Last Updated July 5th
Caspian Tern Colony Counts East Sand Island
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 was observed between 13 May (2004) and 21 May (2000). 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 (27 and 30 June) looking for nesting terns at dredged material disposal sites. During these surveys, no terns were observed roosting or attempting to nest in upland areas of Miller Sands Spit or Rice Island. The total number of terns roosting at sites in the upper estuary has declined by an order of magnitude from what was observed earlier in the season. We have discontinued regular monitoring of Pillar Rock Sands and the other upper estuary dredged material disposal islands for early signs of nesting; the nesting season is progressed sufficiently so there is little chance of Caspian terns initiating nests this late in the season at sites where Caspian terns are not already nesting.
The high and low on-colony counts for the week on Crescent Island were 234 terns (27 June) and 16 terns (1 July), respectively. There have been large fluctuations in the number of adult terns attending the colony site during the day this past week, presumably due to the onset of hot weather (>100 degrees F on some days). Earlier 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. The first fledgling was observed on Crescent Island on 14 June which was 8 days earlier than was observed last year. On 21 June, we observed the first fledged Caspian tern off- colony (near lagoon on Crescent Island to the southeast of the tern colony area). Our current estimate of the number of fledglings on Crescent Island tern colony is 191, or about 0.4 young raised per breeding pair, the lowest nesting success we have ever observed at Crescent Island. On 30 May an aerial survey of Rock Island, one of the Blalock Islands in John Day Pool, revealed that about 110 pairs of Caspian terns were nesting on the island, along with about 200 pairs of ring-billed gulls. At that time the nesting Caspian terns were attending either eggs or chicks. This was a major increase in the size of this Caspian tern colony compared to last year, when only about 6 pairs of Caspian terns attempted to nest on Rock Island. On 18 June, however, when Rock Island was revisited, the Caspian tern colony had experienced complete nesting failure. There were no adult terns attending either eggs or chicks. Dead tern and gull chicks that had died at least 10 days earlier were scattered over the colony. Although 35 dead adult ring-billed gulls were counted on the colony, no dead adult Caspian terns were found. One ring-billed gull carcass and 56 Caspian tern eggs were found cached in a hollow log located near the colony. All the evidence indicated that the colony failure was caused by an American mink. The mink nearly caused complete nesting failure for the ring-billed gull colony as well; only one chick survived and 5-10 pairs of ring-billed gulls have renested on the island and were sitting on eggs on 18 June. 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, 34% of the identifiable fish delivered to the Crescent Island tern colony (N = 153) were salmonids a 50% decline from the previous week. Year to date, 68% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,850) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 74% salmonids in the diet by this time last year.
All chicks captured for banding were released unharmed and no egg loss was observed associated with our banding activities.
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) relative vulnerability of fish in two different size classes to predation by Caspian terns. This past week, 1 trout was removed from the Burbank Slough net pen by Caspian terns; to date a total of 43 trout have been removed from the Burbank Slough net pen by Caspian terns. One trout was removed by Caspian terns from the Peninsula Slough net pen this past week, for a total of 41 trout removed by terns since the study began. We removed both net pens on 28 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. Since then we have detected no evidence of depredation of tern eggs by the raccoon, although fresh raccoon tracks are regularly seen on the north beach below the high tide line. 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. We will continue to attempt to remove the raccoon from East Sand Island. At East Sand Island, three colony disturbances were observed at a rate of 0.3 disturbances/hr of observation. At Crescent Island, one colony disturbance was observed at a rate of 0.1 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. 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 5% 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. This year predation on Caspian tern chicks by California gulls at the Crescent Island tern colony has become a significant source of mortality for tern chicks. In previous years predation by California gulls was limited to take of a few young chicks. This year a few California gulls have apparently developed the skills and the habit of killing even older Caspian tern chicks, and have done so regularly. One California gull nest was observed with the carcasses of at least 8 Caspian tern chicks scattered around it. The raccoon on East Sand Island and the mink on Rock Island are the only indications 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.
Dave Smith assisted with research activities in the Columbia River estuary this past week (thank you, Dave!). |
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The high and low on-colony counts for the week on East Sand Island were 11,770 terns (27 June) and 7,930 terns (28 June), respectively. The weather in the estuary this past week was mostly sunny with moderate winds. The first fledgling was observed on the East Sand Island tern colony on 28 June and the majority of the chicks seen on colony now have black caps. As a result of a large number of failed nesting attempts early in the season and the subsequent renesting of failed breeders, the nesting at the East Sand Island tern colony is more asynchronous than in most previous years.
For the week of 26 June - 2 July, 8% of the identifiable fish delivered to the East Sand Island colony (N = 349) were salmonids. The majority of the identified non-salmonid prey items being delivered to the East Sand Island colony were surfperch (37%) and anchovies (36%). Year to date, 41% of the identifiable fish delivered to the East Sand Island tern colony (N = 4,205) 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. The percentage of the East Sand Island Caspian tern diet that were salmonids has declined dramatically over the last five weeks; after five straight weeks when the diet consisted of greater than 60% juvenile salmonids, the percentage of salmonids in the diet has averaged only 18% during the past five weeks.
On 29 June, a total of 71 tern chicks that were near fledging were banded at the Crescent Island tern colony. Each chick was banded with a numbered U.S. Geological Survey metal leg band. Of the 71 chicks that were banded at Crescent Island, 60 chicks were also banded with a unique color combination of plastic leg bands. Banded fledglings will provide important data on survivorship, dispersal patterns, and recruitment of young terns to breeding colonies in future years.