Altricial bird species, like red-winged blackbirds, hatch at an immature state of functional maturity with limited aerobic capacity and no endothermic capacity. Over the next 10-12 days in the nest, red-winged blackbirds develop increased metabolic capacity before fledging. Although ontogeny of respiration has been described in precocial birds, ontogeny of ventilatory chemosensitivity is unknown in altricial species. Here we examined developmental changes in chemosensitivity of tidal volume (V_T), breathing frequency (f), minute ventilation (V_E), and whole-animal oxygen consumption (Vo₂) from hatching to just before fledging in red-winged blackbirds on days 1, 3, 5, 7, and 9 post-hatching (dph) in response to hypercapnia (2 and 4% CO₂) and hypoxia (15 and 10% O₂). Under control conditions, there was a developmental increase in V_E with age due to increased V_T. Hypercapnic and hypoxic chemosensitivities were present as early as 1 dph. In response to hypoxia, 1, 3, and 9 dph nestlings increased V_E at 10% O₂, by increasing f with some change in V_T in younger animals. In contrast to early neonatal altricial mammals, the hypoxic response of nestling red-winged blackbirds was not biphasic. In response to hypercapnia, 3 dph nestlings increased V_E by increasing both f and V_T. From 5 dph on, the hypercapnic increase in V_E was accounted for by increased V_T and not f. Chemosensitivity to O₂ and CO₂ matures early in nestling red-winged blackbird, well before the ability to increase Vo₂ in response to cooling, and thus does not represent a limitation to the development of endothermy.