Graduate Thesis Or Dissertation
 

Foraging ecology, colony attendance, and chick provisioning of Caspian terns (Sterna caspia) in the Columbia River Estuary

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  • This study was designed to enhance understanding of factors influencing foraging distribution, diet composition, and overall reproductive success of Caspian terns (Sterna caspia) nesting on East Sand Island in the Columbia River estuary. This colony of nearly 10,000 breeding pairs is of concern to regional resource managers because Caspian terns consume large numbers of juvenile salmonids (Oncorhynchus spp.) listed under the Endangered Species Act. In addition, Caspian terns at this colony consume marine forage fishes, the abundance of which fluctuates on annual and decadal scales. I found that average foraging distance from the colony was 6.6 km (38%) greater in 2000 compared to 2001, associated with lower availability of marine forage fishes in the estuary and lower prevalence of marine prey in tern diets. Also, colony attendance was much lower (37.0% vs. 62.5% of daylight hours), average foraging trip duration was 40% longer (38.9 mm), and nesting success was much lower (0.57 young fledged pair⁻¹ vs. 1.40 young fledged pair⁻¹) in 2000 compared to 2001. In 2001, average meal delivery rates to 2-chick broods (0.88 meals hour⁻¹) was 2.6 times greater than to 1-chick broods (0.33 meals hour⁻¹). Parents delivered more juvenile salmonids to their young during ebb tides than during flood tides, suggesting diet composition reflected short-term changes in relative availability of prey near the colony. Foraging trips resulting in delivery of juvenile salmonids took 68% longer than foraging trips resulting in delivery of schooling marine forage fishes, indicating higher availability of marine prey. High proportion of salmonids in the diet was associated with high use of the freshwater zone of the estuary by radio-tagged terns, suggesting that diet composition also reflected the distribution of foraging terns in the estuary. High availability of marine forage fish in 2001 was apparently responsible for high colony attendance, relatively brief foraging trips close to the colony, high food delivery rates to young, and high nesting success of Caspian terns on East Sand Island. Lower availability of marine prey in 2000 apparently limited Caspian tern nesting success by markedly reducing colony attendance and lengthening foraging trips by nesting terns, thereby increasing chick mortality rates from predation, exposure, and starvation. The foraging behavior and nesting success of Caspian terns at the East Sand Island colony is apparently highly dependent on the fluctuating availability of marine forage fishes in the Columbia River estuary. Diet studies indicate that the primary alternative prey for this tern colony are out-migrating juvenile salmonids from throughout the Columbia River basin.
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