Figure 1. Effect of light at night on seasonal variation in testicular width (A) and plasma testosterone levels (B) in captive adult male European blackbirds (Turdus merula).

Urban (triangles, dashed lines) and rural (circles, solid lines) blackbirds were exposed to simulated natural photoperiods but with different light intensities at night. Control birds (blue) experienced nights with light intensity of 0.0001 lux, while experimental birds (red) were exposed to constant light of 0.3 lux at night. Birds were measured from December 2010 to June 2012. Data represent mean ± SEM. Sample sizes: control = 20 (10 rural and 10 urban), experimental = 20 (10 rural and 10 urban). One urban bird in the control group died on April 1^st, 2011.