- Declines in populations of anadromous salmonids (Oncorhynchus spp.) in the Columbia River drainage basin have resulted in extensive programs to annually release large numbers of hatchery-raised juvenile salmonids in an effort to support salmonid restoration. The Pacific Flyway population of Caspian terns (Hydroprogne caspia) has grown from around 3,500 nesting pairs in 1960 to around 13,000 nesting pairs in 2000, and the majority of breeding Caspian terns in the Pacific Flyway population were nesting in the Columbia River basin by 2000. This population growth is in part a result of the stabilization of nesting habitat due to human management of the Columbia River and nearby water bodies, plus the yearly release of over a hundred million hatchery-raised juvenile salmonids into the system. Research on the consumption of juvenile salmonids by piscivorous waterbirds resulted in the identification of Caspian tern predation as a factor limiting salmonid restoration in the basin. Caspian terns nesting in the Columbia Plateau region, which lies within the Columbia River drainage basin, were found to consume a much larger number of juvenile salmonids per capita than Caspian terns nesting elsewhere in the Columbia River basin. This finding resulted in the development of a management plan to reduce mortality of juvenile salmonids in the Columbia Plateau region by preventing nesting of Caspian terns at the two largest colonies in the region, in conjunction with the concurrent creation of alternative nesting habitat for Caspian terns elsewhere in the breeding range of the Pacific Flyway population.
I investigated the response of Caspian terns to the loss of available nesting habitat in the Columbia Plateau region. The management plan, developed by an interagency working group led by the U.S. Army Corps of Engineers, met its goal of preventing Caspian terns from nesting at the sites of the two largest colonies in the Columbia Plateau region, Goose Island in Potholes Reservoir and Crescent Island in McNary Reservoir, Washington State. The goal of lowering the region-wide number of breeding Caspian terns from about 875 nesting pairs to ≤ 200 nesting pairs, however, was not met within the first three years of management implementation. The majority of terns that previously nested at Goose or Crescent islands moved to nest on several low-lying gravel bars in the Blalock Islands, an archipelago downstream from Crescent Island in John Day Reservoir on the Columbia River. A new small tern colony also became established at Lenore Lake, north of Potholes Reservoir. While regional numbers of Caspian tern nesting pairs declined significantly in 2015 and 2016, the second and third years post-management, regional nesting success of terns did not decline. Instead, regional nesting success remained within the range estimated by Suryan et al. (2004) for a stable population of Caspian terns (average of 0.32 - 0.74 young raised per breeding pair per year). Analysis of banded terns that nested at either Goose Island or Crescent Island immediately prior to the initiation of management found that the proportion of terns that returned to breed in 2015 and 2016 declined significantly, while a significantly greater proportion of terns returned as non-breeding floaters in the Columbia Plateau region compared to pre-management. Also, the proportion of terns that went unobserved, either within the Columbia Plateau region or elsewhere in the Pacific Flyway, increased significantly in 2015 and 2016 compared to pre-management.
The management plan resulted in the loss of available nesting space for at least 90% of nesting Caspian terns in the Columbia Plateau region during 2004 - 2013. Despite the capacity of Caspian terns for long-distance breeding dispersal, the birds mostly took advantage of previously active and potential nesting habitat within the region instead of emigrating from the region. My results suggest that, despite the regional drop in numbers of active nesting pairs, Caspian terns did not disperse from the Columbia Plateau region in large numbers. Instead, terns that were unable to successfully compete for nest sites within the region returned as non-breeding floaters. This philopatric response was likely reinforced by drought conditions at alternative nesting habitat in the southern Oregon and northeastern California region, which reduced the availability of suitable nest sites. Also, concurrent management to reduce available nesting habitat for Caspian terns at the large breeding colony in the Columbia River estuary caused a major reduction in space for nesting pairs at this location. Both of these factors apparently played a role in the choice of Caspian terns to remain in the Columbia Plateau region post-management. Following the implementation of management to reduce tern nesting habitat in the Columbia Plateau region, Caspian terns exhibited considerable flexibility and adaptability in their choice of nest site, and may have integrated information on the breeding success of conspecifics at other nesting locations within the region.