Background  In high latitude grassland habitats, altricial nestlings hatching in open-cup nests early in the breeding season must cope with cold temperature challenges. Thyroid hormones (triiodothyronine, T₃ and thyroxine, T₄) and corticosterone play a crucial role in avian thermoregulation response to cold. Investigating the endocrine response of altricial nestlings to temperature variation is important for understanding the adaptive mechanisms of individual variation in the timing of breeding in birds.

Methods  We compared nest temperature, ambient temperature, body temperature, plasma T₃, T₄ and corticosterone levels in Asian Short-toed Lark (Alaudala cheleensis) nestlings hatching in the early-, middle-, and late-stages of the breeding season in Hulunbuir grassland, northeast China.

Results  Mean nest temperature in the early-, middle- and late-stage groups was−1.85, 3.81 and 10.23 ℃, respectively, for the 3-day-old nestlings, and 6.83, 10.41 and 11.81 ℃, respectively, for the 6-day-old nestlings. The nest temperature significantly correlated with body temperature, plasma T₃, T₄ and corticosterone concentrations of nestlings. Body temperature of 3-day-old nestlings in the early and middle groups was significantly lower than that of the late group, but there was no significant difference between the nestlings in the early and middle groups. The T₄ and T₃ concentrations and the ratio of T₃/T₄ of both 3- and 6-day-old nestlings in the early-stage group were significantly higher compared to the middle and late groups. The corticosterone levels of 3-day-old nestlings were significantly higher in the early-stage group compared to the middle- and late-stage groups.

Conclusion  Nestlings hatching early responded to cold temperature by increasing thyroid hormones and corticosterone levels even in the early days of post hatching development when the endothermy has not been established. These hormones may play a physiological role in neonatal nestlings coping with cold temperature challenges.