SEASONAL AND DIURNAL BODY-MASS FLUCTUATIONS FOR TWO NONHOARDING SPECIES OF PARUS IN SWEDEN MODELED USING PATH ANALYSIS

Abstract According to the hypothesis that has been invoked most frequently to explain seasonal fattening patterns for birds—the “adaptive winter-fattening hypothesis”—individuals respond to worsening foraging conditions by increasing body mass and energy reserves. Two hypotheses have been proposed equally frequently to explain daily weight gain patterns for birds: according to the “state-dependent foraging hypothesis,” energy reserves should be amassed early during the day, when starvation risk increases; according to the “mass-dependent predation-risk hypothesis,” mass gain should be delayed for as long as possible, to minimize predation risk. Those hypotheses have been tested previously, using statistical methods (e.g. multiple-regression analysis) that assume independence among environmental variables (e.g. photoperiod and temperature). We conducted path analyses that included four predictor variables (day-in-season, hour-in-day, mean daily temperature, and daily precipitation) to model body-mass fluct...