People on the site have posted pictures of their bluegill x green sunfish hybrids. You can use the search forum feature at the top of the page to find them. As for your pumpkinseed ([SIZE=-1]Lepomis gibbosus) and bluegills ([/SIZE][SIZE=-1]Lepomis macrochirus), it is genetically possibly for them to breed and create a hybrid. Sunfish breeding period in the wild is in the spring time and you can look for nests in gravel during the late spring/early summer.
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Different types of sunfish crossbreed in the wild, result = hybrids, which in-turn crossbreed with other hybrids. Experienced biologists have trouble identifying fish from a hybridized population. It all depends on the area which they come from.
I could be wrong but as i understand it roughly 98% of almost all hybrids are male, and most of those are infertile in some way or another. that is why green/bluegill sunfish hybrid is often recommended for smaller lakes and ponds. The fish cannot breed themselves into stagnation