Do small precocial birds enter torpor to conserve energy during development?

Abstract

Precocial birds hatch feathered and mobile, but when they become fully endothermic soon after hatching, their heat loss is high and they may become energy-depleted. These chicks could benefit from using energy-conserving torpor, which is characterised by controlled reductions of metabolism and body temperature (Tb). We investigated at what age the precocial king quail Cortunix chinensis can defend a high Tb under a mild thermal challenge and whether they can express torpor soon after achieving endothermy to overcome energetic and thermal challenges. Measurements of surface temperature (Ts) using an infrared thermometer showed that king quail chicks are partially endothermic at 2–10 days, but can defend high Tb at a body mass of ∼13 g. Two chicks expressed shallow nocturnal torpor at 14 and 17 days for 4 to 5 hours with a reduction of metabolism by>40% and one approached torpor threshold. Although chicks were able to rewarm endogenously from the first torpor bout, metabolism and Ts decreased again by the end of the night, but they rewarmed passively when removed from the chamber. The total metabolic rate increased with body mass. All chicks measured showed a greater reduction of nocturnal metabolism than previously reported in quails. Our data show that shallow torpor can be expressed during the early postnatal phase of quails, when thermoregulatory efficiency is still developing, but heat loss is high. We suggest that torpor may be a common strategy for overcoming challenging conditions during the development in small precocial and not only altricial birds.