The Severn is home to numerous birds and several mammals. The most prominent nesting bird is the osprey, which builds its large nest of sticks on the fixed navigational marks that are closely approached by boaters. These unique raptors are seen regularly between April and October circling over the Severn, plunging in feet first and usually emerging with a fish. Young ospreys are visible in the nests in June, fledge by August, and in September begin an 18-month sojourn to the Caribbean or South America before returning to the Chesapeake.
Because ospreys were nearly eliminated by the bioaccumulative toxic effects of DDT in the 1960s, the presence of these striking birds is a reminder of environmental problems and the potential to restore depleted species.
Another easily observed Severn bird is the great blue heron, some of which nest in tall trees in the watershed. Unlike ospreys, these large and sometimes noisy herons largely remain here throughout the winter, spearing fish in the shallows with incredibly rapid thrusts of their bills. In the summer their diminutive cousins, the green-backed herons, can also be found around the marshes and shorelines of the Severn. Several species of gulls and terns are present in the open waters of the Severn, and kingfishers patrol its banks.
In the Severn's marshes and swamps mallards and wood ducks raise their young every summer. The breeding plumage male wood duck is a truly spectacular bird that can be observed in quiet Severn coves in the spring and fall. Bufflehead, hooded mergansers, and coots spend winters in the Severn, with common loons, grebes, ruddy ducks, canvasback, scaup, goldeneye, and other ducks stopping during their migration. Tundra swan formerly wintered on the Severn, but have not returned after the demise of the large SAV beds. Non-migratory Canada geese have become established in the Severn in recent years, and double-crested cormorants have been increasing, now routinely seen on the "spider" tower off Greenbury Point.
The muskrat is a common resident of the Severn's marshes, building substantial dens of marsh grasses. River otter are occasionally seen, and in the spring beaver explore the fresher regions of the tidal Severn.
The Severn has been home to several reptiles, including snapping turtles and terrapins, both of which formerly had a place on local menus. Snapping turtles up to 18" long can be seen basking on the surface near marshy areas, submerging when disturbed by humans. Biologists from the Department of Natural Resources have recently found nesting terrapins in the Severn watershed, and efforts are underway to restore terrapins to their former abundance. Another reptile sometimes seen in the Severn is the northern water snake, which swims along the surface of quiet backwaters and tidal ponds. This non-venomous snake is sometimes confused with the water moccasin, a southern serpent whose range does not extend as far north as the Severn.