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 (T_b). We investigated at what age the precocial king quail Cortunix chinensis can defend a high T_b 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 (T_s) using an infrared thermometer showed that king quail chicks are partially endothermic at 2-10 days, but can defend high T_b 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 another approached torpor threshold. Although chicks were able to rewarm endogenously from the first torpor bout, metabolism and T_s 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.

Oxygen consumption measurements using open-flow respirometry. Two animals were measured concurrently and were placed individually into 500 ml metabolic chambers in a temperature-controlled cabinet.  Dried outside air was pumped through these chambers at a rate of approximately 300 ml min^-1. By employing two-way solenoid valves, reference outside air and the air from the metabolic chambers was measured sequentially every 9 minutes, 3 min for each channel. Air exiting the chambers was again dried and flow rate was measured with a mass flow meter (Omega FMA-5606); the oxygen content in a 100 ml min^-1 subsample was then determined with an O₂ analyser (FOX Field oxygen analysis system Version 1.01, Sable System, FXO301-01R). Outputs from the flow meter and the thermocouples from each respirometry chamber were digitized via a 14 bit A/D converter (Data Taker DT100), whereas the O₂ analyser was interfaced with the PC directly via a serial port. Temperature control within the climatic chamber, channel switching, calculations and data storage were performed with a custom program written by G. Körtner in Visual Basic 6 (Microsoft Inc.). Oxygen consumption was calculated based on flow rate and the O₂ differential between reference and chamber air using Eqn. 3a from Withers (1977) assuming a RQ of 0.85. This RQ would result in a maximum error of 3% if the RQ was actually 0.7 or 1 (Withers, 1977). Prior to measurements, the span of the O₂ analyser was set against outside air and the mass flow meter were calibrated with a custom-made bubble meter (Levy 1964).

Withers P.C. (1977). Measurement of VO2, VCO2, and evaporative water loss with a flow-through mask. Journal of Applied Physiology. 42, 120-123

Levy A. (1964). The accuracy of the bubble meter method for gas flow measurements. Journal of Scientific Instruments. 41, 449