Last Updated July 25th
Caspian Tern Colony Counts East Sand Island
The first fledgling (i.e., young-of-the-year capable of flight) was observed at the East Sand Island colony on 28 June. As a result of a large number of failed nesting attempts early in the season and the subsequent renesting of failed breeders, nesting at the East Sand Island tern colony is more asynchronous than in most previous years. It appears that many of these renesting attempts have failed, especially during the past several weeks. A second aerial photo census of the colony was conducted on 7 July. Once these aerial photos are counted (Fall 2006) we will be able to accurately estimate the number of young terns fledged from the East Sand Island colony this year.
We have discontinued our monitoring of upper estuary dredged material disposal islands for early signs of tern nesting for the remainder of the year; 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 41 terns (21 July) and 0 terns (18 July), respectively. Most terns have now dispersed from the island and most of the remaining terns are loafing on the island but off-colony, presumably seeking shade during the heat of the day. 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.
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 this year.
For the week of 17 July - 23 July, 10% of the identifiable fish delivered to the East Sand Island colony (N = 198) were salmonids. The majority of the identified non-salmonid prey items being delivered to the East Sand Island colony were anchovies (58%) and surfperch (25%). Year to date, 34% of the identifiable fish delivered to the East Sand Island tern colony (N = 5,102) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 23% salmonids in the diet by this time last year. In the last half of the breeding season the percent salmonids in the diet of East Sand Island Caspian terns has declined to the levels seen in recent years at this time, suggesting that there has been an influx of anchovies and other marine forage fishes into the estuary concurrent with the decline in out-migrating salmonid smolts. For the week, 53% of the identifiable fish delivered to the Crescent Island tern colony (N = 70) were salmonids. Year to date, 66% of the identifiable fish delivered to the Crescent Island tern colony (N = 2,222) were salmonids (this cumulative percent is an average of the previous weeks’ percentages), the same percentage as was observed in the diet by this time last year.
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.
At East Sand Island, two colony disturbances were observed at a rate of 0.3 disturbances/hr of observation. One colony disturbance was observed at the Crescent Island tern colony this past week at a rate of 0.1 disturbances/hr of observation. Generally, disturbances at the tern colonies 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. No kleptoparasitism was observed during fish watch at the Crescent Island tern colony 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 was 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 and opossum 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 8,142 terns (18 July) and 5,940 terns (23 July), respectively. The annual photo census of the Caspian tern colony on East Sand Island was conducted on 26 May. Visual estimates of colony size from the ground suggest that the East Sand Island colony has increased somewhat since the 2005 nesting season. Once these aerial photos are counted (Fall 2006) we will be able to accurately estimate colony size and determine if, in fact, the colony is larger in 2006 relative to 2005. 
The first fledgling was observed on Crescent Island on 14 June, eight days earlier than was observed last year. 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 20 July, a opossum was captured and removed from East Sand Island. Crew members were unaware that a opossum, another mammalian predator on ground nesting birds, was on the island suggesting that it might have recently arrived there. All live traps were removed from East Sand Island on 21 July.