Migration and the breeding season are periods in which birds face many physical and energetic challenges that may cause changes in stress levels and body condition. Gray Catbirds (Dumetella carolinensis) and Song Sparrows (Melospiza melodia) were captured in 3 consecutive seasons—spring migration, the summer breeding period, and fall migration—in 2014 at a long-term bird-banding station located on the south shore of Lake Ontario to determine patterns of chronic stress and physiological condition related to seasonal shifts in their annual cycles. The heterophil/lymphocyte (H/L) ratio was used to assess chronic stress; and plasma metabolite concentrations (triglyceride and uric acid), along with a scaled body mass index, were used as indicators of the physiological condition and fuel utilization of Gray Catbirds and Song Sparrows. Plasma triglycerides were highest during migration periods for Gray Catbirds, while plasma uric acid was greater in spring and summer than in fall, indicating a seasonal shift in diet, possibly related to seasonal fruit availability. Gray Catbirds had elevated H/L ratios during spring, which suggests that spring migration is a period of modest stress for this species, and this spring stress seemed to be predominantly associated with earlier migration passage through the area. Our results suggest that physiological and immune function can vary across seasons in these species, perhaps in parallel with factors related to food availability at stopover or breeding sites during different times of the year.
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Vol. 135 • No. 1