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Abstract
A "plateau" stage in the rate of embryonic oxygen consumption (M × O2) has been described in some altricial avian species. More recently, it has been suggested that the difference in the shape of the curve describing M × O2 vs. incubation length (I) between altricial and precocial birds is only due to the duration of the "plateau"; in altricials, it is relatively shorter and unnoticeable in comparison to precocials. In this study, we measured the M × O2 of the developing embryos of the griffon vulture (Gyps fulvus), whose chicks are classified as semi-altricial, where the chick hatches with open eyes, is covered with down, but is unable to stand or leave the nest. Therefore, the question of the presence or absence of a "plateau" in the M × O2 curve vs. I was examined. It was found that in eggs of the griffon vulture, there is a 6-7 days plateau from about day 47 until day 54, just prior to the external pipping. The relatively large mass of the egg (239.8 g ± 23.76 SD, n = 9) and the relatively long I (57 d) enabled enough measurements to detect a typical plateau of M× O2 in vulture eggs. A plateau stage, whose length is proportional to I, should now be considered to be present in all bird embryos.