Last Updated May 9th
Caspian Tern Colony Counts East Sand Island The high and low on-colony counts for the week on East Sand Island were 11,480 terns (2 May) and 6,259 terns (1 May), respectively. The high and low counts for this week are 23% and 28% lower, respectively, than during this week last year. The weather in the estuary this past week has continued to improve compared to previous weeks; clear skies and calmer winds prevailed throughout the week.
Three surveys of the upper estuary were conducted this past week (1, 3, and 6 May) looking for nesting terns at dredge disposal sites. During these surveys, no terns were observed roosting or attempting to nest in upland areas of Pillar Rock Sands, Miller Sands Spit, or Rice Island. Furthermore, the total number of terns roosting at sites in the upper estuary has declined considerably from the previous week. 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.
No surveys were conducted this week looking for nesting terns on islands on the mid-Columbia River. Surveys conducted in previous weeks have shown that Caspian terns continue to show interest in nesting on Rock Island in the Blalock Islands group, which is located in John Day pool between the towns of Boardman and Irrigon, Oregon. Eighty Caspian terns were counted on Rock Island during an aerial survey on 26 April. During boat surveys of this island on 19 and 21 April, we observed some nest scrapes but no tern eggs were present. As many as six breeding pairs were observed at Rock Island last year and at least one Caspian tern chick was raised to fledging, the first Caspian tern breeding ever recorded on these islands during our study (1997-2006). It appears that terns may attempt to nest on Rock Island again this year. Nesting Caspian terns have not been observed at other potential colony sites along the Mid-Columbia River so far this year.
For the week of 1 May – 7 May, 70% of the identifiable fish delivered to the East Sand Island colony (N = 350) were salmonids. This dramatic increase in salmonids in the diet of East Sand Island terns over the past two weeks is apparently associated with the onset of the juvenile salmonid out-migration through the Columbia River estuary. The majority of the non-salmonid prey items being delivered to the East Sand Island colony were smelt (11%). During this same week last year, clupeids (herring and sardines) and anchovies made up a combined 35% of the East Sand Island terns diet compared to only 8% this year. Year to date, 48% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,406) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 33% salmonids in the diet by this time last year. For the week, 92% of the identifiable fish delivered to the Crescent Island tern colony (N = 165) were salmonids. Year to date, 79% of the identifiable fish delivered to the Crescent Island tern colony (N = 586) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 85% salmonids in the diet by this time last year.
On 26 April and 27 April, net pens were deployed in Burbank Slough and Peninsula Slough, respectively, just downstream of the confluence of the Snake and Columbia rivers. This will be the third year in which a net pen has been located in Burbank Slough (McNary Wildlife Refuge, 8 km northeast of the Crescent Island tern colony) and the first year in Peninsula Slough (also McNary Wildlife Refuge, 6 km northeast of the Crescent Island tern colony). Roughly 800 PIT-tagged trout in two size class (large and small) were stocked in each net pen. 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. The net pen frames were fitted with wire spike strips to prevent avian predators from roosting on the net pen frames and to deter predators other than terns from foraging on fish held in the net pens. Juvenile rainbow trout in two different size classes (mean fork length 10 cm and 17 cm, respectively) were PIT-tagged and placed in the net pens, with approximately equal numbers of both size classes within each net pen. All fish were certified, disease-free triploids (sterile as adults) obtained from the Trout Lodge Hatchery, WA. The net pens will be monitored 8 hrs/day, 7 days/week until a minimum of 100 fish have been removed from each net pen by Caspian terns. Mesh netting is placed over the net pens after each observation period to prevent terns and other predators from feeding on fish held in the pens when observers are not present. Following the breeding season and after all the terns have left the colony, the Crescent Island tern colony will be scanned for PIT tags by NOAA Fisheries. The number of PIT tags that are removed from the net pens by terns and subsequently detected on the Crescent Island tern colony will be used to address the research objectives listed above.
On 3 May, a great horned owl was captured on East Sand Island using a padded leg hold trap and was removed from the island. The owl was first spotted on East Sand Islands on 24 April. During this 10-day period, this great horned owl repeatedly visited the Caspian tern colony at night and regularly killed adult terns. Great horned owls are known predators of adult Caspian terns and have preyed on terns nesting on the East Sand Island colony in previous years. Great horned owls can cause large disturbances to colonial nesting birds, sometimes leading to colony abandonment. In 1999 and 2003, great horned owls caused serious disruption to the East Sand Island tern colony and in 1999 two owls had to be removed from East Sand Island to prevent the colony from abandoning (in 2003 the owl left the island on its own). Since removal of the owl on 3 May, there has been no further evidence of owl depredation of Caspian terns on East Sand Island. There is considerable evidence over the past week that a raccoon remains on East Sand Island. Previous to this past week, there had been no evidence that the raccoon had made it out onto the tern colony to depredate tern nests. This week (on 7 May), we discovered several depredated tern nests at the periphery of the tern colony. These nest failures were apparently caused by the raccoon, based on fresh raccoon tracks in the vicinity of the nests. Live traps have been set in an effort to capture and remove the raccoon before it causes significant nest failure or colony abandonment. It seems likely that this animal was intentionally released on the island, as the raccoon is trap-shy of live traps and has so far refused to enter one to take the bait. If the raccoon and other mammalian predators are not removed from East Sand Island, it will likely lead to greater numbers of terns from East Sand Island prospecting for nest sites in the upper estuary or further up-river, which would result in higher predation rates on juvenile salmonids from the Columbia River Basin. At East Sand Island, a total of 7 colony disturbances were observed at a rate of 0.7 disturbances/hr of observation. This past week bald eagle activity near the tern colony has increased from what was observed in the previous week. Generally, disturbances at the tern colony are short, causing terns to flush for only 10-30 seconds before returning to the colony. There were no disturbances observed at the Crescent Island tern colony this past week. This past week, gull kleptoparasitism rates at East Sand Island were 4% of all known fates of fish delivered by terns. 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. Other than the raccoon on East Sand Island, there have been no signs to date of mammalian predators on tern colonies monitored as part of this study.
On 4 May, Scott Dunmire, Tim Wik, Ann Setter, and Gary Dahl from the U.S. Army Corps of Engineers, Walla Walla District, plus Brad Ryan from NOAA Fisheries, visited East Sand Island. This past week, Dave Smith assisted with research activities in the Columbia River estuary (thank you, Dave!). |
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The first tern egg was laid at the East Sand Island tern colony on 26 April. This is the latest first-egg date that we have recorded at the East Sand Island tern colony since 1999, and was 12 days later than the first-egg date last year. This delay in nesting chronology at East Sand Island is likely due to poor weather conditions (heavy rain and high winds), frequent disturbance by bald eagles, and an apparent shortage of available forage fish early in the breeding season. 
The high and low on-colony counts for the week on Crescent Island were 721 terns (6 May) and 676 terns (4 May), respectively. These counts are similar to those recorded at this colony last year during the comparable week. The first tern egg was laid at the Crescent Island colony on about 14 April (this is revised from our previous estimate of 16 April for the first egg on Crescent Island in 2006), two days later than the estimated first egg date the previous year. 
This past week, Caspian terns have removed 4 fish from the Burbank Slough net pen; terns have yet to forage on fish held within the Peninsula Slough net pen. We will continue to monitor bird activity at the net pens in the coming weeks.