Increased prolactin in the breeding pair leads to the expression of parental behavior and physiology. The source of the alloparental behavior found in helper birds has been the focus of many studies. A positive correlation was found between increased prolactin levels during the breeding period and helping behavior in non-breeding "Aphelocoma" jays. This suggests that helper birds do not simply respond to the calls of the young, but begin to show parental behavior even before the chicks hatch. These data suggest that natural selection may be acting on cooperative breeding in "Aphelocoma" jays and New World jays in general because the birds are reacting to more than just an environmental stimulus. Studies have also been done on Florida scrub jays ("Aphelocoma coerulescens") and results have confirmed the hypothesis that increased prolactin levels are correlated with an increase in parental behavior in helper birds. Scrub jays are members of the family Corvidae, which are considered the most intelligent of the birds and among the most intelligent of all animals.]