Trends in Avian Roadside Surveys over a 20-Year Period on Saipan, Commonwealth of the Northern Mariana Islands

Abstract

We used roadside surveys to examine abundance trends and spatial patterns for 11 terrestrial bird species from 1991 to 2010 on Saipan, Northern Mariana Islands. Relative abundances of the White-throated Ground-Dove, Mariana Fruit-Dove, Collared Kingfisher, Micronesian Starling, and Micronesian Myzomela all increased during the study period. Abundances of the Golden White-eye and Rufous Fantail followed a curvilinear trend that increased during the first half of the survey period (1991–2000) and decreased in the second half (2001–2010). Abundances of the remaining species (Island Collared-Dove, Bridled White-eye, Nightingale Reed-Warbler, and Eurasian Tree Sparrow) showed no significant trends. The most numerous species (over 40 individuals detected per survey), in decreasing order of abundance, were the Bridled Whiteeye, Rufous Fantail, Golden White-eye, Micronesian Starling, and Micronesian Myzomela. The total abundance of birds increased on average by 0.27 birds per year, per survey station, island-wide. Three stations out of 47 (6.4%) recorded a significant loss of total birds over the time period, and bird numbers increased significantly at only one station (2.1%). Stations at which we detected declines were clustered in the northern, less-populated, region of Saipan. We explored the relationship between typhoon frequency/severity and bird abundance to explain trend patterns but found no significant correlation. We suggest ways to improve roadside surveys as a management tool to aid in detecting avian declines that are of conservation concern.