Seems pretty common that most finches, in fact several species of bird will lay several eggs before starting to incubate them. This confuses me. I have observed my finch pair in the past with their eggs. There may be some difference in behavior once they start the official incubation behavior in terms of sitting time, but I guess my confusion is over the actual physics of the problem. What is different about them being in the nest with the eggs for the first few days vs them officially "incubating" the eggs. Sure they might not be as fluffed up as they are during incubation time, but they both still cram into the nest together over the top of the eggs and their collective body heat must end up at essentially the same result. I just fail to see the official difference thermally. Do they squat over the eggs or something the first few days and we just cant tell the difference? Is the difference only really in behavior? ie once they start incubating, they are more rigorous in their switching off and they really fluff up and smother the eggs. Thoughts on this subject?