Factors Affecting Chick Provisioning by Caspian TernsNesting in the Columbia River Estuary

We investigated factors affecting chick provisioning by radio-tagged Caspian Terns (Sterna caspia)nesting in a large colony on East Sand Island in the Columbia River estuary during 2001. Caspian Tern predationon juvenile salmonids (Oncorhynchusspp.) in the estuary prompted resource managers to relocate ca. 9,000 pairs ofterns nesting on Rice Island (river km 34) to East Sand Island (river km 8), where terns were expected to consumefewer salmonids in favor of marine forage fishes. This study investigated factors influencing foraging success, dietcomposition, and overall reproductive success at the managed Caspian Tern colony. Our results indicated that daytimecolony attendance by nesting terns averaged 64% and decreased throughout the chick-rearing period, whileduration of foraging trips averaged 47 min and increased during the same period; these seasonal changes weremore strongly related to date than chick age. Average meal delivery rates to 2-chick broods (0.88 meals h-1) were 2.6times greater than to 1-chick broods (0.33 meals h-1). Parents delivered more juvenile salmonids to chicks duringebb tides than flood tides, but meal delivery rates to the nest remained constant, suggesting diet composition tracksrelative availability of prey species. Foraging trips resulting in delivery of juvenile salmonids averaged 68% longerthan foraging trips for schooling marine forage fishes, indicating higher availability of marine prey near the colony.High availability of marine forage fish in the Columbia River estuary during 2001 was apparently responsible forhigh colony attendance, short foraging trips, high chick meal delivery rates, and high nesting success of CaspianTerns on East Sand Island.

Scott K. Anderson, Daniel D. Roby, Donald E. lyons, and Ken Collis, Waterbirds 28(1): 95-105, 2005