This subject has always interested me as well. A group of us experimented with this several years ago using both coturnix quail and BCM chickens. Skewing the incubation temps one degree up and down. There was no significant difference in the end. There is lots of evidence that it does effect the sex in some reptiles thou. Alligators seem to be the most commonly studied. We came to the conclusion that if it were effectively possible. The poultry industry would have perfected it by now for a profit! On a side note... While running this experiment, a group of us were all running the same test, in several different states. We decided that to get real results, everyone should be using the same eggs. So we shipped coturnix eggs from the same source. This all happened in warmer summer temps. The biggest factor seemed to be the heat during shipping. The eggs coming into or out of Florida, Mississippi and Texas had lower hatch rates over all and high % of cock birds, while eggs that went to Colorado, Kansas and Wisconsin was normal hatch rates and sexes close to 50/50.... When it was a overall poor hatch rate to begin with, the cock bird percentage was higher.... We concluded that it was likely natures way of matching the population with environmental conditions. Hot dry spring/breeding season = less potential food. It would make sense to eliminate breeding females during periods of low food supply. It was less noticeable with the BCM's. But there were fewer folks working with them as well...
