Breathing while altricial: the ontogeny of ventilatory chemosensitivity in red-winged blackbird (Agelaius phoeniceus) nestlings.

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 (Vt), breathing frequency (ƒ), minute ventilation (V̇e), and whole animal oxygen consumption (V̇o) from hatching to just before fledging in red-winged blackbirds on days 1, 3, 5, 7, and 9 posthatching (dph) in response to hypercapnia (2 and 4% CO) and hypoxia (15 and 10% O). Under control conditions, there was a developmental increase in V̇e with age due to increased Vt Hypercapnic and hypoxic chemosensitivities were present as early as 1 dph. In response to hypoxia, 1, 3, and 9 dph nestlings increased V̇e at 10% O, by increasing ƒ with some change in Vt 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 V̇e by increasing both ƒ and Vt From 5 dph on, the hypercapnic increase in V̇e was accounted for by increased Vt and not ƒ. Chemosensitivity to O and CO matures early in nestling red-winged blackbirds, well before the ability to increase V̇o in response to cooling, and thus does not represent a limitation to the development of endothermy.