Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 15,145 terns (June 4) and 10,455 terns (June 2), respectively. This represents a 12% decrease in the median count of terns on the East Sand Island colony from last week to this week. The aerial photo census of the East Sand Island tern colony was conducted on Friday, May 23. Once the aerial photos are counted (Fall 2003) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2002.

Rice Island and Other Upper Estuary Islands

During this past week, no terns were observed roosting or attempting to nest in upland areas on Rice Island, Miller Sands Spit, Pillar Rock Sands, Fitzpatrick Island, Grassy Island, Crimms Island, and Browns Island (referred to as the upstream end of Puget Island in previous reports). No tern eggs have been found at any of these sites so far this season. We will continue to monitor Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands regularly for the next week looking for early signs of nesting (e.g., scrape digging); subsequently, these dredge material disposal islands will be checked occasionally, as we do not anticipate the formation of new colonies after mid-June.

Washington Coast (Willapa Bay, Grays Harbor)

During aerial surveys conducted along the southern Washington coast this past week (June 4), no terns were observed roosting or attempting to nest in upland areas on any of the islands in Willapa Bay or Grays Harbor. One tern was observed in Willapa Bay and 11 terns were seen in Grays Harbor; both groups were roosting on sand bars below the high tide line.

Up-river Sites (Crescent Island, Three Mile Canyon Island, Miller Rocks)

The high and low on-colony counts for the week on Crescent Island were 774 terns (June 4) and 655 terns (June 7), respectively. This represents a 15% decrease in the median count of terns on the Crescent Island colony from last week to this week. The aerial photo census of the Crescent Island tern colony was conducted on Wednesday, May 21. Once the aerial photos are counted (Fall 2003) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2002.

No terns have been observed at the former colony on Three Mile Canyon Island so far this season; mink predation caused complete colony failure in 2000 and 2001 (no terns attempted to nest there in 2002).

This past week, surveys of gull colonies located on the lower Columbia River at Richland Island and Island 18 (both near Richland, Washington) revealed no nesting Caspian terns.

Diet Composition

For the week of June 2 – June 8, 34% of the identifiable fish delivered to the East Sand Island tern colony (N = 325) were salmonids, a decrease in the percentage as compared to the previous week (36%). Other major prey items being delivered to the East Sand Island colony included clupeids (e.g., herring; 27%) and anchovy (17%). Year to date, 36% of the identifiable fish delivered to the East Sand Island tern colony (N = 3,001) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 46% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2002.

For the week, 80% of the identifiable fish delivered to the Crescent Island tern colony (N = 102) were salmonids, an increase in the percentage as compared to the previous week (66%). Most of the non-salmonids observed being delivered to the Crescent Island colony were cyprinids (e.g., northern pikeminnow, peamouth; 9% of total diet) and bass (7% of total diet). Year to date, 72% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,068) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 73% juvenile salmonids in the diet of terns on Crescent Island by the same date in 2002.

Net Pen Feasibility Study

A feasibility study was conducted this year to determine if Caspian terns will feed on fish held in a net pen. If so, this approach could be used to assess the relative vulnerability of various stocks of juvenile salmonids and other fish to Caspian tern predation in a controlled environment.

On May 7, a 6-meter circular net pen was anchored in a backwater slough off the Columbia River near Crescent Island and a total of 2,000 juvenile rainbow trout (approximately 10-15 cm in length) were placed in the net pen. All fish were certified, disease-free triploids (sterile as adults) obtained from the Trout Lodge Hatchery, WA. The net pen and its vicinity were monitored for foraging activity by fish-eating birds for between 4 and 8 hrs/day until the net pen was removed last Friday, June 6.

Early on Monday, June 2, the net pen frame was refitted with wire spike strips to prevent avian predators from roosting on the net pen frame and to deter predators other than terns from foraging on the fish held in the net pen. Caspian terns were observed foraging on fish within the net pen on each of the five days between fitting the net pen with wire spike strips and the termination of the pilot study. This past week, a total of 16 plunge dives by Caspian terns were observed, 6 of which were successful (i.e., resulting in a fish being captured). As many as 7 Caspian terns have been observed foraging in the vicinity of the net pen at one time and many more commuted past the site each day to other foraging areas. Following the reinstallation of the wire spike strips on the net pen frame, Forster’s terns no longer roosted on the net pen and their foraging activity on fish within the net pen declined compared to previous weeks. The net pen feasibility study demonstrated conclusively that Caspian terns can learn to forage for fish confined to a net pen that is equipped with deterrents that prevent other avian predators from using the net pen.

Off-Colony Distribution

Radio-Tagging

On May 13 and 14, a total of 12 adult Caspian terns were captured and radio-tagged at the Crescent Island tern colony. These radio-tagged terns will help determine the foraging patterns of terns nesting on Crescent Island (see previous weekly report for further details on the radio-tagging operation).

Resightings of Radio-Tagged Terns

One radio-tracking flight (June 4) was conducted this past week. The area surveyed included: the lower Columbia River from one mile below McNary Dam to Richland, Washington; the lower Snake River from the Columbia River confluence to 10 miles above Ice Harbor Dam; the Walla Walla River from the Columbia River confluence to Touchet, Washington; and the Yakima River from the Columbia River confluence to river mile 7. Ten of the 12 radio-tagged terns (83%) were detected during the early evening flight and of these, 4 were on the Crescent Island tern colony (40%), 3 were on the mainstem Columbia River downstream of Crescent Island (30%), one was on the mainstem Columbia River upstream of Crescent Island (10%), and one was at a pond adjacent to the Walla Walla River (10%). All ten of these terns are confirmed breeders on Crescent Island based on our colony observations from the blind. This past week, one tern that was radio-tagged on Crescent Island on May 13th was observed at the East Sand Island tern colony and one tern was not detected at any of the areas surveyed. We plan to conduct one radio-tracking flight per week for the remainder of the breeding season.

Predator Activity

Disturbance rates at the East Sand Island tern colony were lower this week (0.1 disturbances/hr of observation) compared to the previous week (0.3 disturbances/hr of observation). Disturbance to the East Sand Island tern colony this past week was caused by bald eagles. There were no disturbances observed at the Crescent Island tern colony this past week.

We observed no predation on tern eggs or chicks at the East Sand or Crescent island tern colonies this past week.

This past week, gull kleptoparasitism rates at East Sand Island were 1% of all known fates of fish delivered by terns, a decrease in the kleptoparasitism rate compared to the previous week (2% of all known fish fates). At Crescent Island, gull kleptoparasitism rates this past week were 15% of all known fates of fish delivered by terns, an increase in the kleptoparasitism rate compared to the previous week (9% of all known fish fates).

Media & Other Visitors

There were no visitors to the East Sand Island tern colony site this past week.

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