Last Updated May 13th

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 13,031 terns (May 10) and 10,208 terns (May 10), respectively. This represents an 8% increase in the median count of terns on the East Sand Island colony from last week to this week. Compared to last year at this time, there are an estimated 1,200 (9%) fewer terns on the East Sand tern colony this year (based on median counts). The aerial photo census of the East Sand Island tern colony is scheduled for next week. Once the aerial photos are counted we will be able to estimate colony size with greater accuracy and determine whether there has been a significant change in colony size since 2001.

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, or Pillar Rock Sands. On Friday (May 10), there was a bald eagle perched near the upland area used previously by prospecting terns on Pillar Rock Sands. On that same date, there were no terns observed anywhere on Pillar Rock Sands (in upland areas or on the beaches). We will continue to monitor Rice Island and other upper estuary dredged material disposal islands 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 885 terns (May 8) and 850 terns (May 6), respectively. This represents a 7% increase in the median count of terns on the Crescent Island colony from last week to this week. The median counts of terns on the Crescent Island colony were roughly the same this year (868 terns), as compared to last year at this time (874 terns).

Diet Composition

For the week of May 6 - May 12, 47% of the identifiable fish delivered to the East Sand Island tern colony (N = 366) were salmonids. Similar to last week, the majority of the non-salmonid prey items being delivered to the East Sand Island colony were surfperch and anchovy. Year to date, 39% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,671) were salmonids (this cumulative percent is an average of the previous weeks' percentages). This compares to 47% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2001.

For the week, 95% of the identifiable fish delivered to the Crescent Island tern colony (N = 159) were salmonids. This represents a 12% increase in the percent of the diet that were salmonids for terns on the Crescent Island colony from last week to this week. Year to date, 73% of the identifiable fish delivered to the Crescent Island tern colony (N = 608) were salmonids (this cumulative percent is an average of the previous weeks' percentages). This compares to 68% juvenile salmonids in the diet of terns on Crescent Island by the same date in 2001.

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 at McNary Dam revealed the apparent effectiveness of avian predation abatement methods, primarily pyrotechnics, implemented at the dam by USDA - Wildlife Services. We conducted our surveys at McNary Dam on Tuesday (May 7; when birds were not being actively hazed) and Thursday (May 9; when birds were being actively hazed). The average number of gulls and white pelicans counted near McNary Dam were 2-3 times higher when hazing activities were not taking place, as compared to when hazing activities were taking place. Similarly, the number of foraging attempts by gulls in the tailrace was roughly six times higher without hazing activities (42.2 attempts per hour), as compared to with hazing activities (7.7 attempts per hour). The success rate of foraging gulls (% of foraging attempts that resulted in the bird catching a fish) did not vary with predator abatement activities; the rate with and without hazing was roughly 50%. Thus far, double-crested cormorants and Caspian terns have not been observed at McNary Dam in appreciable numbers.

Predator Activity

Bald eagle disturbances to the East Sand Island tern colony were similar this last week (0.24 disturbances/hr of observation), as compared to the previous week (0.27 disturbances/hr of observation). These disturbances are generally short, causing terns to leave the colony for only 10-15 seconds before returning.

Although the disturbance rate to the East Sand Island tern colony was slightly lower this past week as compared to the previous week, gull predation on tern eggs increased once again; the second straight weekly increase. We observed 1.5 egg predation events per disturbance this last week, compared to 0.4 egg predation events per disturbance the previous week. We will continue to monitor the level of nest predation by gulls at the East Sand Island tern colony and present 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