What starts up these chemical processes of embryonic growth and then keeps development going?
Chemical reactions won’t start or proceed to completion unless the ingredients are warm enough. Embryonic development requires heat. In Tree Swallow eggs embryonic development won’t even begin unless egg temperatures are over about 26 degrees C (80 degrees F). And for optimum embryonic development bird eggs must reach temperatures of about 38 degrees C (100 degrees F). This is much higher than normal spring air temperatures. But embryos within eggs can’t generate or control their own heat. So adult Tree Swallows must add heat to get their eggs to develop. And once development has started egg temperatures must be kept within a certain range; too high or too low can kill the embryo.
