Is there a built-in conflict in the function of the avian brood patch?

The avian embryo is sealed in the egg and is anatomically separated from the incubating bird by the eggshell (which contains gas-filled pores) and the fibers of the shell membranes (which contain gas spaces). In addition, the chorioallantoic membrane (CAM), which is analogous to the mammalian placenta and is rich in blood vessels, is attached to the inner side of the shell membranes. Respiratory gas exchange takes place by diffusion between the blood vessels in the CAM and the environment through the gas spaces in the membranes and the pores in the eggshell. During active incubation, the incubating bird covers about 20% of the eggshell-surface area with its brood patch in order to regulate egg temperature. It is believed that the gas pressures under the shell are uniform at all points because of the high diffusion coefficients of gases in the gas phase in the shell membranes (∼106 times higher than in water). How far does covering a fraction of the shell influence the partial pressures of oxygen (PO₂) and carbon dioxide (PCO₂) under it and, thus, the respiratory gas exchange of the embryo through the CAM? To answer this question, eggs of the domestic goose Anser anser domesticus were covered with an artificial brood patch, and gas composition in the gas spaces of the shell membranes under it was determined. Eggs were incubated in 37.5 ± 1 °C and 45 ± 2.5% RH. On day 21, eggs were removed and 2 holes (4 mm in diameter) were drilled, one on each side of the "waist" of the egg to a depth exposing the eggshell membranes. Sawed syringe needle hubs were glued around the holes. The open hubs were corked, and eggs were returned to the incubator. On day 22, the eggs were divided into 4 experimental groups. A round patch of epoxy was then glued around one of the 2 hubs on the eggshell, to serve as an artificial brood patch, therefore preventing gas exchange in that area. The areas of these patches constituted 1.5 ± 0.29 (n = 11), 5.0 ± 0.43 (n = 9), 9.3 ± 0.50 (n = 7), and 14.5 ± 1.16 (n = 13) percent of the eggshell area. The other hubbed side served as control. Syringes opened to the 20 ml mark were attached to all hubs, and eggs were returned to the incubator to equilibrate for 24 hours with the gas composition under the shell. The gas composition in all the syringes was determined daily on days 23-27. Only normally hatching eggs (on day 28) were included in the analysis. Control values for PO₂ dropped with time from 106 to 93 Torr, between days 23 and 26, and rose to 98 Torr after internal pipping on day 27. The corresponding changes in PCO₂ with time were from 42 to 46, and 45 Torr, respectively. Even a 1.5% coverage changed the gas pressures relative to the control. The changes in PO₂ and PCO₂ were significant from shell coverage of 5% onwards. The PO₂ level dropped to 60%, and that of PCO₂ increased to 130% in comparison to the control. These differences show that there is only a limited degree of lateral diffusion in the membranes under the shell. Under coverage conditions corresponding to natural incubation, gas exchange with the blood is ineffective in the covered area. There is a functional contradiction between the local respiratory strain caused by the brood patch and its role in thermoregulation of the developing embryo.