between mean hematocrit and elevation for males (r = 0.94, N = 8 sites, P less than 0.001), females (r = 0.96, N = 7 sites, P less than 0.001) and juveniles (r = 0.99, N = 4 sites, P less than 0.01).