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Chick Survival, Fledgling Residency and Evaluation of Methods for Estimating Fledging Success in Least Terns

Abstract Fledgling counts are often used to estimate fledgling production or calculate an index of chick survival (i.e., fledging success) in the Least Tern (Sterna antillarum). Timing of counts is critical because numbers of fledglings present may be influenced by temporal patterns in fledgling departures from colonies and post-fledging mortality. The objectives of the present study were to measure chick survival and fledgling residence time (FRT) at three Least Tern colonies in Maine and examine the accuracy of single or multiple counts of fledglings for estimating fledgling production and fledging success. Chicks were color-banded at age zero to one d in 2002-2003 to estimate chick survival using mark-resighting methods and to examine relationships between fledging date and FRT. Colony sites were searched for banded chicks and fledglings every one to three d from just prior to fledging until their disappearance from the vicinity of nesting areas at three colonies. Data on numbers of banded fledglings known alive on each survey date were used to simulate single or periodic counts of banded fledglings for the purpose of comparing estimates of fledgling production and success based on count methods with known values. Chick survival estimates were 0.73, 0.74, and 0.14 for the three colonies. Fledgling residence times decreased relative to fledging date at all colonies, and the trend for maximum values of FRT, which may be an indicator of the maximum time to departure, varied from approximately 30 d in mid-July to 15 d in early to mid-August. The highest one-d counts of fledglings underestimated fledgling production by 21-30% for the two sites with high chick survival. Methods based on multiple counts also tended to underestimate fledgling production. Fledgling residency patterns limit the utility of count methods for estimating fledgling production and fledging success.