Last Updated May 16th
Caspian Tern Colony Counts East Sand Island
Two surveys of the upper estuary were conducted this past week (8 and 10 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. As was reported last week, the total number of terns roosting at sites in the upper estuary has declined considerably from what was observed earlier in the season. 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.
The high and low on-colony counts for the week on Crescent Island were 739 terns (8 May) and 617 terns (14 May), respectively. The first tern chick hatched on Crescent Island on 7 May, 2 days earlier than the first chick was seen on the Crescent Island tern colony in 2005. Nesting chronology at Crescent Island seems to be proceeding on a normal schedule this season. The former colony sites at Three Mile Canyon Island and Miller Rocks were visited on 10 May and no indications of terns nesting at either site were observed. 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 8 May – 14 May, 75% of the identifiable fish delivered to the East Sand Island colony (N = 350) were salmonids. The high proportion of salmonids in the diet of East Sand Island terns over the past three weeks is considerably higher than during this period last year, and is apparently associated with the peak of the juvenile salmonid out-migration through the Columbia River estuary. The majority of the identified non-salmonid prey items being delivered to the East Sand Island colony were clupeids (herring and sardines; 5%), anchovies (5%), surfperch (4%) and smelt (4%). Year to date, 54% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,756) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 37% salmonids in the diet by this time last year. For the week, 83% of the identifiable fish delivered to the Crescent Island tern colony (N = 156) were salmonids. Year to date, 80% of the identifiable fish delivered to the Crescent Island tern colony (N = 742) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 81% salmonids in the diet by this time last year.
On 26-27 April, net pens were deployed in Burbank Slough and Peninsula Slough, 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 National Wildlife Refuge, 8 km northeast of the Crescent Island tern colony) and the first year in Peninsula Slough (also McNary NWR, 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. This past week, 3 trout were removed from the Burbank Slough net pen by Caspian terns; to date a total of 7 trout have been removed from the Burbank Slough net pen by Caspian terns. The first trout was removed from the Peninsula Slough net pen by a Caspian tern this past week. We will continue to monitor bird activity at the net pens in the coming weeks. Adult Banding and Force-Feeding Capturing and marking breeding adult Caspian terns with plastic leg bands that are engraved with easily readable alpha-numeric code is being used to measure annual adult survival and dispersal of breeding adults to other nesting colonies. These are important data for modeling the demography of the Caspian tern population in the Pacific Northwest.
On 13-14 May, 60 breeding adult terns were captured on the Crescent Island colony late in the egg incubation period using noose-mat traps placed over active nests. Each captured adult was weighed, measured, banded with an engraved plastic leg band plus a federal numbered metal leg band, and force-fed one fish (4-5 cm in length) containing one PIT tag. All fish were certified, disease-free triploid trout obtained from the Trout Lodge Hatchery, WA. Following banding and force-feeding, terns were released back onto the colony. Individually-marked terns were observed following release to determine if and when normal on-colony behavior was resumed. Of the 60 terns that were force-fed fish, 98% successfully ingested the fish. Six of the 60 terns captured (10%) already had PIT tags in their digestive track at the time of capture (i.e., from a naturally ingested PIT-tagged fish). A total of 28 tern eggs and one tern chick were lost as a result of this research activity, the majority due to adult terns crushing eggs (n = 11) during capture in noose mats or by depredation by gulls (n = 11). No adult terns were injured during this activity. The capture of adult terns at Crescent Island for banding and force-feeding is now completed for the 2006 breeding season. Adult capture for banding and force-feeding is scheduled to take place at East Sand Island next week (25-26 May). Following the breeding season and after all the terns have left their respective colonies, East Sand and Crescent islands will be scanned for PIT tags by NOAA Fisheries and the proportion of force-fed tags that are detected on-colony will be determined.
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. The previous week (May 1-7) we reported that the raccoon on East Sand Island had depredated tern nests, as evidenced by raccoon tracks and broken tern eggs on the East Sand Island tern colony. This past week no evidence of the raccoon depredating tern eggs was discovered. 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 any 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 the East Sand Island tern colony, a total of 5 colony disturbances were observed at a rate of 0.4 disturbances/hr of observation. A total of 2 colony disturbances were observed at the Crescent Island tern colony at a rate of 0.1 disturbances/hr of observation. These disturbance data for the Crescent Island tern colony do not include disturbances associated with efforts to catch adult terns for banding and force feeding (see above). Generally, disturbances at the tern colony are short, causing terns to flush for only 10-30 seconds before returning to the colony.
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. However, it should be noted that in the effort to trap and remove the raccoon from East Sand Island, several Norway rats have been captured. Rats have apparently inhabited East Sand Island at least since the 1970s. Although they may cause some nest failure to Caspian terns and other breeding colonial waterbirds on East Sand Island, their impact is evidently small.
This past week, Gary Miller assisted with research activities in the Columbia River estuary (thank you, Gary!). |
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The high and low on-colony counts for the week on East Sand Island were 12,500 terns (10 May) and 8,235 terns (8 May), respectively. Clear skies and calmer winds have prevailed at East Sand Island over the past several weeks. This past week, we have observed an expansion in the area used by nesting terns to the lower regions of the colony, northeast of the core colony area. The counts of incubating terns on our plots for sampling productivity suggest that most breeding adult terns have laid eggs and are incubating. The annual photo census of the Caspian tern colony on East Sand Island is scheduled to occur on 24 May. Once the aerial photos are counted (Fall 2006) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2005.
The recovery of PIT tags on piscivorous waterbird colonies has been used as a direct measure of predation rates on ESA-listed salmonid populations from the Columbia and Snake rivers. Predation rates based on PIT tag recoveries are, however, underestimates; some proportion of tags consumed by terns is deposited off-colony, weather and other environmental factors remove PIT tags that are deposited on-colony, and some PIT tags are damaged and rendered unreadable before or after egestion on the colony. Understanding the magnitude of the bias in estimates of tern predation on smolts based on PIT tag recoveries on-colony is critical for deriving ESU-specific tern predation rates. One method we are using to estimate PIT tag deposition rates on-colony is to force-feed PIT-tagged fish to adult terns captured on the East Sand and Crescent island tern colonies and then determine the proportion of force-fed tags that are subsequently deposited at each colony. 
This past week, gull kleptoparasitism rates at East Sand Island were 7% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 6% 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.