Diurnal variations in body mass and visible fat scores were measured for seasonally acclimatized Mountain Chickadees (Poecile gambeli) and Juniper Titmice (Baeolophus ridgwayi) to examine if they undergo winter fattening. Body mass varied with time of day and was highest in evening for both species in summer and winter. Body mass, expressed as percent mass increase from morning to evening, was 7.3% for summer chickadees, 7.6% for summer titmice, 9.1% for winter chickadees, and 6.1% for winter titmice. Body mass was not significantly higher in winter-acclimatized birds compared to summer-acclimatized birds. Visible fat scores were significantly elevated in winter-acclimatized Mountain Chickadees relative to summer. Mountain Chickadees and Juniper Titmice appear to have seasonally constant body mass rather than undergoing winter fattening. These data are similar to other North American species in the family Paridae but contrast with data on European parids.