SEASONAL POPULATION SURVEYS AND NATURAL HISTORY OF A MICRONESIAN BIRD COMMUNITY

摘要

I replicated quarterly population surveys of landbirds on Saipan, Mariana Islands at two environmental scales: habitat specific and island-wide. I determined population densities and the degree of seasonal fluctuation in counts to compare densities in native vs disturbed habitat and to observe whether populations exhibited characteristics of those at either saturation or below saturation densities. I also gathered new data on the natural history of largely unknown species. For seven of the nine forest birds examined, inter-seasonal census variation was greater than intra-seasonal variation, suggesting that most of the species undergo seasonal shifts in population or breeding status (the latter case was indicated for four forest species). The principal difference uncovered between the two census scales was that the Micronesian Honeyeater (Myzomela rubrata) was relatively uncommon in native forest but regular on island-wide counts. Otherwise, forest species showed numerous similarities in count trends at both scales. However, habitat-specific data showed that for many species, counts and computed densities were greater in native forest than in disturbed habitat. Independent density assessment (based on a new procedure) for the Bridled White-eye (Zosterops conspicillatus) was of the same order of magnitude as that obtained through censusing. The densities reported here, particularly for the Rufous Fantail (Rhipi-dura rufifrons). Bridled White-eye, and Golden White-eye (Cleptomis marchei), are among the highest ever reported for birds (> 1900/km~2) and are almost certainly at habitat saturation. Interspecific competition is expected in such a case, and interspecific aggression was prevalent, particularly among ecologically similar species.