Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 14,920 terns (May 17) and 11,570 terns (May 18), respectively. The first tern chick likely hatched at the East Sand Island tern colony on May 18th. The number of terns counted on colony appears to have reached a maximum and will likely decrease as the breeding season progresses. Most of the terns on the East Sand Island colony are incubating eggs.

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. The continuous hazing of terns prospecting at Pillar Rock Sands and Miller Rocks Sands by USACE contractors was discontinued on May 11 and May 14, respectively. 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 due to poor weather conditions. 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 880 terns (May 17) and 829 terns (May 12), respectively. The first tern chick was observed at the Crescent Island tern colony on May 15th. 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 Miller Rocks (just above the Deschutes River confluence) and Little Memaloose Island (just above The Dalles Dam) revealed no nesting Caspian terns.

Diet Composition

For the week of May 12 – May 18, 47% of the identifiable fish delivered to the East Sand Island tern colony (N = 353) were salmonids, a decrease in the percentage compared to the previous week (67%). Other major prey items being delivered to the East Sand Island colony included smelt (21%) and clupeids (e.g., herring; 14%). Year to date, 35% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,971) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 41% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2002.

For the week, 82% of the identifiable fish delivered to the Crescent Island tern colony (N = 111) were salmonids, nearly the same as the percentage for the previous week (83%). Most of the non-salmonids observed being delivered to the Crescent Island colony were cyprinids (e.g., northern pikeminnow, peamouth; 14% of total diet). Year to date, 73% of the identifiable fish delivered to the Crescent Island tern colony (N = 701) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 71% 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. The net pen frame was fitted 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. 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).

In the twelve days following the stocking of fish in the net pen, Forster’s terns have been observed repeatedly feeding on fish within the net pen. Caspian terns, although showing an interest in fish within the net pen (i.e., circling and hovering over the pen), have not attempted to plunge dive for fish within the pen. These observations led us to suspect that the wire spikes on the net pen frame might be deterring feeding by Caspian terns at the net pen. Caspian terns are much larger than Forster’s terns, and may be more wary of potential obstacles near feeding areas. The wire spike strips were removed from the net pen on May 16, allowing the Forster’s terns to roost, conduct courtship feeding, and even copulate on the net pen frame. So far, Caspian terns have still not foraged on fish held in the net pen. We will continue to monitor bird activity at the net pen until Caspian terns are observed foraging on fish held in the net pen or the remaining trout in the pen die due to thermal stress.

Off-Colony Distribution

Radio-Tagging

On May 13 and 14, a total of 16 terns were captured using noose mats placed over nests with eggs on the Crescent Island tern colony. Twelve of the captured terns were weighed and measured, color-banded, and radio-tagged. In addition, 11 of these were dyed pink with Rhodamine, a dye commonly used to mark the plumage of birds for identification at a distance. These radio-tagged terns will help determine the foraging patterns of terns nesting on Crescent Island. We observed that four Caspian tern eggs were damaged during the trapping efforts on the colony. No predation on tern eggs or chicks was observed during trapping on Crescent Island and all captured terns were released unharmed. Following release of the 12 radio-tagged terns, 9 have been resighted on nests at the Crescent Island tern colony. One tern that was radio-tagged on Crescent Island on May 13th was observed digging a nest scrape at the East Sand Island tern colony on May 19th, 310 miles downstream of the Crescent Island colony.

Resightings of Radio-Tagged Terns

A stationary tracking station was set up at Crescent Island to monitor attendance of radio-tagged terns at the colony site. Off-colony distributions of radio-tagged terns will be monitored using fixed-wing aircraft and road-based surveys beginning next week.

Predator Activity

The great horned owl that was observed on East Sand Island on May 12 appears to have left the island. Great horned owls are known predators of adult Caspian terns and have preyed on terns nesting on the East Sand Island colony in previous years. Owls, which hunt at night, can cause large disturbances to colonial nesting birds, sometimes leading to colony abandonment. In 1999, two great horned owls caused serious disruption to the East Sand Island tern colony and had to be removed to prevent the colony from abandoning. We have no evidence to suggest that the great horned owl that was on the island early in the week disturbed or preyed upon terns at the colony. 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 lower this week (0.5 disturbances/hr of observation) compared to the previous week (0.7 disturbances/hr of observation). The majority of the disturbances to the East Sand Island tern colony this past week were caused by bald eagles (60%).

Researchers caused disturbances to the Crescent Island tern colony during the above mentioned tern trapping and radio-tagging operation.

We observed predation on 6 tern eggs by gulls at the East Sand Island colony this past week or 0.6 eggs depredated/hr of observation, an increase in the egg predation rate compared to the previous week (0.2 eggs depredated/hr of observation). We observed predation on 1 tern egg by gulls at the Crescent Island tern colony this past week, or 0.1 eggs depredated/hr of observation. This predation event was not associated with the disturbance caused by capturing and radio-tagging terns.

This past week, gull kleptoparasitism rates at East Sand Island were 32% of all known fates of fish delivered by terns, twice the rate that was observed the previous week (16% of all known fish fates). At Crescent Island, gull kleptoparasitism rates this past week were 34% of all known fates of fish delivered by terns, nearly the same rate as was observed the previous week (29% of all known fish fates). At the East Sand Island tern colony, kleptoparasitism is by glaucous-winged/western gulls, while at Crescent Island kleptoparasitism is by the smaller California gull.

Media & Other Visitors

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

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