Experimental Reduction of Hematocrit Affects Reproductive Performance in European Starlings, Sturnus Vulgaris

Given the function of hemoglobin and observed increases in hematocrit during periods of increased energetic demands, hematocrit and hemoglobin are assumed to be related to aerobic capacity. Reductions in hematocrit and hemoglobin during reproduction are similar in magnitude to increases associated with aerobically demanding activities and therefore we sought to investigate whether these reductions in hematology have consequences for reproductive performance.We analyzed associations between natural variation in hematology in free-living European starlings (Sturnus vulgaris) and reproductive performance. To test whether transient reductions in hematology during different stages of reproduction (egg production and late incubation/early chick rearing) affected measures of reproductive performance, we also manipulated hematology using phenylhydrazine (PHZ), which lyses red blood cells.To investigate effects of reductions of hematology during egg-laying, we treated females with PHZ or saline (control) upon completion of their unmanipulated first clutch and removed eggs to induce the production and rearing of a replacement clutch. To investigate effects of reductions of hematology during chick rearing, we treated females during incubation of the unmanipulated first clutch and then monitored the subsequent hatching and rearing of the clutch.Individuals with higher hematocrit and hemoglobin initiated nesting earlier. Furthermore, higher hemoglobin levels during incubation were associated with a greater number of chicks fledged. PHZ treatment prior to egg production resulted in a significant delay in the laying of replacement clutches, but had no effect on provisioning rate or the size or number of chicks fledged. PHZ treatment during incubation and early chick rearing resulted in decreased hatchling mass in all years and a decrease in the size and number of fledglings in one of two years. The year that the effect of PHZ was observed appeared to be a particularly difficult year, since hatchling mass, brood size at hatching and at fledging were low among control females compared to other years.Our results suggest that a reduction in hematology during reproduction can be functionally significant, but that these costs are context-dependent.