THE FAT CONDITION OF SOME ISLAND BIRDS

Several resident passerine species of birds on the Tres Marias islands, Mexico, are in a fatter condition than their mainland counterparts just prior to, and at the beginning of, the breeding season. At least two non-passerine species on the islands are also fat at this time of the year. The significance of the fat reserves of the island birds is discussed, and it is concluded that the fat may serve the functions of energy-storage and possibly water-storage, to be used later at the climatically most unfavourable time of the year.