Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 16,310 terns (May 24) and 12,395 terns (May 19), respectively. This represents an 8% increase in the median count of terns on the East Sand Island colony from last week to this week. The first tern chick was observed at the East Sand Island tern colony on May 20th, however we expect the first chick may have hatched on May 18th. 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 3 times each week looking for early signs of nesting (e.g., scrape digging) and will promptly inform resource managers of any such activity.

Washington Coast (Willapa Bay, Grays Harbor)

No aerial surveys were conducted along the southern Washington coast this past week. We plan to fly periodic surveys in these areas looking for nesting terns.

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 925 terns (May 21) and 845 terns (May 20), respectively. 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 May 19 – May 25, 47% of the identifiable fish delivered to the East Sand Island tern colony (N = 353) were salmonids, the same percentage as the previous week. Other major prey items being delivered to the East Sand Island colony included smelt (22%) and clupeids (e.g., herring; 15%). Year to date, 37% of the identifiable fish delivered to the East Sand Island tern colony (N = 2,324) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 43% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2002.

For the week, 63% of the identifiable fish delivered to the Crescent Island tern colony (N = 155) were salmonids, a decrease in the percentage as compared to the previous week (82%). Most of the non-salmonids observed being delivered to the Crescent Island colony were cyprinids (e.g., northern pikeminnow, peamouth; 20% of total diet) and bass (14% of total diet). Year to date, 71% of the identifiable fish delivered to the Crescent Island tern colony (N = 856) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 70% juvenile salmonids in the diet of terns on Crescent Island by the same date in 2002.

Net Pen Feasibility Study

On May 7, a feasibility study was initiated to determine if Caspian terns will feed on fish held in a net pen. If so, this approach can potentially be used to assess the relative vulnerability of various stocks of juvenile salmonids and other fish to Caspian tern predation in a controlled environment.

A 6-meter circular net pen was anchored in a backwater slough off the Columbia River near Crescent Island. 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 the surrounding slough were monitored both before and after the fish were placed in the net pen (approximately 4 hrs/day and 8 hrs/day, respectively).

On May 26, 2 Caspian terns were observed foraging on fish within the net pen. A total of 8 plunge dives were observed, 3 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 commute past the site each day to other foraging areas. As was reported last week, Forster’s terns continue to use the net pen frame as a roost and as many as four individuals have been observed repeatedly feeding on fish within the net pen. We have observed Forster’s terns defending the net pen site by mobbing any bird, including Caspian terns, attempting to forage in or near the net pen. If this behavior continues, we will install wire spike strips on the net pen frame to prevent Forster’s terns and other avian predators from roosting on the net pen frame. These spike strips were initially installed on May 7 and removed nine days later because we suspected they might be deterring feeding by Caspian terns at the net pen. We will continue to monitor bird activity at the net pen for a few more weeks or until the remaining trout in the pen die due to thermal stress.

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 (4/23) 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 radio-tagged terns (83%) were detected during the morning flight and all were on the Crescent Island tern colony. All ten of these terns were confirmed breeders on Crescent Island based on our on-colony blind observations. This past week, one tern (8%) that was radio-tagged on Crescent Island on May 13th was observed at the East Sand Island tern colony and one tern (8%) 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 and will alter the flight schedule in order to maximize the number of off-colony detections of radio-tagged terns (e.g., late afternoon).

Predator Activity

The great horned owl that was observed on East Sand Island on May 12 appears to have left the island. If the great horned owl returns to East Sand Island and it is determined that it is frequenting the colony to prey on terns, efforts will be made to remove the owl from the island as soon as possible.

Disturbance rates at the East Sand Island tern colony were higher this week (0.6 disturbances/hr of observation) compared to the previous week (0.5 disturbances/hr of observation). Most disturbances to the East Sand Island tern colony this past week were either of an unknown cause (40%) or caused by bald eagles (30%).

Researchers caused a disturbance to the East Sand Island tern colony during a brief visit onto the colony (2-3 minutes) to collect tern eggs (20) for later contaminant analysis. We observed 5 eggs being taken by predatory gulls as a result of this disturbance.

We observed predation on 9 tern eggs and 2 tern chicks by gulls at the East Sand Island colony this past week or 0.7 eggs/chicks depredated/hr of observation, an increase in the predation rate compared to the previous week (0.6 eggs/chicks depredated/hr of observation). Five of the 9 egg predations observed this past week were associated with the disturbance caused by our egg collections (see above). We observed no predation on tern eggs or chicks by gulls at the Crescent Island tern colony this past week.

This past week, gull kleptoparasitism rates at East Sand Island were 7% of all known fates of fish delivered by terns, nearly a five-fold decrease in the rate that was observed the previous week (32% of all known fish fates). At Crescent Island, gull kleptoparasitism rates this past week were 26% of all known fates of fish delivered by terns, a decrease in the rate that was observed the previous week (34% of all known fish fates).

Media & Other Visitors

Bernie Taylor (freelance author) visited the East Sand Island tern colony site on Wednesday, May 21.

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