A passage from the book Mammals of North America, by Dr. Adrian Forsyth,
states:

"...they (river otters) launch submarine attacks on floating waterbirds;
and they plunder birds' nests. The otter is also a fighter. It will
follow a muskrat into its burrow and kill it, and it is one of the few
animals that may be able to kill a beaver in its lodge."

Andy Sorensen
Anchorage, Alaska
ajstudio at ptialaska.net