Observations on the White-Faced Ibis, Plegadis Chihi, in Louisiana

Abstract

The eight-month study deals with the White-faced Ibis, Plegadis chihi, in southwestern Louisiana. It affirms the absence of the closely related Glossy Ibis, Plegadis falcinellus, as a breeding bird in that area today. Data on feeding habits, social behavior, nesting cycles, and plumage changes in chihi are presented as a step toward clarifying the relationship of these two puzzling forms of ibis; but the full significance of these findings cannot be ascertained until more is known about the habits of falcinellus. The most important conclusions concern plumage changes. It is shown that published descriptions of plumage and molt in chihl contain serious errors. For most of the year individuals of the two forms, both adult and immature, are practically indistinguishable in the field, while some unidentifiable birds are present at all seasons. During the breeding cycle, however, chihi assumes a striking facial patch lacking in falcinellus and probably also exhibits differently colored soft parts. The very fact that the really trenchant differences between the two forms are confined to the breeding season lends these differences added taxonomic significance, suggesting that they might function as reproductive isolating mechanisms should falcinellus and chihl meet in the same range. Therefore, it would seem that, in the present state of knowledge falcinellus and chihi should be maintained as separate species despite their similarities. Measurements of carefully sexed birds indicate marked sexual dimorphism in size in White-faced Ibis—a situation hitherto obscured by the seemingly erroneous labeling or sexing of some specimens.