Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 5,455 terns (April 17) and 2,080 terns (April 14), respectively. Early breeding behaviors (i.e., scrape digging, courtship feeding, copulation) increased in frequency at the colony site throughout the week.

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

On April 15, Caspian terns were observed loafing on Pillar Rock Sands (a dredged material disposal island in the upper estuary) in upland areas (12 terns) and on the beach (154 terns). On April 18, no terns were observed in upland areas and 120 terns were observed loafing on the beach at Pillar Rock Sands. Recently (April 21), as many as 332 terns were seen on the upland area of Pillar Rocks Sands during low tide. This is significant because if the terns were just loafing near a foraging site they would likely use the beach during low tide. Other indications of their intention to nest on Pillar Rock Sands were courtship displays, exchange of courtship meals, and copulations, although no nest scrapes were found on April 21. We think that taken together, these observations suggest that egg-laying on Pillar Rock Sands may occur as early as next week (April 27). Resource managers were informed of the situation and the USACE has plans to attempt to dissuade terns from nesting on Pillar Rocks Sands.

No aggregations of terns have been observed at other upland areas in the upper estuary (Rice Island, Miller Sands Spit) this week. Given that the entire former colony area on Rice Island is now vegetated, we do not expect that terns will attempt to nest at the former colony site on Rice Island in 2003. We will continue to monitor Rice Island, Pillar Rock Sands, 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 316 terns (April 17) and 145 terns (April 15), respectively. The first tern egg was laid at the Crescent Island colony on April 20th or April 21st, 5-6 days later than the first egg date on the Crescent Island tern colony in 2002. No terns have been observed at the former colony on Three Mile Canyon Island so far this season; mink predation caused complete colony failure in 2000 and 2001 (no terns attempted to nest there in 2002).

Diet Composition

For the week of April 14 - 20, 19% of the identifiable fish delivered to the East Sand Island tern colony (N = 368) were salmonids. The majority of the prey items being delivered to the East Sand Island colony were smelt (41%) and sand lance (20%). Year to date, 16% of the identifiable fish delivered to the East Sand Island tern colony (N = 551) were salmonids (this cumulative percent is an average of the previous weeks’ percentages).

For the week, 81% of the identifiable fish delivered to the Crescent Island tern colony (N = 77) were salmonids. Most of the non-salmonids observed being delivered to the Crescent Island colony were cyprinids (e.g., northern pikeminnow, peamouth; 8% of total diet) and bass (5% of total diet). Year to date, 65% of the identifiable fish delivered to the Crescent Island tern colony (N = 121) were salmonids (this cumulative percent is an average of the previous weeks’ percentages).

Predator Activity

Disturbances to the East Sand Island tern colony declined (0.6 disturbances/hr of observation) as compared to the previous week (0.8 disturbances/hr of observation). The majority (80%) of these disturbances this past week were caused by bald eagles. Terns appear to be much more settled on the colony and are less likely to take flight when an eagle is flying high over the colony, as compared to earlier in the breeding season. These disturbances are short, causing terns to leave the colony for only 10-15 seconds before returning.

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 14% of all known fates of fish delivered by terns, twice the rate observed the previous week. At Crescent Island, gull kleptoparasitism rates were 29% 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

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

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