Last Updated June 24th

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 10,042 terns (June 20) and 7,951 terns (June 20), respectively. This represents a 21% decrease in the median count of terns on the East Sand Island colony from last week to this week. This decline in colony attendance is likely attributable to three factors: (1) failed breeders and non-breeders dispersing from the colony site, (2) older chicks have higher food requirements and are more independent of parental care, allowing parents to spend more time out foraging, and (3) declines in food availability cause terns to spend more time foraging off-colony.

The first fledgling (i.e., young-of-the-year capable of flight) was observed at the East Sand Island on Friday, June 21. 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

During this past week, no terns were observed roosting or attempting to nest in upland areas on Rice Island, Miller Sands Spit, or Pillar Rock Sands. This week as compared to the previous week, there was a decline in the number of terns observed roosting on the beaches at Rice Island, Miller Sands Spit, and Pillar Rock Sands. In total, 7 terns were observed roosting on the beaches of these three upper estuary dredge spoil islands on June 20. Because of the stage of the Caspian tern nesting season (late chick-rearing) and the absence of any nesting behavior at the upper estuary dredge spoil islands for over a month, we are confident that nesting by terns will not occur on these islands this season. We will continue to monitor these islands on a less frequent basis (weekly) to count numbers of terns roosting and foraging in the upper estuary.

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 415 terns (June 19) and 326 terns (June 20), respectively. This represents a 48% decrease in the median count of terns on the Crescent Island colony from last week to this week. This decline in colony attendance is likely attributable to the same factors listed above for the East Sand Island tern colony.

To date no fledgling terns have been seen flying at the Crescent Island colony, but nesting success so far this year appears to be similar to the last two years.

Diet Composition

For the week, 18% of the identifiable fish delivered to the East Sand Island tern colony (N = 379) were salmonids. This represents a 51% decrease in the percent of the diet that was salmonids for terns on the East Sand Island colony from last week to this week. The majority of non-salmonid prey items delivered to the East Sand Island colony were anchovies and clupeids (i.e., herring). Year to date, 43% of the identifiable fish delivered to the East Sand Island tern colony (N = 3,845) were salmonids (this cumulative percent is an average of the previous weeks' percentages), the same percentage as was recorded by this date in 2001.

For the week, 84% of the identifiable fish delivered to the Crescent Island tern colony (N = 153) were salmonids. This represents a 17% increase in the percent of the diet that was 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 = 1,508) were salmonids (this cumulative percent is an average of the previous weeks' percentages). This compares to 74% juvenile salmonids in the diet of terns on Crescent Island by the same date in 2001.

Observations at McNary Dam

We conducted surveys for fish-eating birds at McNary Dam on Monday, June 17 and Tuesday, June 18. This past week, the average numbers of foraging Caspian terns, gulls, and white pelicans counted in the tailrace of McNary Dam were 13, 9, and 5, respectively. There was a dramatic increase in the number of Caspian terns observed foraging near McNary Dam this week (maximum count = 38 terns), concurrent with large declines in colony attendance at both the Crescent and East Sand island tern colonies. This past week, the number of tern and gull foraging attempts below McNary Dam was roughly 13 and 7 attempts per hour, respectively. Over half of the tern foraging attempts were successful, compared to roughly a third for foraging gulls.

Predator Activity

Bald eagle disturbance rate to the East Sand Island tern colony was 0.10 disturbances/hr of observation, the same disturbance rate as was recorded last week. These disturbances were generally short, causing terns to leave the colony for only 10-15 seconds before returning.

Gull predation rates on tern eggs and chicks decreased this week (0.8 predation events/disturbance), as compared to the previous week (1.0 predation events/disturbance). Gull kleptoparasitism rates have also remained low over the past several weeks (i.e., 0.8%, 0.0%, 0.0%, and 0.6% of all known fates of fish delivered by terns in each of the last four weeks, respectively).

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

Media & Other Visitors

Andy Duncan and Lynn Ketchum from Extension and Experiment Station Communications at OSU visited the East Sand Island tern and cormorant colonies this past week (June 20-21).