Survival and Recruitment of Rehabilitated Caspian Terns in Southern California

Every year thousands of birds are brought into care centers for rehabilitation in hopes that they can be treated and returned to the wild (McRuer et al. 2017). Many are victims of oil spills and other anthropogenic activities (Duerr et al. 2016; Henkel and Ziccardi 2018). In southern California, International Bird Rescue, Los Angeles (hereafter “IBR”) rehabilitates and releases 300 to 500 aquatic birds annually. Caspian Terns (Hydroprogne caspia) are among the many species of birds treated at IBR. Caspian Terns currently breed at three colony sites in southern California (Cuthbert and Wires 1999; Suryan et al. 2004; Collins 2006). Two of these sites—Pier 400 in the Port of Los Angeles, Los Angeles County (“Pier 400”), and Bolsa Chica State Ecological Reserve in northern Orange County (“Bolsa Chica”)—are approximately 18 km apart. The third is 180 km south on salt evaporation pond levees in the South San Diego Bay Unit of the San Diego Bay National Wildlife Refuge (“Salt-Works”) (Collins 2006). Since 2001, when IBR opened its Los Angeles facility, 69 Caspian Terns have been rehabilitated. Forty-four (63%) of these terns came from two separate incidents on barges anchored in Long Beach Harbor; nine others (13%) had injuries related to fish hooks and entanglement in fishing line. These barges presented an island-like habitat for nesting in an area that historically had islands available. Caspian Terns are known to have a prolonged post-fledging period during which young terns accompany their parents on foraging trips and migration (Cuthbert and Wires 1999), and thus it was not certain whether chicks rescued from these events and raised in captivity would survive and subsequently recruit into the breeding population. The first barge incident took place in 2006 when an estimated 360 pairs of Elegant Terns (Sterna elegans) and 586 pairs of Caspian Terns nested on two barges anchored in the outer Long Beach Harbor (California Department of Fish and Wildlife, 2007 unpublished data). The barges’ owners directed their employees to wash the nests and small chicks overboard prior to moving the barges out of the area. This was an illegal activity, since these species are protected under the Migratory Bird Treaty Act of 1918 (the owners were successfully prosecuted and damages were assessed). Most of the chicks died; 413 dead chicks were recovered (California Department of Fish and Wildlife, 2007 unpublished data). Twentysix chicks were rescued and taken to IBR for rehabilitation and release. One Caspian Tern chick was euthanized after 44 d in care because of an elbow infection; one Elegant Tern chick was euthanized on arrival because of a wing fracture. On average the chicks were in care at IBR for 46 d (range was 43–52 d). Nine Elegant Tern and 15 Caspian Tern chicks survived and were deemed strong enough to be safely released to the wild (Table 1). Since