Postnatal temperature triggers predictable thermoregulatory shifts in birds without a trade-off between heat and cold tolerance.

Thermoregulatory competence is key to maintaining fitness in thermally unstable environments. Previous research found that development in the warmth renders birds better able to handle mild heat stress, whereas cold postnatal development gives no apparent benefit upon mild cold exposure. It is still not known how developmental temperature affects maximal temperature tolerance limits, and even less is known about any physiological trade-off between heat and cold tolerance. We investigated how postnatal development under simulated cold snap or heatwave-like conditions from hatching until reproductive maturity impacted the ontogeny of maximal heat and cold tolerance, and the trade-off between them, in Japanese quail (Coturnix japonica). To study whether any such effects are reversible or permanently programmed, we transferred half of each treatment group to intermediate common garden conditions once reproductive maturity was reached and repeated the measurements several weeks later. Development in heatwave-like conditions increased evaporative water loss rate and moved heat tolerance limits upwards, whereas cold snap-like development rendered more thermogenic birds with improved cold tolerance limits. However, we found no evidence for a trade-off between heat and cold tolerance. The common garden birds converged in nearly all thermoregulatory traits at the end of the study, suggesting that the prior emergence of temperature-dependent phenotypes reflected reversible plasticity. We suggest that improved temperature tolerance limits improve performance in matched thermal conditions, reducing the rate at which thermal injury accrues. Yet, in the short term, we found markedly lower capacity to acclimate heat tolerance compared with cold tolerance.