-Both Bank and Barn swallow embryos show an exponential increase in oxygen consumption (M02) during development with a substantial rise in Mo02 occurring upon hatching. The pattern observed in these species is similar to that observed in other altricial birds. Measured pre-pipping M02 values (1.45 and 1.75 cm3O2h-1, respectively) agree well with those predicted on the basis of egg weight, but diverge markedly from predictions based on incubation time and water vapor conductance. The findings here support the recent proposal of Hoyt et al. (1978b) that air cell oxygen tensions are positively related to initial egg weight and that the relationships predicting prepipping M02 on the basis of incubation time and water vapor conductance need to be re-examined. The developmental pattern of oxygen consumption (Mo2) and the magnitude of the pre-pipping M02 has been of great interest to physiologists studying avian eggs (Romanoff 1967, Rahn et al. 1974, Hoyt et al. 1978a, b). It now appears that there are two basic patterns in the ontogeny of oxygen consumption, each associated with the type of young hatched: altricial or precocial. Altricial birds show an exponential increase in oxygen consumption throughout development with a substantial rise in 402 ,upon hatching (Vleck et al. 1979). Precocial species also show an exponential increase in metabolism during development, but before hatching there is a "plateau" in Mo0 (Romanoff 1967, Drent 1970, Rahn et al. 1974, Hoyt et al. 1978b). This stabilization of M0o4 in precocial bird embryos results from a similar stabilization of growth rate during late incubation (Vleck et al. 1979). The pre-pipping XMo0 is also of considerable importance in birds as it is the maximum rate attained while the chick remains within the shell and determines, at least in part, the air cell gas tensions which have been proposed as the stimuli for hatching (Visschedijk 1968, Rahn et al. 1974). Air cell oxygen tension in avian eggs is a function of both No02 of the embryo and egg 0, conductance and is supposedly similar (about 104 torr) for all eggs at the time just prior to pipping (Rahn et al. 1974). Constancy of air cell oxygen tensions assumes that if embryonic M0o2 changes then there will be compensatory changes in egg-shell conductance. This suggestion has recently been challenged by Hoyt et al. (1978b). They have postulated that since embryonic metabolic rate is proportional to the 0.73 power and shell conductance to the 0.81 power of fresh egg mass (the gradient in partial pressure of oxygen is therefore proportional to the -0.08 power of fresh egg mass), air cell oxygen tensions should increase with increased egg mass. Eggs of large birds do have higher calculated air cell oxygen tensions (Hoyt et al. 1978b) so conversely we should expect eggs of small birds to have low air cell oxygen tensions. Air cell oxygen tensions in the eggs of small birds can be estimated from measurements of shell conductance and "plateau" or pre-pipping Mo2. We report here on the ontogeny and pre-pipping values of Mo, in eggs of two species of altricial birds, Bank (Riparia riparia) and Barn (Hirundo rustica) swallows, which also lay small eggs (<2 g). Pre-pipping M402 values are then compared with those predicted from the allometric relationships reported by Rahn et al. (1974), Hoyt et al. (1978a) and Ar and Rahn (1978). Also from these data and those reported on water conductance of eggs from these species (Birchard and Kilgore 1980) air cell oxygen tensions are estimated for a test of Hoyt's hypothesis. MATERIALS AND METHODS Eggs of Bank Swallows and Barn Swallows were collected in June 1978, in southwestern Montana and returned to the laboratory in Missoula. Eggs were then weighed, placed in a numbered cotton nest, and incubated at 37 t 1.50C. Oxygen consumption of each egg was measured daily at 370C using closed-system microrespirometers constructed using the design of Davies (1966). Tem-