A lot depends on the parents. Homers have a longer period between sets of eggs, and I have never had problems with their squabs getting chilled. Birmingham rollers on the other hand can be down on a new set of eggs as soon as their squabs are two weeks old. The strain that I had were excellent parents, and the hen would brood the squabs during the day while the cock was on the eggs. In the evening they would switch and the cock would brood the squabs. I would pair the birds up on Valentine's day and generally have them on eggs by the end of Feb. First rounds could be a bit iffy, but I rarely lost any squabs on subsequent round.