AVIAN REPRODUCTION OVER AN ALTITUDINAL GRADIENT: INCUBATION PERIOD, HATCHLING MASS, AND EMBRYONIC OXYGEN CONSUMPTION CYNTHIA CAREY, 1 EDWARD L. THOMPSON, 1 CAROLM. VLECK, 2'3 AND FRANCES C. JAMES 4 •Department of EPO Biology, University of Colorado, Boulder, Colorado 80309 USA, 2Department of Zoology, University of Washington, Seattle, Washington 98105 USA,and 4Department of Biological Science, Florida StateUniversity, Tallahassee, Florida 32306USA ABsTK•CT.--Embryonic oxygen consumption, incubationperiod, and hatchling mass do not vary significantly among populationsof Red-winged Blackbirds (Agelaius phoeniceus) breeding overa 2,900-m altitudinal gradient, despite a 28% decrease in oxygen tension from the lowest to the highestaltitude. Calculations suggest that embryosdevelopnormally at 2,900m at air-cell O2 tensions of 7.1 kPa. The regulation of lossof water vapor or CO2by a reduction of eggshell conductance appears to have been more importantfor populations breeding in montane environments than maximizing O2 availability to the embryo. The superior tolerance to hypoxia demonstrated by embryos of wild birds, compared to that of embryos of domestic chickens, may relate to the modification of the diffusiveresistance of O2 inside the shell. Received 28 September 1981, accepted 24 April 1982. THEability of a species to invade a new hab- itat and to establish permanent residency there depends importantly upon successful repro- duction. Avian species have exploited a re- markable diversity of habitats. This success is particularly striking in view of the potential vulnerabilityof avian embryos to environmen- tal stresses. Because avian embryos develop externally from the body of the adult, they are less protectedby the homeostatic systems of the adult than are embryos of viviparous ver- tebrates. Although the physiology of avianem- bryos has received considerable attentiondur- ing the last decade, embryonic tolerances of and adaptationsto harsh environments have received little study. Avian embryos exchange gases with the en- vironment by diffusion (Wangensteen et al. 1970/71). The diffusive flux of O2 into the egg, and CO2 and water vapor outwards, is de- scribed by a modification of the Fick equation (Wangensteen et al. 1970/71, Paganelli et al. 1975): 19I = (D/RT).(Ap/L)'•IP (1) a Present address: Department of Ecology and Evo- lutionary Biology,University of Arizona, Tucson, Arizona 85721 USA. 710 where/9I = gas flux (cm a STPD' s-•), D = dif- fusion coefficient (cm 2' s •), Ap = effective pore area (cm2), L = length of diffusion pathor shell thickness (cm), RT = gas constant and abso- lute temperature (cm a STPD' cm -a'kPa-•), and •P = partial pressure difference of gas across shell (kPa). The terms (D/RT)' (Ap/L) are often combined into the term "G" (cm -a' s -•' kPa •), representing the conductance of theeggshell to eachgas (Ar et al. 1974). The progressive decrease in barometric pres- sure, Pn, in montane habitats is an environ- mental gradient that posesdistinct problems for diffusive respiratory systems. Equation 1 suggests two factors that should become pro- gressively more important to avian develop- ment as Pn falls with increasing altitude. First, O2 flux (/9Ic•) is influenced by thePc• difference across the eggshell (APo2). Because ambientPo2 falls as Pn decreases, the driving force for O2 diffusion will decrease unless internal Po• is lowered commensurately. Therefore,an em- bryo maynot obtainsufficient O2above certain altitudes to sustain normal metabolism and growth. Second, theoretical predictions and empiricalresults show that D is inversely pro- portional to Pn (Reid and Sherwood 1966,Pa- ganelli et al. 1975). Therefore, gases exchanged between the embryo and the environment will diffuse more rapidly as Pn decreases with al- The Auk 99: 710-718. October 1982