Seasonality in circadian locomotor activity and serum testosterone level in the subtropical tree sparrow (Passer montanus)

• Tree sparrow exhibits circadian rhythmicity in activity that is confined to light periods.
• They show season-dependent differences in characteristics of circadian activity rhythm.
• Entrained activity rhythm in 12L/12D free runs under LLdim without seasonal change in τ.
• Seasonal changes in activity followed testosterone levels with peaks in spring.
• Testosterone influences circadian system indicating adaptation to local photoperiod.

Seasonality in daily locomotor activity pattern was investigated in the subtropical tree sparrow by exposing a group of birds to natural day lengths (NDL) for 30 days and another group to 12L/12D for 14 days followed by transfer to constant dim light (LLdim) for another 15 days in four different seasons of the year. Serum testosterone levels were also measured during different seasons. Sparrows, under NDL, exhibited distinct circadian rhythmicity in their locomotor activity with almost similar general pattern in different seasons that restricted mainly to the light hours. However, they showed season-dependent differences in the characteristics of circadian locomotor activity rhythm. Birds, when exposed to 12L/12D, showed entrainment of their locomotor activity rhythm with the activity confined mainly during the light phase. Though, tau (τ) under free run conditions did not show any significant difference, the activity period varied significantly in different seasons. The highest level of testosterone was recorded in the spring season that corresponded with the maximum locomotor activity in spring months. The seasonality in daily locomotor activity correlates with the seasonal changes in testosterone levels suggesting the influence of gonadal steroids on endogenous circadian system which is indicative of adaptation of tree sparrow to local photoperiodic conditions.