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Recent studies suggest that a linear correlation exists between the age of individual birds and accumulation of the advanced glycation endproduct pentosidine (Ps). We compared Ps accumulation in skin tissue of Ruffed Grouse (Bonasa umbellus) and Double-crested Cormorants (Phalacrocorax auritus). Skin samples were taken from the breasts of 52 Ruffed Grouse of known ages ranging from a few days to 10 years. Skin was cleaned of feathers and adipose tissue, acid hydrolyzed, and measured for collagen content via hydroxyproline spectrophotometric assay. Pentosidine concentrations were determined using reverse-phase high-performance liquid chromatography. Fluorescent detection (excitation: 310 nm, emission: 385 nm) revealed that concentrations of Ps per milligram of collagen increased curvilinearly with age (r2 = 0.87, P < 0.0001). Additionally, a blind study of 19 Double-crested Cormorants revealed a linear relationship between Ps concentrations and age (r2 = 0.93). Comparison of these two species revealed a more rapid accumulation of Ps in Ruffed Grouse. Accurate age information could aid species recovery programs and provide insights into longevity, now understood entirely from banding programs and captive birds.