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 fluctuations for two small, nonhoarding (noncaching) passerine species that inhabit central eastern Sweden. Data were partitioned hierarchically into species, age class, gender, and season subgroups. As reported in many small passerine species studies, body mass increased during the day and maximized at dusk; over seasons, body mass increased during autumn, maximized by midwinter, and declined toward breeding in spring. Path analysis models accounted for 9.5–49.9% (mean 26.3%) for Blue Tit (Parus caeruleus) body mass variance and 1.8–52.3% (mean 16.8%) for Great Tit (P. major) body mass variance; for both species, accountability was lowest for autumn (Blue Tit,12.2%; Great Tit, 7.3%), highest for winter (Blue Tit, 33.4%; Great Tit, 21.9%), and intermediate for spring (Blue Tit, 22.7%; Great Tit, 11.8%); for Blue Tits, it was greater for adults than for juveniles (33.2 and 21.7%); whereas negligible for Great Tits (15.9 and 17.3%) and slightly greater for males than for females (Blue Tit, 27.4 and 23.5%; Great Tit, 23.1 and 21.3%). Those results are consistent with predictions formulated on the basis of the adaptive winter-fattening, partially with state-dependent foraging, and, possibly, mass-dependent predation-risk hypothesies and reveal that body-mass fluctuations are associated to a greater extent with photoperiod than with temperature.