Variation in body condition, corticosterone response and immune function is related to the timing of nesting in Franklin’s Gull

Understanding individual variation in adult condition is necessary for developing hypotheses on how nest initiation, chick development and recruitment are related in many migratory birds. We quantified attributes of condition among Franklin's Gull (<i>Lecuophaeus pipixcan</i>) adults initiating nesting at different dates during the nesting period using four metrics: body measurements recorded from live-trapped birds, the corticosterone levels measured from blood samples collected serially from live-trapped birds, heterophil/lymphocyte ratios determined from blood smears and antimicrobial capacity of plasma. Variation in physiological condition was related to the timing of nesting such that individuals nesting later in the season had lower mass relative to skeletal size, increasing corticosterone concentrations measured 3-, 20- and 30-minute post-capture and reduced immune performance. Specifically, residual body mass decreased and keel bone exposure increased with laying date. Additionally, birds nesting later in the season show higher maximum corticosterone concentrations after exposure to acute capture stress along with reduced bacteria-killing capability of their plasma. Our findings indicate that timing of nesting is significantly related to the physiological condition of Franklin's Gull. Individual 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. Quantifying variation in physiological condition within the breeding season will aid in modelling population-level response to shifts in nesting phenology.