Low recruitment of Common Terns Sterna hirundo in the declining Barnegat Bay population

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Abstract
Common Tern Sterna hirundo populations have declined in the southern portion of their breeding range along the Atlantic coast of the United States. The Barnegat Bay (New Jersey, USA) population has been declining, with sea-level rise increasing the frequency of flooding of salt marsh islands believed to be a contributory factor. Productivity is typically very poor, relative to studies undertaken elsewhere, and a previous analysis suggested permanent emigration of breeding adults out of Barnegat Bay. At Pettit Island, a long-term study site in the bay, the number of ringed chicks recaptured as adults was extremely low, even when accounting for mortality prior to fledging. Of 1,314 chicks ringed at Pettit Island from 2006 to 2014, only 23 were recaptured as adults at Pettit from 2010 to 2017 (1.8%, or 3.9% of presumed fledglings). Correcting for the proportion of adults captured, recruitment by four years of age was estimated at 8.1 to 9.3% of fledglings, or 3.5 to 4.0% of all chicks. Recruits comprise a small percentage of breeders in the colony. Of 34 adults captured in 2016, 10 were previously ringed and only three of these had been ringed as chicks (8.8% of total, 30% of ringed birds). It is unlikely that the small number of returns at Pettit Island simply reflects natal dispersal within the bay, because no terns ringed as chicks at Pettit Island were recaptured as young breeders at other colonies. Whether poor recruitment reflects low postfledging or subadult survival, or emigration out of the population is unknown.

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