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 (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 posthatching (dph) in response to hypercapnia (2 and 4% CO2) and hypoxia (15 and 10% O-2). Under control conditions, there was a developmental increase in V-E 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 V-E at 10% O-2, by increasing f 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 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 O-2 and CO2 matures early in nestling red-winged blackbirds, well before the ability to increase VO2 in response to cooling, and thus does not represent a limitation to the development of endothermy.