The population responds to environmental variability largely determined by the dynamic interactions between fitness components within‐ and among‐individual variation in the expression of the environmentally sensitive phenotype. The study was conducted on daily and seasonal changes in the expression of steroidogenic gene markers and corresponding seasonal changes in the physiological characters in adult male tree sparrows. Two experiments were performed. In experiment one, birds (n = 5/time points) were sampled during the breeding season at 6‐time points, i.e., ZT1, ZT5, ZT9, ZT13, ZT17, and ZT21 [Zeitgeber time (ZT) 0 = sun rise time at the respective time of the year], and daily variation in expression of steroidogenic markers was observed in hypothalamus and testes tissues. In experiment two, birds (n = 5/month) were sampled every month at mid‐day for a year. Body mass, bill color, testes size, and molt in feathers were recorded. The hypothalamus and testes tissues were used for gene expression studies. Blood plasma cholesterol and testosterone levels were measured. Higher testicular volumes were recorded from March to May, whereas maximum molt was observed during the post‐breeding phase. Plasma cholesterol levels were highest before the breeding phase. Higher testosterone levels corresponded with the breeding phase. Higher expressions of thyroid‐stimulating hormone subunit beta (tshβ), type 2 deiodinase (dio2), and gonadotropin‐releasing hormone (gnrh) during the breeding phase and higher expression of type 3 deiodinase (dio3) and gonadotropin‐inhibitory hormone (gnih) were observed during the non‐breeding phase. The steroidogenic transcripts showed seasonal changes in their expression in the hypothalamic and testicular tissue and were upregulated either during the pre‐breeding or breeding phase. The study reveals that mRNA levels of steroidogenic enzymes exhibit daily rhythmicity both in the hypothalamus and testis tissues. Further, steroidogenic transcripts show seasonal variations that correspond to the annual reproductive cycle of the tree sparrow (Passer montanus).
