Understanding seasonal variation in adult physiological condition is important for developing hypotheses on how nest initiation, adult condition, chick development and recruitment are related in Franklin's gull and other migratory species of the northern plains. The purpose of this study was to profile physiological condition during the breeding season in nesting Franklin's gull (Luecophaeus pipixcan) adults. Physiological condition was quantified in nesting adults through four metrics: body measurements recorded from live-trapped birds, the corticosterone stress response measured from blood samples collected serially from live-trapped birds, and two measures of immune function (antimicrobial capacity of plasma from blood samples and heterophil/lymphocyte ratios based on blood smears, both taken from live-trapped birds). Physiological condition declined across the breeding season, as shown by a decline in body condition, stress tolerance and immune performance. Specifically, residual body mass decreased and exposure of the sternum keel increased with the progression of the breeding season. Additionally, birds nesting later in the season showed greater maximum corticosterone concentrations in the stress profile along with lower antimicrobial capacity. These results suggest two hypotheses: 1) that timing of nesting has a significant impact on the physiological condition of Franklin's gull and 2) that birds in poorer condition initiate breeding later in the season. Seasonal variation in condition may be related to time constraints observed in temperate latitudes and whether these birds are capital (i.e., acquiring resources outside the breeding area) or income (i.e., acquiring resources locally) breeders. Thus, determining physiological condition during the breeding season is an important step in elucidating how nest initiation, adult condition, chick development and recruitment are related in Franklin's gull.