Last Updated June 14th
Caspian Tern Colony Counts East Sand Island
The annual photo census of the Caspian tern colony on East Sand Island was conducted on 26 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.
Two surveys of the upper estuary were conducted this past week (6 and 9 June) 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. 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 376 terns (6 June) and 254 terns (9 June), respectively. Counts of incubating or brooding adult terns on the Crescent Island colony indicate that about 450 pairs of Caspian terns attempted to nest at the colony this year. This represents about a 5% decline in the size of the colony compared to 2005, when about 475 pairs nested on Crescent Island. More than 70 Caspian terns were counted on Rock Island during a boat survey conducted on 26 May. During that survey, we observed 20-40 active Caspian tern nests with eggs and some (3) with chicks. 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). With the exception of Rock Island, 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 5 June - 11 June, 21% of the identifiable fish delivered to the East Sand Island colony (N = 350) were salmonids. The majority of the identified non-salmonid prey items being delivered to the East Sand Island colony were anchovies (49%) and surfperch (16%). Year to date, 51% of the identifiable fish delivered to the East Sand Island tern colony (N = 3,156) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 34% salmonids in the diet by this time last year. The diet composition of Caspian terns nesting on East Sand Island has changed dramatically in the last two weeks; after five straight weeks when the diet consisted of greater than 60% juvenile salmonids, the percentage salmonids in the diet has been less than 40% for the last two weeks. For the week, 59% of the identifiable fish delivered to the Crescent Island tern colony (N = 157) were salmonids. Year to date, 73% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,386) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 75% salmonids in the diet by this time last year. For the last two weeks juvenile salmonids have comprised less than 60% of prey items for the Crescent Island tern colony, which is an unusually low proportion of the diet at this time of 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. Trout held in the Burbank Slough net pen have been experiencing a great deal of mortality, presumably due to high water temperatures in that slough and predation by great blue herons and black-crowned night-herons. Efforts to dissuade herons from foraging on trout at that net pen (i.e., wire spike strips fitted to the net pen frame and mesh netting placed over the net pens after each observation period) have been ineffective. On 6 June, 118 PIT-tagged trout were transferred from the Peninsula Slough net pen to the Burbank Slough net pen in hopes that we might increase the sample size of fish removed from that net pen by Caspian terns. Following the transfer of trout to the Burbank Slough net pen this past week, 4 trout were removed from that net pen by Caspian terns; to date a total of 31 trout have been removed from the Burbank Slough net pen by Caspian terns. Three trout were removed by Caspian terns from the Peninsula Slough net pen this past week, or a total of 39 trout since the study began. We will continue to monitor bird activity at the net pens until the end of June.
For the week ending 7 May 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 depredation of tern eggs by the raccoon was observed. 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 East Sand Island, a total of 6 colony disturbances were observed at a rate of 0.6 disturbances/hr of observation. Generally, disturbances at the tern colony are short, causing terns to flush for only 10-30 seconds before returning to the colony. At Crescent Island, no colony disturbances were observed this past week. This past week, gull kleptoparasitism rates at East Sand Island were 1% 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. 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, Dave Smith assisted with research activities in the Columbia River estuary (thank you, Dave!). |
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The high and low on-colony counts for the week on East Sand Island were 12,115 terns (11 June) and 8,315 terns (10 June), respectively. The weather in the estuary this past week was mostly rainy with some clearing over the weekend. The first tern chick hatched on East Sand Island on 23 May, 7 days later than the first chick was seen on the East Sand Island tern colony in 2005. Nesting chronology at East Sand Island is delayed compared to previous years (2000-2005), when the first chick dates ranged from 13 May (2004) to 21 May (2000). 
This past week, as many as 84 brown pelicans were observed on the East Sand Island Caspian tern colony near the Southwest blind. This is the first time we have observed brown pelicans entering the tern colony area when terns are still nesting at the colony. The pelicans were seen picking up tern chicks in their bills and throwing them in the air. Some chicks were held in the pelican’s pouches and a few may have been swallowed. During three different periods, we observed a total of 49 tern chicks handled by brown pelicans, and we suspect most of these chicks died as a result of this disturbance. In addition, at least one tern egg was taken by a pelican.