Last Updated April 22th

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 7,905 terns (April 20) and 1,445 terns (April 16), respectively. The first tern egg was observed on the East Sand Island colony on April 18th, a day earlier than the first egg date on the East Sand Island tern colony in 2001.

All the areas on colony that were flooded after the heavy rains last week are now completely dried out and are almost entirely occupied by nesting terns.

Rice Island and Other Upper Estuary Islands

No evidence of nesting by Caspian terns (i.e., nest scrapes) has been observed at Rice Island, Miller Sand Spit, or Pillar Rock. On April 20, 78 and 53 terns were observed roosting below the high tide line on Miller Sands Spit and Pillar Rock, respectively.

Given that the entire former colony area on Rice Island is now either flagged or vegetated, we do not expect that terns will attempt to nest at the former colony site on Rice Island in 2002. We will continue to monitor Rice Island and other upper estuary dredge 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 690 terns (April 21) and 285 terns (April 17), respectively. The first tern egg was observed on the Crescent Island colony on April 15th, a day earlier than the first egg date on the Crescent Island tern colony in 2001. 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 15 - 21, 36% of the identifiable fish delivered to the East Sand Island tern colony (N = 388) were salmonids. The majority of the prey items being delivered to the East Sand Island colony were sand lance, clupeids (i.e., herring, sardines), and smelt. Year to date, 33% of the identifiable fish delivered to the East Sand Island tern colony (N = 575) were salmonids (this cumulative percent is an average of the previous weeks' percentages).

For the week, 71% of the identifiable fish delivered to the Crescent Island tern colony (N = 106) were salmonids. Most of the non-salmonids observed being delivered to the Crescent Island colony were yellow perch. Year to date, 48% of the identifiable fish delivered to the Crescent Island tern colony (N = 134) 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. Periodic surveys during the past week revealed very few piscivorous waterbirds in the immediate vicinity of McNary Dam. Other studies of fish-eating birds at Columbia River dams have shown that the greatest level of activity is in late April and May, presumably associated with peak numbers of juvenile salmonids passing the dams at that time.

Predator Activity

At East Sand Island, bald eagles continue to cause disturbances to the tern colony, although the rate of these disturbances (0.16 disturbances/hr of observation) has declined by roughly half compared to the previous week (0.34 disturbances/hr of observation). Generally, these disturbances are short, causing terns to leave the colony for only 10-15 seconds before returning.

Bald eagles disturbances have also declined at the Crescent Island tern colony. On April 20 and 21, no disturbances to the tern colony were observed. Most of the migratory waterfowl in the vicinity of Crescent Island have left the area, which may explain the observed decrease in eagle activity near the Crescent Island tern colony.

Media & Other Visitors

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