Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 14,670 terns (June 27) and 7,615 terns (June 29), respectively. This represents a 23% decrease in the median count of terns on the East Sand Island colony from last week to this week. The aerial photo census of the East Sand Island tern colony was conducted on May 23. Once the aerial photos are counted (Fall 2003) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2002.

The first fledgling (i.e., young-of-the-year capable of flight) was observed at the East Sand Island tern colony on Tuesday, June 24. 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

Our weekly surveys of Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands for nesting terns has concluded for the season. These dredge material disposal islands will only be checked occasionally for the remainder of this season, as we do not anticipate the formation of new colonies at this late date.

Washington Coast (Willapa Bay, Grays Harbor)

Our aerial surveys along the southern Washington coast for nesting terns have concluded for the season as we do not anticipate the formation of new colonies at this late date.

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 393 terns (June 24) and 241 terns (June 27), respectively. This represents a 9% decrease in the median count of terns on the Crescent Island colony from last week to this week. The aerial photo census of the Crescent Island tern colony was conducted on May 21. Once the aerial photos are counted (Fall 2003) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2002.

The first fledgling (i.e., young-of-the-year capable of flight) was observed at the Crescent Island tern colony on Tuesday, June 24. Nesting success at the Crescent Island tern colony appears high so far this year.

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 June 23 – June 29, 6% of the identifiable fish delivered to the East Sand Island tern colony (N = 351) were salmonids, a decrease in the percentage as compared to the previous week (15%). Other major prey items being delivered to the East Sand Island colony included anchovy (40%) and clupeids (e.g., herring; 28%). Year to date, 31% of the identifiable fish delivered to the East Sand Island tern colony (N = 4,052) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 40% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2002.

For the week, 57% of the identifiable fish delivered to the Crescent Island tern colony (N = 152) were salmonids, a decrease in the percentage as compared to the previous week (63%). Most of the non-salmonids observed being delivered to the Crescent Island colony were bass (20% of total diet) and cyprinids (e.g., northern pikeminnow, peamouth; 17% of total diet). Year to date, 70% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,522) 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 2002.

Net Pen Feasibility Study

The net pen feasibility study was concluded on June 6. This study demonstrated conclusively that Caspian terns can learn to forage for fish confined to a net pen that is equipped with deterrents that prevent other avian predators from using the net pen.

Off-Colony Distribution

Radio-Tagging

On May 13 and 14, a total of 12 adult Caspian terns were captured and radio-tagged at the Crescent Island tern colony. These radio-tagged terns will help determine the foraging patterns of terns nesting on Crescent Island (see previous weekly report for further details on the radio-tagging operation).

Resightings of Radio-Tagged Terns

One radio-tracking flight (June 26) was conducted this past week. The area surveyed included: the lower Columbia River from one mile below McNary Dam to Richland, Washington; the lower Snake River from the Columbia River confluence to 10 miles above Ice Harbor Dam; the Walla Walla River from the Columbia River confluence to Touchet, Washington; and the Yakima River from the Columbia River confluence to river mile 7. Five of the 12 radio-tagged terns (42%) were detected during the morning flight and of these, 2 were on the Crescent Island tern colony (40%), 2 were on the mainstem Columbia River downstream of Crescent Island (40%), and 1 was at McNary Dam (20%). All five of these terns are confirmed breeders on Crescent Island based on our colony observations from the blind. This past week, one tern that was radio-tagged on Crescent Island on May 13th was observed at the East Sand Island tern colony, two terns have lost their transmitters, and four terns (3 of which are confirmed nesters on Crescent Island) were not detected at any of the areas surveyed. We plan to conduct one radio-tracking flight per week for the remainder of the breeding season.

Predator Activity

Disturbance rates at the East Sand Island tern colony were higher this week (0.3 disturbances/hr of observation) compared to the previous week (0.0 disturbances/hr of observation). The disturbances to the East Sand Island tern colony this past week were either of an unknown cause (67%) or caused by bald eagles (33%). There were no disturbances observed at the Crescent Island tern colony this past week.

We observed predation on 1 tern egg and 0 tern chicks by gulls at the East Sand Island colony this past week or 0.1 eggs/chicks depredated/hr of observation, an increase in the predation rate compared to the previous week (0.0 eggs/chicks depredated/hr of observation). We observed no predation on tern eggs or chicks by gulls at the Crescent Island tern colony this past week.

This past week, no gull kleptoparasitism was observed at East Sand Island, a decrease in the kleptoparasitism rate compared to the previous week (0.4% of all known fish fates). At Crescent Island, gull kleptoparasitism rates this past week were 22% of all known fates of fish delivered by terns, an increase in the kleptoparasitism rate compared to the previous week (8% of all known fish fates).

Media & Other Visitors

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

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