Hematocrits in Montane Sparrows in Relation to Reproductive Schedule

Hematocrits were measured in migratory Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) on their high altitude (3,000 m) breeding grounds during seven consecutive summers. Contrary to expectations, hematocrits were highest in newly arrived individuals then declined thereafter until the end of postnuptial molt. They then increased prior to autumnal migration. Hematocrits were higher in males than in females until onset of parental care but not thereafter. A large, rapid decrease in hematocrit was observed in females prior to ovulation and it remained low during oviposition. Hematocrits were at their lowest in both sexes during postnuptial molt. These changes in relative quantity of erythrocytes may involve a variety of underlying causes. Among them are the metabolic demands associated with migration and thermoregulation, differential effects of sex steroids on erythropoiesis, osmotic effects on plasma volume when nutrient loads in plasma are high, such as during ovogenesis and perhaps molt, and transient changes in blood volume during molt.