Songbirds Adjust Oxygen-Carrying Capacity through Modulation of the Number Rather than the Size of Erythrocytes

AbstractOver a lifetime, birds experience changing energy demands, requiring adjustment of their metabolism. Modulation of oxygen-carrying capacity is one way to adjust metabolism to new energy demands, and such modulation depends on hemoglobin concentration. However, little is known about whether the underlying cellular mechanisms controlling hemoglobin concentration are achieved by varying the number and/or size of erythrocytes. Here, we investigated the relationship between four directly measured hematological variables-hemoglobin concentration, hematocrit, erythrocyte number, and erythrocyte size. We used data on 697 individuals from three avian species, zebra finches (Taeniopygia guttata), great tits (Parus major), and common starlings (Sturnus vulgaris), to test our hypothesis that hemoglobin concentration is controlled by erythrocyte number rather than their size. We found that hemoglobin concentration correlates positively with erythrocyte number and hematocrit but that there is no correlation with erythrocyte size. We also observed a weak negative correlation between erythrocyte number and erythrocyte size. Our results support the hypothesis that songbirds adjust their hemoglobin concentration through changes in erythrocyte number, not their size. This finding suggests that modulation of erythrocyte number may serve as a physiological mechanism songbirds apply to regulate oxygen-carrying capacity when energy demands change due to reproduction, physical training, acclimatization, and other demanding activities.