The Monarch Grove Sanctuary in Pacific Grove, California is a gathering site where Monarch Butterflies congregate to spend the winter. The Monarchs are protected from harassment by humans. Congregating is a defense against predation by birds who find the monarchs distasteful due to toxic cardenolides that are concentrated in the wings. Volunteer docents explain the natural history to visitors and help with census of the monarch population.
One of the docents, Connie Masotti, found dying Monarchs that were missing their abdomens. Search for the cause, she noted a fox squirrel (an invasive species in this area of California) was capturing Monarchs, eating the abdomens and discarding the rest. Presumably, the abdomens are less well defended than the wings. Connie has posted photo documentation of the squirrel on her weblog, CapturedByConnie that features her exquisite nature photos.
In addition to eating the monarchs, the squirrel disturbs resting adults causing them to take flight and use extra essential energy. Efforts to trap and relocate the squirrel are needed to protect the overwintering monarchs. This is one more example of invasive species having detrimental affects on native species.