Left: incubating female blue tit. Blue tits are common inhabitants of north European forests and gardens. A female lays about 12 eggs on which she sits tightly for some two weeks. Right: blue tit eggs in one of the artificial incubators used in the study. The pale yellowish eggs in the middle have just been put into the incubator, whereas the darker eggs in the bottom right corner have been incubated for about a week. The striking color difference gets more pronounced each day as the chick-to-be grows and matures inside the egg.