Lipid metabolites as markers of fattening rate in a non-migratory passerine: effects of ambient temperature and individual variation.

Plasma lipid metabolites triglycerides (TRIG) and glycerol (GLY) are used as indicators of fattening rate and nutritional condition in migratory birds. Requiring only one blood sample, they could also be used for studying daily and seasonal fattening rates in relation with habitat quality or weather variations in species wintering in cold climates. Using black-capped chickadees exposed to three experimental temperatures (0 °C, 15 °C, and 30 °C), the goal of this experiment was to determine the relationship between plasma levels of TRIG and GLY and fattening rate measured over periods from a few hours to the previous two days. Results showed that birds maintained in the cold had metabolite levels 39-81% higher than those at thermoneutrality, likely reflecting the size of their fat reserves, and that TRIG and total GLY were highly correlated across treatments. Fattening rate was also higher at 0 °C (+35%) and 30 °C (+24%) relative to that measured at 15 °C and, as expected, was positively correlated with metabolite levels across treatments. However, despite fattening rates similar to that observed at the other temperatures, the relationships were uncoupled at 30 °C, implying that the technique may not be easily applicable at temperatures within or close to thermoneutrality. We also found a strong individual effect in the relationships between fattening rate and TRIG levels, suggesting high individual consistency in these parameters in conditions of unrestricted food access such as in captivity. Our study confirms that plasma TRIG and GLY levels can be used as relative indexes of condition and fattening rates in wintering passerines.