Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 8,880 terns (4 July) and 3,490 terns (10 July), respectively. The annual photo census of the Caspian tern colony on East Sand Island was conducted on 17 May. Visual estimates of colony size from the ground suggest that the East Sand Island colony has declined somewhat since the 2004 nesting season. Once these aerial photos are counted (Fall 2005) we will be able to accurately estimate colony size and determine whether there has been a significant decline in colony size since 2004.

The first fledgling (i.e., young-of-the-year capable of flight) was observed at the East Sand Island colony on 23 June. Nesting success at the East Sand Island tern colony appears somewhat lower than in recent years. A second aerial photo census of the colony was conducted on 7 July. Once these aerial photos are counted (Fall 2005) we will be able to accurately estimate the number of young terns fledged from the East Sand Island colony this year.

Rice Island and Other Upper Estuary Islands

Our weekly boat-based surveys to monitor Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands looking for early signs of nesting (e.g., scrape digging) have been discontinued; we may check these dredged material disposal islands occasionally, but we do not anticipate the formation of any new colonies this late in the season.

Upper Columbia River Sites (Crescent Island, Three Mile Canyon, Miller Rocks)

The high and low on-colony counts for the week on Crescent Island were 185 terns (7 July) and 60 terns (10 July), respectively. The annual photo census of the Caspian tern colony on Crescent Island was conducted on 17 May. Preliminary visual counts on the ground suggest that the Crescent Island tern colony has declined by about 10% since the 2004 nesting season. Once these aerial photos are counted (Fall 2005) we will be able to accurately estimate colony size and determine whether there has been a significant decline in colony size since 2004.

The first fledgling (i.e., young-of-the-year capable of flight) was observed at the Crescent Island tern colony on 22 June. Nesting success at the Crescent Island tern colony appears to be lower this year compared to last.

Nesting Caspian terns have not been observed at former colony sites along the Mid-Columbia River at Three Mile Canyon Island (John Day Pool), Miller Rocks (The Dalles Pool), and Cabin Island (Priest Rapids Pool).

A small, new Caspian tern breeding colony has been discovered on Rock Island in the Blalock Islands, John Day pool (between the towns of Boardman and Irrigon, Oregon). On 5 July, there were 3 adults and one black-capped fledgling observed on the island. As many as six breeding pairs were observed during previous visits to the island. Nesting Caspian terns shared the island with nesting California gulls and Forster’s terns (ca. 60 pairs).

Diet Composition

For the week of 4 July – 10 July, 5% of the identifiable fish delivered to the East Sand Island colony (N = 366) were salmonids. The majority of the prey items being delivered to the East Sand Island colony were anchovies (49%) and surfperch (30%). Year to date, 26% of the identifiable fish delivered to the East Sand Island tern colony (N = 4,656) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 18% salmonids in the diet at this time last year. In the last six weeks the percent salmonids in the diet of East Sand Island Caspian terns has declined to the levels seen last year at this time, suggesting that there has been an influx of anchovies and other marine forage fishes into the estuary.

For the week, 40% of the identifiable fish delivered to the Crescent Island tern colony (N = 200) were salmonids, a significant drop in the salmonid percentage from previous weeks. Year to date, 72% of the identifiable fish delivered to the Crescent Island tern colony (N = 2,678) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 73% salmonids in the diet at this time last year.

Predator Activity

At East Sand Island, three colony disturbances were observed at a rate of 0.3 disturbances/hr of observation. Generally, disturbances of the East Sand Island tern colony by bald eagles have been more frequent and more intense than last year, including observations of eagles roosting on the colony and preying on adult Caspian terns on or near the colony. During these eagle disturbances, a large proportion of the adult terns on the colony may flush, leaving tern eggs and chicks vulnerable to predation by gulls nesting near the tern colony. Gull predation on Caspian tern eggs and chicks at the East Sand Island colony has been more frequent this year than in any other year since the colony was established in 1999.

At Crescent Island, no colony disturbances were observed this past week.

This past week, gull kleptoparasitism rates at East Sand Island were 1% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 3% 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

On 5-6 July, Tyler Leake visited East Sand Island. Another visitor to East Sand Island caused quite a stir on 7 July. Although the visit of a black-tailed deer to East Sand Island is amusing, it is a sobering reminder that large mammalian predators, such as coyotes, could reach the island and cause a major disruption of nesting Caspian terns and other seabirds breeding and roosting on East Sand Island.

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