Summary: The (invisible) behaviour called Internal Pipping (IP, the puncturing of the inner egg membrane to bring the bill tip inside the air-cell to breathe air) is considered as a regular and obligatory element in the normal hatching process of a bird. It does, indeed, frequently occur when about 90 % of the incubation period has elapsed, shortly before the embryo cracks the eggshell with its bill (External Pipping, EP). In some bird species (e. g. pigeons, shearwaters, petrels, gulls and terns) EP is reported to precede IP. However, in the Rock Dove (Columba livia) and the Diamond Dove (Geopelia cuneata) we could not confirm this observation. In many papers IP is said normally to be especially correlated with a change in embryonic oxygen consumption, namely a transition to a plateau phase. At this time respiration changes from chorioallantoic membrane to lungs; the lungs are filled with air which is thought to be provided exclusively from the air-cell. Many authors describe this behaviour accordingly. Nevertheless, there exist no direct observations or even experimental data to substantiate this claim. Our paper deals with this problem: During our long-term experiments on embryonic metabolic rate we first observed that embryos of at least 17 different bird species frequently hatched and developed successfully without any signs of a general IP. The membrane of the air-cell was not pipped (see Table 1). Additional and systematic observations in a professional breeding station on 1040 eggs ofGallus gallus domesticus showed that in 7–13,5 % of cases embryos hatched without any signs of IP (see Table 2). To determine whether this phenomenon is a common event we performed special experiments in four different bird species (Domestic FowlGallus gallus domesticus, Rock Dove, Common QuailCoturnix coturnix, Brown KiwiApteryx mantelli). These tests showed that hatching without IP occurs in 7 to 65 % (mean value 35.7 %) of cases in all four species without any negative effects on subsequent ontogenetic development. Evidently, IP is not an obligatory but a facultative behaviour in these cases, and probably in general. The aeration of the lungs can be done a different way within the space of the chorioallantoic membrane; for this reason no air-cell is needed. [ABSTRACT FROM AUTHOR]