I just stumbled onto an article about global warming and the early hatching of eggs in wild bird clutches. It appears it is possible for extreme ambient temperatures to trigger embryo development. There are consequences for this. As we know, our broody hens start sitting their eggs once they've laid the final egg in the clutch so all the chicks will hatch around the same time. If the hatch is staggered over several days, the chicks will be of a different size and some may be smaller and less able to compete for food. This is especially critical in the case of wild birds.
So I was wrong to dismiss the possibility too quickly. Here's the article and a quote from it. www.forbes.com/sites/grrlscientist/2016/02/18/the-heat-is-on-climate-change-causes-birds-to-hatch-early/#3519a63f0cbe
“If all chicks hatch at the same time, the competition is much more equal,” said Professor Griffith. However, if eggs are exposed to temperatures that are warm enough to trigger embryonic development before incubation begins, they will hatch over a period of several days. The first chick to hatch will be able to monopolize the food provided by its parents, potentially reducing the survival of its younger and smaller brothers and sisters."
So I was wrong to dismiss the possibility too quickly. Here's the article and a quote from it. www.forbes.com/sites/grrlscientist/2016/02/18/the-heat-is-on-climate-change-causes-birds-to-hatch-early/#3519a63f0cbe
“If all chicks hatch at the same time, the competition is much more equal,” said Professor Griffith. However, if eggs are exposed to temperatures that are warm enough to trigger embryonic development before incubation begins, they will hatch over a period of several days. The first chick to hatch will be able to monopolize the food provided by its parents, potentially reducing the survival of its younger and smaller brothers and sisters."