Lake Pepin was once known as a major clamming center before becoming celebrated as a resort area. In the late 1800s and early 1900s, more than 500 clammers worked the lake from flat-bottomed johnboats, using giant combs called crowfoot bars to rake abundant mussel beds and gather shells for sale to button factories at Lake City. With thirty-two species in its waters, Lake Pepin was unusually rich in mussels, including species known as pig-toe, pimpleback, pocketbook, washboard, elephant ear, heelsplitter, spectacle-case, sheepnose, and wartyback, and clammers sometimes also found pearls. By 1898, nearly 50 button factories operated in cities along the Mississippi River, but the industry expanded so quickly that it began to exhaust the mussel supply. In 1914, Lake Pepin produced eight million pounds of marketable shell, but by 1929 the harvest had fallen to less than one-twentieth of that amount. In recent years, commercial clamming in Lake Pepin has revived, with pellets made from mussel shells used by the Japanese cultured pearl industry to induce oysters to form pearls. At the same time, many freshwater mussel species face danger from changes in water temperature, water flow, and sedimentation, as well as pollution, eroded soil, pesticides, fertilizers, sewage effluent, other contaminants, channel dredging, and bridge construction. More than 100 songbird species also migrate north to the Midwest in warm weather when food is abundant, then return south in fall to Mexico, the West Indies, and Central and South America.