That's chickens with an above average egg production.

超级鸡指的是产蛋量高于平均水平的鸡。

In this experiment, researchers compared the egg production of a group of average chickens to a group of super-chickens.

在这个实验中，研究者将一组普通鸡的产蛋量和一组超级鸡的产蛋量进行比较。