Size of the bursa of fabricius in relation to gonad size and age in laysan and black-footed albatrosses

Age determination can be difficult for birds that undergo little or no plumage change during life. This is the case for Laysan and Black-footed Albatrosses (Diomedea immutabilis and D. nigripes). The juvenile plumage for both these North Pacific albatrosses is completely grown by about five to six months of age, just prior to their first flight, and is largely indistinguishable from the definitive basic plumage. Consequently, no well-documented methods of distinguishing newly fledged birds, older pre-breeders, or breeding-aged adults has been described for these species (Harrison 1985).