Developing young are particularly sensitive to environmental challenges, often with life-long consequences, but the risks of climate warming during this period are not well understood. To quantify the physiological effects of increasing temperatures for developing endotherms, we measured metabolic rate, water loss, and heat dissipation behaviours between 25-45°C in nestlings of a free-living songbird. The thermoneutral zone ranged from 33.1–42.3°C – higher and narrower than adults – and metabolic rate increased sharply above and below this range. Water loss was constant below 33.5°C, above which it increased sharply, when nestlings also drooped their wings to lose heat. Despite nestlings initiating panting (p50) around 40°C, water loss was never sufficient to dissipate metabolically produced heat, indicating poor cooling capabilities. Our data show that while developing young appear relatively tolerant to higher temperatures, they are at a high risk of dehydration and hyperthermia, with limited ability to mitigate these risks of increasing temperatures.