Internal Pipping (IP): obligatory or facultative behaviour for successful hatching?

The (invisible) behaviour called Internal Pipping (IP, the puncturing of th e inner egg membrane to bring the bill tip inside the air-cell to breathe a ir) is considered as a regular and obligatory element in the normal hatchin g process of a bird. It does, indeed, frequently occur when about 90 % of t he incubation period has elapsed, shortly before the embryo cracks the eggs hell with its bill (External Pipping, EP). In some bird species (e. g. pige ons, shearwaters, petrels, gulls and terns) EP is reported to precede IP. H owever, in the Rock Dove (Columba livia) and the Diamond Dove (Geopelia cun eata) we could not confirm this observation. In many papers IP is said normally to be especially correlated with a chang e in embryonic oxygen consumption, namely a transition to a plateau phase. At this time respiration changes from chorioallantoic membrane to lungs; th e lungs are filled with air which is thought to be provided exclusively fro m the air-cell. Many authors describe this behaviour accordingly. Neverthel ess, there exist no direct observations or even experimental data to substa ntiate this claim. Our paper deals with this problem: During our long-term experiments on embryonic metabolic rate we first observed that embryos of a t least 17 different bird species frequently hatched and developed successf ully without any signs of a general IP. The membrane of the air-cell was no t pipped (see Table 1). Additional and systematic observations in a professional breeding station o n 1040 eggs of Gallus gallus domesticus showed that in 7-13,5 % of cases em bryos 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 Fowl Gallus gallus do mesticus, Rock Dove, Common Quail Coturnix coturnix, Brown Kiwi Apteryx man telli). These test showed that hatching without IP occurs in 7 to 65 % (mea n value 35.7 %) of cases in all four species without any negative effects o n subsequent ontogenetic development. Evidently, IP is not an obligatory bu t a facultative behaviour in these cases, and probably in general. The aera tion of the lungs can be done a different way within the space of the chori oallantoic membrane; for this reason no air-cell is needed.