Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 5,544 terns (17 April) and 1,150 terns (11 April), respectively. The colony has continued to increase in size despite the wet and cool weather in the estuary. Eight inches of rain have fallen in Astoria so far this April, with over five inches falling this past week. This has resulted in considerable standing water on the East Sand Island tern colony, up to 60-70% of the tern colony area at the height of the rainfall. After heavy rains subside, most of the standing water drains from the colony within a few hours. One exception is an area roughly 100 m² in size that is located just north of the SW blind, which has yet to drain entirely. Flooding on the colony has caused early nesting terns to occupy the higher ground around the edges of the colony; the core area of the colony is filling in with nesting terns as this area dries out.

The first Caspian tern egg of the season on East Sand Island was seen on 14 April. This is 2 days earlier than the first egg was observed on the East Sand Island colony last year, and the earliest lay date so far recorded for Caspian terns in the Columbia River estuary.

Rice Island and Other Upper Estuary Islands

On 13 April, Caspian terns (36) were observed loafing in upland areas on Pillar Rock Sands (a dredged material disposal island in the upper estuary). During high tide on 17 April, a total of 396 terns were seen on upland areas of Pillar Rocks Sands and 37 terns were seen on upland areas of Miller Sands Spit (another dredge disposal island in the upper estuary). No breeding behaviors (i.e., courtship displays, exchange of courtship meals, copulations, or digging of nest scrapes) have been observed at these sites. Resource managers were informed of the situation and the USACE plans to dissuade terns from nesting in the upper estuary using flagging, eagle decoys, and, if necessary, active hazing.

During this past week, no terns were observed roosting or attempting to nest in upland areas of Rice Island. Given that the entire former colony site on Rice Island is now vegetated, we do not expect that terns will attempt to nest there in 2005.

We will continue to monitor Rice Island and the other upper estuary dredged material disposal islands for early signs of nesting (e.g., nest scrape digging) and will promptly inform resource managers of any such activity.

Up-river Sites (Crescent Island)

The high and low on-colony counts for the week on Crescent Island were 452 terns (17 April) and 205 terns (13 April), respectively. The first tern egg was laid at the Crescent Island colony on 12 April, the same date the first egg was observed on Crescent Island the previous year.

Nesting terns have not been observed at other sites along the Mid-Columbia River.

Diet Composition

For the week of 11–17 April, 27% of the identifiable fish delivered to the East Sand Island colony (N = 275) were salmonids. The majority of the prey items being delivered to the East Sand Island colony were clupeids (i.e., herring; 29%). Year to date, 26% of the identifiable fish delivered to the East Sand Island tern colony (N = 362) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 8% salmonids in the diet by this time last year.

For the week, 79% of the identifiable fish delivered to the Crescent Island tern colony (N = 167) were salmonids. Year to date, 88% of the identifiable fish delivered to the Crescent Island tern colony (N = 202) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 80% salmonids in the diet by this time last year.

Predator Activity

At East Sand Island, bald eagles have been observed at the colony throughout the week, frequently causing terns to take flight. A total of 20 colony disturbances were observed (all causes) at a rate of 0.8 disturbances/hr of observation. Generally, these disturbances are short, causing terns to flush for only 10-30 seconds before returning to the colony.

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

This past week, gull kleptoparasitism rates at East Sand Island were 3% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 13% 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 California gull.

To date, there have been no signs of mammalian predators on any of the tern colonies monitored as part of this study.

Media & Other Visitors

During 15-17 April, Tyler Leake visited East Sand Island and helped the colony monitors build tent platforms and walkways (thank you, Tyler!).

On 13 April, Scott Dunmire visited the Crescent Island Caspian tern colony.

Site developed and maintained by Real Time Research, Inc.

Comments about the site? Contact clientservices@realtimeresearch.org