I found this artical, maybe it will help.

BLUEGILL

Biologist discover a third type of male bluegill.

In spring, male bluegill make depressions in the floor of lakes and ponds to form nests. He fiercely defends the nest by attacking any intruders that don't resemble female bluegills. The males that build nests are big and have bright colors that attract the smaller, drabber females.

These dominant males have competition from younger, smaller males, called "sneakers", that linger around the edge of the nest and attempt to mate with the females in his harem when he's not looking.

Biologist have discovered a third type of male bluegill. Instead of growing up to be a typical territorial male, some sneakers take on the drab colors of female bluegills. Because of this, they can sneak undetected into a big male's nest to mingle and mate with the females.