The Pelican

  A wonderful bird is the pelican,
  His bill can hold more than his beli-can.
       He can take in his beak
       Food enough for the week;
  But I'm damned if I see how the heli-can.

Often attributed to Ogden Nash
(but some maintain it's by Dixon Lanier Merritt).
No matter, it's clever whether by Nash or Merritt.