POPULATION TRENDS OF SOME NORTHEASTERN NORTH-AMERICAN LANDBIRDS - A HALF-CENTURY OF DATA

We analyzed population trends of 26 species of North American migratory landbirds using 53 years of spring migration count data collected in eastern Massachusetts from 1937 to 1989, inclusive. These data span twice the time period of most landbird monitoring data sets, allowing us to examine how accurately trends based on shorter-term investigations of population trends represent longer ones. We found that 18 species (64%) have declined significantly, three species (12%) have increased significantly, and five species (19%) have shown no trend over this time period. Using the most recent ten years of our data, the trend matched that of the full 53-year data set for 10 species (38%). Using the most recent 20 years of data, the number of matching trends increased to 18 (69%). Using a 30-year subset of data did not improve the percentage of matching trends and showed a greater number of non-significant trends than did analysis of the most recent 20-year subset of data. This pattern resulted because many species appeared to be at low population levels in the 1950s and early 1960s, before the time when most current avian censuses and surveys began. Our results indicate that many species are in fact declining, but that declines in the past 20 years might be partly a result of normal, short-term population fluctuations.