Estimating the effectiveness of a survey window for monitoring least tern (Sterna antillarum) population size in Massachusetts

Lahaise, Diane M.
Lou Verne
Department of Environmental Studies
The Massachusetts Division of Fisheries and Wildlife surveys Least Terns on their breeding grounds during a brief window of time every year (June 5-20, inclusive) and uses the information collected from the survey to estimate population size and trend. In this study, I evaluated the survey window for estimating the Least Tern population by analyzing detailed information on the timing of colony formation from a breeding colony at Plymouth Beach, Massachusetts, for the year 1988 to 1996. I found significant differences in the mean nest start date (start date = date when first egg in nest is laid) of the colony during the study period. In addition, there were large differences in the number of nests present at the end of the survey period versus the beginning. Standard deviation of start dates and skewness and kurtosis of nest initiation curves also indicated a high degree of variability in colony synchrony during the study period. Predation and instability of the nesting habitat are believed to be the most important de-synchronizing factors in the Least Tern colonies. Analysis of state and Plymouth Beach population data show that the present survey window of June 5-20 is adequate for documenting Least Tern population trend in Massachusetts. However, based on my analysis I make the following suggestions that may improve the accuracy of survey: (1) Least Tern colonies should be visited as late in the survey period as possible; 92) colonies should be visited more than once during the survey period; and (3) additional counts should be conducted after June 20 (“B-counts”) whenever possible to detect late peaks of nesting activity resulting from de-synchronizing events.

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