Endocrine patterns during aging in the common tern (Sterna hirundo)

Blood samples were taken from breeding common terns (Sterna hirundo) of known age during two successive breeding seasons to determine if plasma concentrations of estradiol, progesterone, androgen, and luteinizing hormone (LH) reflect the process of aging. Males and females were trapped and sampled once between laying of the first and the second eggs in the clutch. The transient capture and sampling did not disrupt incubation or the timing of the subsequent oviposition. Ages ranged from 2 to 21 years, with most in the range 4-11 years. There was a strong inverse correlation (P < 0.0001) between age and laying date in both females and males, with older birds nesting earlier in the season. Plasma LH levels increased (P < 0.005) among mature (>5 year) females. Plasma progesterone and E2 were positively correlated with each other among mature females, but did not change in an age-dependent manner. In males, plasma progesterone levels showed no age-related changes. However, maturing (3-5 year) males showed increasing plasma androgen levels (P < 0.01); mature males showed no change with age. Plasma LH levels were correlated with androgen levels in both maturing and mature males. Plasma progesterone declined with age in maturing males (P < 0.001). These data provide evidence for changing endocrine status with age, but the patterns differ for younger individuals (