Last Updated April 24th
Caspian Tern Colony Counts East Sand Island The high and low on-colony counts for the past week on East Sand Island were 7,990 terns (19 April) and 905 terns (16 April), respectively. The average number of terns on-colony this week (4,338 terns) was slightly lower than similar counts conducted at the colony during this time last year (5,035 terns). This past week, the weather in the Columbia River estuary was more typical for this time of year (i.e., some high winds and rain) compared to the previous week when the weather was mostly dry and warmer temperatures. In previous years, significant rainfall resulted in standing water on the East Sand Island tern colony, with the majority of the colony area appearing wet following periods of heavy rainfall. This has yet to happen at the East Sand Island tern colony this year. The first Caspian tern egg of the season on East Sand Island was seen on 19 April. This is 7 days earlier than the first egg was observed on the East Sand Island colony last year, but later than the first-egg dates in 2005 (14 April) and 2004 (16 April).
Two surveys were conducted this past week (18 and 22 April) looking for nesting terns at dredged material disposal sites in the upper estuary; small numbers of terns were observed briefly roosting in upland areas of Pillar Rock Sands (3 terns on 18 April) and Miller Sands (1 tern on 22 April). No courtship or nesting behaviors were observed at either site during these visits. No terns were observed in upland areas on Rice Island during these surveys. We will continue to monitor Rice Island and the other upper estuary dredge 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 405 terns (21 April) and 68 terns (17 April), respectively. This represents more than a three-fold increase in the average number of terns counted on colony this week as compared to the previous week. Still, the numbers of Caspian terns counted on the Crescent Island colony significantly lag the numbers counted on the colony at this time last year.
Nesting Caspian terns have not been observed at other sites along the Mid-Columbia River. For the week of 16–22 April, 43% of the identifiable fish delivered to the East Sand Island colony (N = 332 prey items) were salmonids. The majority of the non-salmonid prey items being delivered to the East Sand Island colony were smelt (16%) and clupeids (i.e., herring or sardines; 12%). Year to date, 44% of the identifiable fish delivered to the East Sand Island tern colony (N = 708) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 28% salmonids in the diet by this time last year.  For the week, 73% of the identifiable fish delivered to the Crescent Island tern colony (N = 157) were salmonids. The majority of the non-salmonid prey items being delivered to the Crescent Island colony were centrachids (i.e., bass and sunfish; 14%) and yellow perch (5%). Year to date, 64% of the identifiable fish delivered to the Crescent Island tern colony (N = 247) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 69% salmonids in the diet by this time last year.
Avian predation rates often vary by salmonid species, run-type, stock, and rearing type (hatchery vs. wild). Furthermore, predation rates for specific groups of fish can vary greatly from one year to the next. Despite this well-documented variation, limited data are available to determine which biotic and abiotic factors account for differences in smolt vulnerability to avian predators. One method to evaluate the potential intrinsic and extrinsic factors affecting relative vulnerability of smolts to avian predation is to PIT-tag smolts and subsequently recover their tags on bird colonies following the breeding season. In 2007, we initiated a pilot study whereby run-of-the-river steelhead smolts encountered at the juvenile fish facility at Lower Monumental and Ice Harbor dams on the lower Snake River are PIT-tagged, weighed, measured, photographed (for condition scoring), and released back into the river to resume their out-migration. Following the breeding season, PIT tags will be recovered from various bird colonies in order to test hypotheses concerning how differences in smolt morphology, condition, abundance, origin, river conditions, and dam operations are associated with differences in smolt vulnerability to avian predation. PIT-tagging commenced on 5 April at Lower Monumental Dam and on 10 April at Ice Harbor Dam. This past week, 166 steelhead were PIT-tagged at Lower Monumental Dam. The majority of fish were in either good (46%) or fair (47%) physical condition, with some fish classified as being in poor (7%) condition (more detailed results on fish condition will be provided in a subsequent report). To date, a total of 250 steelhead have been PIT-tagged at either Lower Monumental or Ice Harbor dams as part of this study.
The number of terns attending the Crescent Island tern colony has been increasing rapidly this past week. Although there has been a juvenile bald eagle observed near Crescent Island, no significant disturbances were observed at the Crescent Island tern colony this past week. This past week, gull kleptoparasitism rates at East Sand Island were 2% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 7% of all known fish fates observed at that colony. 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.
This past week, Brendan Courtot, a past crew member, visited East Sand Island to assist with data collection and crew training (thank you Brendan!).
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The first Caspian tern egg of the season on Crescent Island was seen on 19 April, the same first-egg date observed at East Sand Island this year. This is 3 days later than the first egg was observed on the Crescent Island colony last year. 
At East Sand Island, peregrine falcons and bald eagles continue to disturb the Caspian terns at the East Sand Island colony, causing frequent and lengthy disturbances to nesting terns (1.3 disturbances per hour of observation). Most of the known-cause disturbances to the East Sand Island tern colony were by peregrine falcons (61%) this past week, with at least one individual repeatedly visiting the colony to prey on the carcass of an adult tern that was killed by a peregrine falcon 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.