Mille Lacs National Wildlife Refuge hosts one of only five common tern breeding colonies in Minnesota. The common tern is currently listed as a state threatened species, a Minnesota Species in Greatest Conservation Need, and a U.S. Fish and Wildlife Service, Region 3 Conservation Priority Species.  Lake Mille Lacs offers an abundance of minnows which common terns require to survive. Hennepin Island provides a remote and secure place to nest, free from predators such as fox or mink that eat eggs and nestlings.

Double-crested cormorants began nesting on Mille Lacs National Wildlife Refuge in 1998 and now boast a thriving colony. Their population has been rebounding since the 1970s. DDT and other contaminants had a detrimental effect on these fish-eating birds. After they were banned, birds like cormorants have recovered across most of their population range.

Pea gravel is the nesting substrate for common terns on Hennepin Island.  Common terns nest on the ground and lay speckled eggs, so their eggs are well-camouflaged among pea gravel. Periodically, pea-sized gravel is hauled to Hennepin Island over the ice. This gravel augmentation replaces a portion of what is lost to ice scouring and wave action.