Last Updated May 6th

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 12,516 terns (May 4) and 8,977 terns (April 29), respectively. This represents a 37% increase in the median count of terns on the East Sand Island colony from last week to this week. Although the colony disturbance rate increased from last week to this week (see below), in general terns have become more settled on the colony and are distributing themselves more evenly across the entire colony area.

Rice Island and Other Upper Estuary Islands

During this past week, no terns have been observed roosting or attempting to nest in upland areas on Rice Island, Miller Sands Spit, Pillar Rock Sands, or Puget Island. On Wednesday (May 1), the continuous monitoring and hazing of terns prospecting at the Pillar Rock Sands site was discontinued by the USACE (terns had not attempted to settle in upland sites on Pillar Rock Sands since Monday, April 29). We will continue to monitor Rice Island and other upper estuary dredged material disposal islands (3 times a week) looking for early signs of nesting (e.g., scrape digging) and will promptly inform resource managers of any such activity.

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 850 terns (May 1) and 770 terns (April 29), respectively. This represents a 14% increase in the median count of terns on the Crescent Island colony from last week to this week. No terns have been observed on the former colony on Three Mile Canyon Island so far this season; mink predation caused complete failure of the tern colony on Three Mile Canyon Island in the previous two years. Thus far in 2002, terns have not been observed at Miller Rocks either, the site of a new tern colony (roughly 10-15 pairs) in 2001.

Diet Composition

For the week of April 29 - May 5, 42% of the identifiable fish delivered to the East Sand Island tern colony (N = 363) were salmonids. Similar to last week, the majority of the non-salmonid prey items being delivered to the East Sand Island colony were smelt, anchovies, and clupeids (i.e., herring, sardines). Year to date, 37% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,305) were salmonids (this cumulative percent is an average of the previous weeks' percentages).

For the week, 85% of the identifiable fish delivered to the Crescent Island tern colony (N = 162) were salmonids. As was the case last week, most of the non-salmonids observed being delivered to the Crescent Island colony were bluegill. Year to date, 67% of the identifiable fish delivered to the Crescent Island tern colony (N = 449) were salmonids (this cumulative percent is an average of the previous weeks' percentages).

Off-Colony Distribution

In 2002, researchers will be investigating the distribution and foraging behavior of piscivorous waterbirds (primarily gulls, terns, cormorants, and pelicans) at McNary Dam. Surveys during the past week revealed a notable increase in the number of gulls and white pelicans observed in the immediate vicinity of McNary Dam. During two days of observation at McNary Dam this past week, as many as 30 gulls and 9 white pelicans were observed near the dam, compared to maximum counts of 15 gulls and 4 white pelicans the previous week. Most foraging activity continues to be concentrated in tailrace areas at McNary Dam.

Predator Activity

Bald eagle disturbances to the East Sand Island tern colony increased this past week (0.27 disturbances/hr of observation) as compared to the previous week (0.06 disturbances/hr of observation). On Friday (May 3), 13 bald eagles were counted on the north beach adjacent the tern colony. Terns on colony did not flush while the eagles were on the beach; however, eagles flying over the colony did cause terns to take flight. These disturbances are generally short, causing terns to leave the colony for only 10-15 seconds before returning.

Associated with the increase in bald eagle disturbance to the East Sand Island tern colony this past week, gull predation on tern eggs also increased, as compared to the previous week. Typically, when nesting terns take flight during a disturbance, gulls will move in to take eggs (and chicks during the chick rearing period) from unattended tern nests. In previous years, disturbances by bald eagles varied week to week and generally declined as the nesting season progressed. We will continue to monitor the level of nest predation by gulls at the East Sand Island tern colony to see if there is an increasing trend and report those results in subsequent weekly reports.

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

Media & Other Visitors

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