COMPARISON OF METHODS TO ESTIMATE FLEDGING RATES FOR CALIFORNIA LEAST TERNS

Jeanette T Boylan; San Diego Zoo Institute for Conservation Research; jboylan@sandiegozoo.org; Katrina Murbock, Travis Wooten, Rachel Smith, Maggie L. Post, Lisa Nordstrom, Ron Swaisgood

Fledgling numbers are notoriously difficult to determine for California Least Terns (Sternula antillarum browni) because of the mobility of chicks soon after hatching and the difficulty of finding signs of predation. We compared two new methods for estimating fledging rates: use of small enclosures and use of radio tags on chicks. The enclosures were 60 x 60 or 90 m, constructed of 15-20 cm high fencing, and erected before the nesting season. The small radio tags were attached to the intrascapular region of chicks at ~ 5 days old. After trialing both methods and comparing to the results from traditional visual surveys, the use of small enclosures proved to be the better option. The radio tags had two primary drawbacks: young chicks in the nest were too small to attach the transmitters and the tags frequently fell off. A critical component of the enclosure design was the requirement to be large enough to encompass a sufficient number of nests. In addition to obtaining information on yearly chick survival, data on growth was also obtained which can be compared across years to investigate factors influencing growth and survival.