Note: The following data are preliminary and may change upon further review.

Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 17,966 terns (17 June) and 8,752 terns (14 June), respectively. The annual photo census of the Caspian tern colony on East Sand Island was conducted on 17 May. Once the aerial photos are counted (Fall 2004) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2003.

The first fledgling (i.e., young-of-the-year capable of flight) was observed at the East Sand Island on Saturday, 19 June. Nesting success at the East Sand Island tern colony appears high so far this year. A second aerial photo census of the colony will be conducted in early July in order to estimate the number of young terns fledged from the East Sand Island colony this year.

Rice Island and Other Upper Estuary Islands

Our weekly boat-based surveys to monitor Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands looking for early signs of nesting (e.g., scrape digging) have been discontinued; occasionally, we may check these dredged material disposal islands, but we do not anticipate the formation of any new colonies this late in the season.


Washington Coast (Willapa Bay, Grays Harbor)

No surveys for Caspian terns in Willapa Bay and Grays Harbor were conducted this past week; occasionally, we may check these areas for terns, but we do not anticipate the formation of any new colonies this late in the season.

Up-river Sites (Crescent Island)

The high and low on-colony counts for the week on Crescent Island were 348 terns (16 June) and 301 terns (15 June), respectively. Based on six independent ground counts of incubating adult terns on the Crescent Island colony at the onset of hatching, we estimate that approximately 530 breeding pairs attempted to nest at the Crescent Island tern colony in 2004, or about 20 more pairs than in 2003.

The first fledgling (i.e., young-of-the-year capable of flight) was observed at the Crescent Island tern colony on Monday, 21 June. Nesting success at the Crescent Island tern colony appears fair so far this year.

We have detected no Caspian terns attempting to nest at other colony sites on the Mid-Columbia River, Lower Snake River, or Lower Yakima River this breeding season. Caspian terns were nesting at two sites on islands in Potholes Reservoir near Moses Lake; terns nesting at one of these sites (Solstice Is.) failed due to rising water levels that flooded low lying nests and black-crowned night heron predation on the remaining nests (C. Moranto, University of Washington, pers. comm.).

Diet Composition

For the week of 14 June – 20 June, 10% of the identifiable fish delivered to the East Sand Island tern colony (N = 356) were salmonids, a lower percentage as compared to the previous week (18%). Most of the non-salmonids observed being delivered to the East Sand Island colony were anchovy (29% of the total diet) and surfperch (28% of the total diet). Year to date, 22% of the identifiable fish delivered to the East Sand Island tern colony (N = 3,679) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 33% salmonids in the diet by this time last year.

For the week, 66% of the identifiable fish delivered to the Crescent Island tern colony (N = 225) were salmonids, a higher percentage as compared to the previous week (65%). Most of the non-salmonids observed being delivered to the Crescent Island colony were centrarchids (e.g., bass, sunfish; 24%) and cyprinids (e.g., northern pikeminnow, peamouth; 6%). Year to date, 74% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,999) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 71% salmonids in the diet of Crescent Island terns by this time last year.

Net Pen Study

Earlier this season, a net pen was deployed near Crescent Island and stocked with approximately 1,000 PIT-tagged juvenile rainbow trout to investigate (1) PIT tag deposition rates at the Crescent Island tern colony, (2) vulnerability of fish in different size classes to tern predation, and (3) gull kleptoparasitism rates on fish captured by terns at the net pen.

The net pen and the surrounding slough were monitored 8 – 15 hrs/day, 7 days/week, from 24 April to 4 June. A net was placed over the net pen after each observation period to prevent terns and other predators from feeding on fish held in the pen when observers were not present. On 26 April, Caspian terns began foraging on fish within the net pen. During the 40-day study period, total of 94 PIT-tagged trout were removed from the net pen by Caspian terns during 207 attempts (i.e., plunge dives into the net pen). No other predators were observed removing PIT-tagged trout from the net pen, with the exception of Forster’s terns, which removed a total of 43 fish from the net pen.

Following the breeding season and after all the terns and gulls have left their respective colonies, the Crescent Island tern and gull colonies will be scanned for PIT tags by NOAA Fisheries. The number of PIT tags that were removed from the net pen by terns and subsequently detected on the Crescent Island tern and gull colonies will be used to address the research objectives listed above.

Predator Activity

At East Sand Island, two colony disturbances (bald eagles) were observed at a rate of 0.2 disturbances/hr of observation. Generally, these disturbances are short, causing terns to flush for only 10-30 seconds before returning to the colony.

There were no disturbances observed at the Crescent Island tern colony this past week.

During plot watches, no nest predation was observed at the East Sand Island tern colony this past week. One tern egg was depredated by a California gull at the Crescent Island tern colony this past week, or 0.1 nest depredations/hr of observation.

This past week, gull kleptoparasitism rates at East Sand Island were 3% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 9% of all known fates of fish delivered by terns this past week. At the East Sand Island tern colony, kleptoparasitism is by glaucous-winged/western gulls, while at Crescent Island kleptoparasitism is by the smaller Californiagull.

So far this season there have been no signs of mammalian predators on any of the tern colonies monitored as part of this study.

Captive Rearing Experiments

On 1 June, 40 recently-hatched Caspian tern chicks were collected from the East Sand Island colony for captive-rearing experiments. These experiments were designed to evaluate the Quantitative Fatty Acid Signature Analysis method for nondestructive determination of diet composition in piscivorous waterbirds. We will also use these captive-reared chicks to better understand PIT tag passage rates in Caspian terns. The tern chicks are being raised on different diets of forage fish, and fat will be biopsied from the chicks periodically to validate the use of this method to detect dietary differences among free-ranging Caspian terns. So far, all captive chicks are displaying healthy appetites and thriving, with the exception of one chick that suffered from a congenital developmental abnormality.

Media & Other Visitors

Kathleen O’Reilly and six of her students (Andy Maggi, Toni Boesiger, Devon Johnstone, Maria Thaiveettil, Kyoung Hwan Oh, and Sara St. James) from the University of Portland visited the East Sand Island tern colony site on 14 June.

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