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.

Concern about biodiversity loss has often focused on rare species in danger of extinction, but many ‘species of least concern’ (according to International Union for the Conservation of Nature criteria) at a global level have experienced steep declines in numbers, losses of local populations, and range contractions (Ceballos et al. 2017). Seabird populations have decreased world-wide, with especially large declines among cosmopolitan long distance migrants in general, and among terns in particular (Paleczny et al. 2015). Common Terns Sterna hirundo, although widespread and abundant, have experienced notable population declines in several regions, such as the Great Lakes in North America (Morris et al. 2012; Nisbet et al. 2017) and the Wadden Sea in Europe (Szostek & Becker 2012). Along the Atlantic coast of the United States, Common Tern populations in the northeast appear to be stable (Nisbet et al. 2017), but those from about Jamaica Bay, New York, south have declined (Erwin et al. 2011; Burger & Gochfeld 2016; Nisbet et al. 2017).

In long-lived birds, such as terns, the population growth rate is most sensitive to changes in adult survival, but when adult survival varies little, other demographic parameters may actually be more likely to affect population dynamics (Sæther & Bakke 2000). There is increasing awareness of the importance of variation in juvenile survival and recruitment, immigration, and emigration to the dynamics of seabird populations (Frederiksen & Petersen 2000; Lebreton et al. 2003; Coulson & Coulson 2008; Ledwoń et al. 2014; García-Quismondo et al. 2018), including in the Common Tern (Tims et al. 2004; Szostek & Becker 2012; Szostek et al. 2014).

In Barnegat Bay, New Jersey, the number of Common Tern individuals has decreased since the mid 1980s and the number of colonies has decreased since the mid 1970s (Burger & Gochfeld 2016). Since about 2007, increased frequency and intensity of flooding presumably due to sea level rise (Sallenger et al. 2012; Kopp et al., 2016), has caused low productivity at extant colonies and made former colony sites unsuitable (Palestis 2009; Palestis & Hines 2015; Burger & Gochfeld 2016). An analysis of apparent adult survival of Common Terns suggested high rates of emigration out of Barnegat Bay (Palestis & Hines 2015). Here I analyse recruitment of Common Terns at Pettit Island, a long-term study site in the bay. Although apparent adult survival at Pettit Island (0.88, Palestis & Hines 2015) is similar to estimates from stable populations (Nisbet et al. 2017), the number of breeding pairs has declined and there may be too few recruits entering the breeding population to sustain this colony.

Funding was provided by the Eastern Bird-Banding Association, Biosocial Research Foundation, Mollica Family Fund, John Deane Fund for Environmental Studies, the Megerle Endowment and Wagner College. I thank K. Eppinger, M. Fealey, J. Husic, S. O’Neill, M. Shaw, M. Stanton, M. Valero, and A. Zummo for assistance in the field, and Joanna Burger and Mike Gochfeld for providing traps and sharing information on Barnegat Bay tern colonies. I also thank Richard Buzby and the Edwin B. Forsythe National Wildlife Refuge for allowing fieldwork on their properties, and the borough of Surf City, New Jersey, for providing a location for boat mooring.

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