flight feathers
...that some species of waterfowl lose all their flight feathers at once while moulting, rendering them incapable of flight?
...that some species of <a href="./Anseriformes" id="mwAa0" rel="mw:WikiLink" title="Anseriformes">waterfowl</a> lose all their <b id="mwAa4"><a href="./Flight_feather" id="mwAa8" rel="mw:WikiLink" title="Flight feather">flight feathers</a></b> <i id="mwAbA">(pictured)</i> at once while <a class="mw-redirect" href="./Moult" id="mwAbE" rel="mw:WikiLink" title="Moult">moulting</a>, rendering them incapable of <a href="./Bird_flight" id="mwAbI" rel="mw:WikiLink" title="Bird flight">flight</a>?