Interactions Between Common Terns and Ground-Nesting Ospreys Brian Palestis Wagner College, Staten Island, NY USA A typical, above-ground osprey nest Pettit Island Common Tern colony Introduction & Methods Results Figures Conclusions An Osprey nest was present on the ground at Pettit Island on the first day of field work in 2013 and 2014 (16 and 15 May), before terns began nesting. The Osprey nest occupied an area near the center of the island that is usually safe from flooding. This area is typically densely packed with tern nests (e.g. 15 nests within 5m of the center of the former Osprey nest in 2016). Fig. 1 shows the area of the island from which terns were excluded when the Ospreys were present (area ~ 340m2). No terns nested here in 2013 until the Osprey nest was abandoned. In 2014, 7 nests were in this area; 0 in the first 2 weeks of incubation. In 2015 and 2016, with no Ospreys present, 25 – 35 tern nests were located here. The number of tern nests decreased in both years that the Ospreys were present, reaching 80 pairs in 2014, the smallest number recorded at this site since 1981. The Ospreys did not return in 2015 and the number of tern breeding pairs increased to 205, typical for this site. However, the number of breeding pairs at Pettit Island began decreasing before 2013 and decreased again in 2016 (Fig. 2). The presence of the Ospreys caused prolonged upflights, as an Osprey would be chased away then quickly return and be mobbed again repeatedly. These cycles lasted at least 15–20 min. Despite this frequent disturbance, presence of the Ospreys did not appear to affect productivity of the terns (Fig. 3) and tern hatching success was high other than typical early losses to flooding. No eggs in the Osprey nest hatched in either year with no obvious cause. Frequent mobbing by the terns may have prevented the Ospreys from incubating effectively. The Osprey nest was abandoned before 4 June in 2013 and 11 June in 2014. Terns then began nesting near the abandoned nest, with one pair nesting in it. Although Common Terns generally discriminate predators from nonpredators and are flexible in their antipredator behavior, their responses to the piscivorous Ospreys suggest that they treat any raptor as a threat. Such a strategy is probably adaptive, as most raptors would be potential predators on young or adults. Harassment by the terns may have resulted in the failure and abandonment of the Osprey nest. The Ospreys occupied an area that terns normally use for nesting. Whether their presence contributed to changes in the number of breeding pairs on Pettit Island is unclear, however. The pattern of change may reflect the decline in the Common Tern population in Barnegat Bay as a whole, coupled with abandonment and recolonization of other islands in the bay. Movements away from or back to these other sites could cause fluctuations in the number of terns nesting at Pettit Island. Fig. 1 Ospreys (Pandion haliaetus) typically nest above ground on structures, but occasional ground-nesting may occur on islands free of mammalian predators. In 2013 and 2014 a pair of Ospreys nested on the ground in the middle of the Common Tern (Sterna hirundo) colony on Pettit Island, in Barnegat Bay, New Jersey USA. This island is very small (0.3 ha) with limited nesting substrate. The Barnegat Bay Common Tern population has been declining in both number of breeding pairs and number of colonies. Although they are piscivorous, as large raptors, Ospreys may be perceived as predators by terns. I recorded behavioral interactions between the two species and tracked the fate of the Osprey nest. I also recorded the number and general location of tern nests and estimated fledging success to compare years with and without ground-nesting Ospreys. The number of Common Tern breeding pairs was estimated by the peak number of nests, and fledgling success by chick survival >2 weeks. 30m Map of Pettit Island showing location of Osprey nest and exclusion zone Fig. 2 Bibliography & Acknowledgments I thank Alex Zummo and Maggie Shaw for field assistance, and the Deane Fund and Wagner College for funding. Burger & Gochfeld. 1991. The Common Tern: Its Breeding Biology and Social Behavior. Columbia University Press. Burger & Gochfeld. 2016. Habitat, Population Dynamics, and Metal Levels in Colonial Waterbirds: A Food Chain Approach. CRC Press. Palestis & Hines. 2015. Adult survival and breeding dispersal of Common Terns (Sterna hirundo) in a declining population. Waterbirds 38: 221-228. Poole et al. 2002. Osprey (Pandion haliaetus), Birds of North America Online http://bna.birds.cornell.edu/bna/species/683 Fig. 3 Osprey nest on the ground, surrounded by Spartina Arrows indicate years with osprey nest Order online at https://www.postersession.com/order/