Mother birds fed the lower quality diet, however, later produced far more sons than daughters.

“In this case, it is adaptive for mothers to produce more sons when conditions are poor because sons are much less vulnerable to nutritional stress than daughters,” Pryke explained. “For example, sons reared on poor quality diets grew faster, were healthier, fledged earlier and were much more likely to survive than daughters. Indeed, more than 51.5 percent of daughters reared on low quality diets died before reaching parental independence compared to only 7.3 percent of sons.”

It is unclear whether or not human moms would produce more sons or daughters when environmental conditions are poor. That will probably remain a mystery for quite a while, since, as Pryke said, “researchers can’t do experimental manipulations, like in the current study” on humans.