Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 7,120 terns (July 14) and 4,840 terns (July 16), respectively. This represents a 39% decrease in the median count of terns on the East Sand Island colony from last week to this week. Most young terns have fledged from the colony and have joined the post-breeding season dispersal with their parents (Caspian terns provide their young with post-fledging care).

The aerial photo census to estimate the number of breeding pairs at the East Sand Island tern colony was conducted on May 23. A second aerial photo census of the colony was conducted on July 9 in order to estimate the number of young terns fledged from the East Sand Island colony this year. Once the aerial photos are counted (Fall 2003) we will be able to accurately estimate colony size and productivity to determine whether there has been a significant change since 2002.

Rice Island and Other Upper Estuary Islands

Our weekly surveys for nesting terns on Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands have concluded for the season; we do not anticipate the formation of new colonies at this late date.

Washington Coast (Willapa Bay, Grays Harbor)

Our aerial surveys for nesting terns along the southern Washington coast have concluded for the season; 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 117 terns (July 16) and 56 terns (July 14), respectively. This represents a 39% 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.

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 July 14 – July 20, 8% of the identifiable fish delivered to the East Sand Island tern colony (N = 350) were salmonids, an increase in the percentage as compared to the previous week (6%). Other major prey items being delivered to the East Sand Island colony included anchovy (54%) and clupeids (e.g., herring; 20%). Year to date, 26% of the identifiable fish delivered to the East Sand Island tern colony (N = 5,114) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 34% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2002.

For the week, 63% of the identifiable fish delivered to the Crescent Island tern colony (N = 155) were salmonids, a decrease in the percentage as compared to the previous week (68%). Most of the non-salmonids observed being delivered to the Crescent Island colony were cyprinids (e.g., northern pikeminnow, peamouth; 21% of total diet) and bass (16% of total diet). Year to date, 69% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,979) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 70% 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. The success of the feasibility study indicates that Caspian tern diet preferences can be assessed in controlled field experiments.

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 the weekly report for May 12-18 for further details on the radio-tagging operation).

Resightings of Radio-Tagged Terns

No radio-tracking flights were conducted this past week due to mechanical problems with the aircraft. This ends the weekly aerial surveys for radio-tracking terns nesting at Crescent Island, as all radio-tagged terns have left the colony.

Predator Activity

There were no disturbances observed at either the East Sand Island or Crescent Island tern colonies this past week.

We observed predation on 1 tern egg and 2 tern chicks by gulls at the East Sand Island colony this past week or 0.4 eggs/chicks depredated/hr of observation, an increase in the predation rate compared to the previous week (0.2 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.

At East Sand Island, gull kleptoparasitism rates this past week were 3% of all known fates of fish delivered by terns, an increase in the kleptoparasitism rate compared to the previous week (1% of all known fish fates). At Crescent Island, gull kleptoparasitism rates this past week were 39% of all known fates of fish delivered by terns, an increase in the kleptoparasitism rate compared to the previous week (34% 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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