Last Updated July 20th
Note: The following data are preliminary and may change upon further review. East Sand Island The first fledgling (i.e., young-of-the-year capable of flight) was observed at the East Sand Island colony on 19 June. Most tern fledglings have now left the colony, and are either gathered in large groups on the beach or have flown from the island with their parents. Nesting success at the East Sand Island tern colony appears high this year. A second aerial photo census of the Caspian tern colony on East Sand Island was conducted on 3 July. Once these aerial photos are counted and verified we will be able to accurately estimate the number of young terns fledged from the East Sand Island colony this year. 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 for this year. No active Caspian tern nests (nest scrapes containing eggs) were detected at any of these upper estuary sites this year, and we do not anticipate the formation of any new colonies this late in the season. The first fledgling (i.e., young-of-the-year capable of flight) was observed at the Crescent Island tern colony on 21 June. Based on preliminary estimates, nesting success at the Crescent Island tern colony has been only fair this year (0.62 fledglings raised per breeding pair), although slightly higher than productivity at this colony last 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, WA; 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.). Nesting success at the other colony on Potholes Reservoir (Goose Is.) was low this year.
For the week, 54% of the identifiable fish delivered to the Crescent Island tern colony (N = 250) were salmonids, a lower percentage as compared to the previous week (78%). Most of the non-salmonids observed being delivered to the Crescent Island colony were centrarchids (e.g., bass, sunfish; 31%). Year to date, 71% of the identifiable fish delivered to the Crescent Island tern colony (N = 2,950) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 69% salmonids in the diet of Crescent Island terns by this time last year.
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, a 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.
At East Sand Island , no colony disturbances were observed this past week. At Crescent Island, one colony disturbance was observed, for a rate of 0.1 disturbances/hr of observation. Generally, these disturbances are short, causing terns to flush for only 10-30 seconds before returning to the colony. During plot watches, no nest predation was observed at the East Sand Island or Crescent Island tern colonies this past week. This past week, gull kleptoparasitism rates at East Sand Island were 5% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 12% 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.
Some of the captive-reared fledglings are now subjects in an experiment designed to better understand the passage rates and route of egestion for PIT tags consumed when Caspian terns eat a PIT-tagged salmonid smolt. In the first six days of the 8-day experiment, all PIT tags consumed by the tern fledglings have been regurgitated in castings of indigestible material, instead of defecated. Some PIT tags have been regurgitated less than four hours after they were consumed in a fish. Estimates of the mean retention time for ingested PIT tags, together with information on when and how PIT tags are egested will help us calculate predation rates on salmonid smolts by Caspian terns based on recovery rates of smolt PIT tags on tern nesting colonies. Alex Morgan, Richard Huffman, and Barron Cato from Seattle Audubon visited East Sand Island this past week. |
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The high and low on-colony counts for the week on East Sand Island were 7,940 terns (13 July) and 5,258 terns (15 July), respectively. The first aerial photo census of the Caspian tern colony on East Sand Island was conducted on 17 May. Once these aerial photos are counted and verified we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2003. 
The high and low on-colony counts for the week on Crescent Island were 224 terns (16 July) and 165 terns (12 July), 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, similar to 2003. 
For the week of 12 July - 18 July, 6% of the identifiable fish delivered to the East Sand Island tern colony (N = 350) were salmonids, a higher percentage as compared to the previous week (3%).. Most of the non-salmonids observed being delivered to the East Sand Island colony were clupeids (e.g., herring and sardines; 39% of prey items), anchovy (28% of prey items), and surfperch (16% of prey items). Year to date, 17% of the identifiable fish delivered to the East Sand Island tern colony (N = 5,143) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 26% salmonids in the diet of East Sand Island terns by this time last year. The proportion of juvenile salmonids in the diet of East Sand Island terns this year promises to be the lowest since we began collecting diet data in 1997.