Last Updated May 31st
Note: The following data are preliminary and may change upon further review.
Caspian Tern Colony Counts East Sand Island 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 may have declined somewhat since the 2004 nesting season. Once the aerial photos are counted (Fall 2005) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2004.
During this past week, no terns were observed roosting or attempting to nest in upland areas of Pillar Rock Sands, Miller Sands Spit, or 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.
Nesting Caspian terns have not been observed at other potential colony sites along the Mid-Columbia River, including the former colony sites at Three Mile Canyon Island (John Day Pool), Miller Rocks (The Dalles Pool), and Cabin Island (Priest Rapids Pool). For the week of 23 May – 29 May, 36% of the identifiable fish delivered to the East Sand Island colony (N = 368) were salmonids. The majority of the non-salmonid prey items being delivered to the East Sand Island colony were anchovies (24%) and clupeids (e.g., herring; 15%). Year to date, 39% of the identifiable fish delivered to the East Sand Island tern colony (N = 2,512) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 23% salmonids in the diet at this time last year; consequently, there has been a 70% increase in the proportion of salmonids in the diet so far in 2005 compared to 2004. For the week, 66% of the identifiable fish delivered to the Crescent Island tern colony (N = 245) were salmonids. Year to date, 77% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,349) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 76% salmonids in the diet by this time last year.
This past week, we supplemented the Burbank Slough net pen with approximately 100 PIT-tagged trout removed from the Ice Harbor net pen. The cover was removed from the Ice Harbor net pen and observations of the pen were reduced to just two hours every other day due to the lack of tern foraging activity at the site. A total of 15 fish were removed from the Burbank Slough net pen by Caspian terns this past week, thee times the number removed from the pen in the previous 5 weeks (n = 5). This increase in foraging activity at the Burbank Slough net pen may have been associated with the increase in the density of fish within the net pen (see above), decreasing numbers of alternative prey for terns near Crescent Island, and the clear weather that may have increased the visibility of fish within the pen. We will continue our daily monitoring of the Burbank Slough net pen until mid-June.
At East Sand Island, a total of 10 colony disturbances were observed at a rate of 0.6 disturbances/hr of observation. Generally, disturbances at the tern colony were short, causing terns to flush for only 10-30 seconds before returning to the colony. The colony monitors observed a bald eagle prey upon an adult Caspian tern that was sitting on its nest.on May 23. This is the first time that an adult Caspian tern has been observed being taken by a bald eagle on the East Sand Island colony. At Crescent Island, no colony disturbances were observed this past week. This past week, there was no gull kleptoparasitism observed during fish watch at the East Sand Island Caspian tern colony. At Crescent Island, gull kleptoparasitism rates were 2% 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.
On 27 May, Grant McOmie and Kevin Eyers (KATU Channel 2, Portland) visited East Sand Island. |
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The high and low on-colony counts for the week on East Sand Island were 13,910 terns (28 May) and 8,050 terns (26 May), respectively. The first tern chick was observed on East Sand Island on 16 May, 3 days later than the first chick was seen on the East Sand Island tern colony in 2004. Colony disturbances, mostly by bald eagles, continue to be common, more so than in previous years. Many new nesting attempts have been initiated by terns on East Sand Island this past week, presumably by failed breeders. 
The high and low on-colony counts for the week on Crescent Island were 669 terns (24 May) and 573 terns (28 May), respectively. The first tern chick was observed on Crescent Island on 9 May, 1 day earlier than the first chick was seen on the Crescent Island tern colony in 2004. 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 may have declined slightly since the 2004 nesting season. Once the aerial photos are counted (Fall 2005) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2004. 
On 19 April and 20 April, net pens were deployed in the forebay of Ice Harbor Dam and in Burbank Slough, respectively. Roughly 650 and 850 PIT-tagged trout were stocked in the Ice Harbor and Burbank Slough net pens, respectively. PIT-tagged fish removed from the net pens by Caspian terns will be used to determine (1) PIT tag deposition rates at the Crescent Island tern colony and (2) vulnerability of fish in different size classes to tern predation.