Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 13,290 terns (July 7) and 6,385 terns (July 12), respectively. This represents a 26% decline in the median count of terns on the East Sand Island colony from last week to this week.

The aerial photo census to estimate the number of breeding pairs at the East Sand Island tern colony was conducted on May 23. A second aerial photo census of the colony was conducted on July 9 in order to estimate the number of young terns fledged from the East Sand Island colony this year. Once the aerial photos are counted (Fall 2003) we will be able to accurately estimate colony size and productivity to determine whether there has been a significant change since 2002. This year the productivity of the East Sand Island tern colony has been high.

Rice Island and Other Upper Estuary Islands

Our weekly surveys for nesting terns on Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands have concluded for the season. These dredge material disposal islands will only be checked occasionally for the remainder of this season, as we do not anticipate the formation of new colonies at this late date.

Washington Coast (Willapa Bay, Grays Harbor)

Our aerial surveys for nesting terns along the southern Washington coast have concluded for the season; we do not anticipate the formation of new colonies at this late date.

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 158 terns (July 11) and 125 terns (July 8), respectively. This represents a 17% decline 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 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).

Diet Composition

For the week of July 7 – July 13, 6% of the identifiable fish delivered to the East Sand Island tern colony (N = 360) were salmonids, a decrease in the percentage as compared to the previous week (8%). Other major prey items being delivered to the East Sand Island colony included anchovy (58%) and clupeids (e.g., herring; 19%). Year to date, 28% of the identifiable fish delivered to the East Sand Island tern colony (N = 4,764) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 36% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2002.

For the week, 68% of the identifiable fish delivered to the Crescent Island tern colony (N = 152) were salmonids, an increase in the percentage as compared to the previous week (65%). Most of the non-salmonids observed being delivered to the Crescent Island colony were cyprinids (e.g., northern pikeminnow, peamouth; 19% of total diet) and bass (10% of total diet). Year to date, 69% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,824) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 72% juvenile salmonids in the diet of terns on Crescent Island by the same date in 2002.

Chick Banding

On July 10 and 11, a total of 450 tern chicks that were near fledging were banded at the East Sand Island tern colony. Each chick was banded with a numbered U.S. Geological Survey metal leg band and 438 of the chicks were also banded with a unique color combination of plastic leg bands.

No mortality or injuries occurred to the banded tern chicks during the above-mentioned banding operations. These banded fledglings will provide important data on survivorship, dispersal patterns, and recruitment of young terns to breeding colonies in future years.

Net Pen Feasibility Study

The net pen feasibility study was concluded on June 6. This 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 was conducted this past week (July 10). The area surveyed included the lower Columbia River from one mile below McNary Dam to the Snake River confluence and the Walla Walla River from the Columbia River confluence to river mile 1. Five of the 12 radio-tagged terns (42%) were detected during the afternoon flight and of these, 4 were on the mainstem Columbia River downstream of Crescent Island (80%) and 1 was on the mainstem Snake River below Ice Harbor Dam (10%). Four of the 5 radio-tagged terns that were resighted during the flight this past week are confirmed breeders on Crescent Island and are still caring for chicks at the colony. Of the 12 adult terns that were radio tagged on Crescent Island on May 13 and 14, 4 terns still have active nests on Crescent Island (33%), two terns have lost their transmitters (17%), two terns attempted to nest on Crescent Island and failed (17%), 2 terns have successfully fledged young from Crescent Island (17%), 1 tern was observed at the East Sand Island tern colony this past week (8%), and 1 tern has not been resighted in any of the surveys conducted since tagging (8%). We plan to conduct one more radio-tracking flight this 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 (0.4 disturbances/hr of observation). The disturbance to the East Sand Island tern colony this past week was from an unknown cause. There were no disturbances observed at the Crescent Island tern colony this past week.

We observed no predation on tern eggs or chicks by gulls at the East Sand Island tern colony this past week. We observed predation on 0 tern eggs and 1 tern chick by gulls at the Crescent Island colony this past week or 0.2 chicks depredated/hr of observation, an increase in the predation rate compared to the previous week (0.0 chicks depredated/hr of observation).

At East Sand Island, gull kleptoparasitism rates this past week were 1% of all known fates of fish delivered by terns, the same kleptoparasitism rate as was observed the previous week. At Crescent Island, gull kleptoparasitism rates this past week were 34% of all known fates of fish delivered by terns, a decrease in the kleptoparasitism rate compared to the previous week (39% of all known fish fates).

Media & Other Visitors

Nanette Seto (USFWS), Howard Browers (USFWS), Tara Zimmerman (USFWS), Geoff Dorsey (USCAE), Cathy Tortorici (NOAA Fisheries), and Jim Bottom (NOAA Fisheries) visited the Crescent Island tern colony this past week.

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