Geographic variation in the ecological energetics and physiology of the northern cardinal (Cardinalis cardinalis) in winter.

Abstract

I collected Northern Cardinals during the winters of 1993 and 1994 in IA, MO, and LA (western transect) and MI, IN, and AL (eastern transect). Live body; lean dry; lipid; and dry pectoralis muscle, heart, liver, kidney, and intestine masses were determined for 175 males, and basal metabolic rate (BMR) was determined for 36 other males to assess how cardinal physiology varies in response to a range of winter conditions. Overnight depletion of lipid confirmed that cardinals use energy at rates similar to those predicted from ambient temperature, nightlength, and body mass. Live body and lean dry masses varied geographically in accordance with temperature; birds were larger where it was colder. Lipid mass, in contrast, increased with latitude, but reached a plateau at mid-latitude and northern sites. As a result, cardinals at northern sites were predicted to have less fasting endurance and lower survival during an all-day fast than cardinals living farther south. Lipid indices (LI, g lipid/g lean dry mass) were more strongly correlated with long-term temperature and daylength variables than with short-term temperature variables, suggesting that temperature is the ultimate, and daylength the major proximate, cue to fattening. Temperature and snow cover did not explain why LI plateau or decline in the north. Masses of intestine, kidney, and liver, both absolute and mass-adjusted, increased with latitude in accordance with energy requirements. Pectoralis muscle and heart masses, and BMR increased with latitude, but peaked at mid-latitude sites. Plasma triiodothyronine increased with latitude in accordance with decreasing ambient temperature and increasing energy requirements, while plasma thyroxine consistently dipped at mid-latitudes and was not strongly correlated with ambient temperature. Neither hormone was significantly related to body fat or BMR in free-living or captive cardinals. These results suggest that cardinals living near the northern range edge experience limited food availability and are more susceptible to starvation during inclement weather than cardinals living elsewhere. Thus, an energetic limitation at higher latitudes may determine the northern edge of the cardinal's range.