Last Updated June 20th
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.
During aerial surveys conducted along the southern Washington coast this past week (13 June), roughly 75 Caspian terns were counted on islands in Willapa Bay and no terns were seen in Grays Harbor. The Caspian terns seen in Willapa Bay were all observed roosting below the high tide line. The numbers of terns counted in Willapa Bay and Grays Harbor on 13 June were lower than those counted during the previous survey on 17 May (89 and 54, respectively). The aerial survey conducted on 13 June was the last for this season; 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 307 terns (12 June) and 162 terns (17 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. The first fledgling was observed on Crescent Island on 14 June which was 8 days earlier than was observed last year. On 30 May an aerial survey of the Rock Island, one of the Blalock Island 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, 52% of the identifiable fish delivered to the Crescent Island tern colony (N = 153) were salmonids. Year to date, 71% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,539) 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 three 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) relative vulnerability of fish in two different size classes to predation by Caspian terns. This past week, 10 trout were removed from the Burbank Slough net pen by Caspian terns; to date a total of 41 trout have been removed from the Burbank Slough net pen by Caspian terns. No trout were removed by Caspian terns from the Peninsula Slough net pen this past week, for a total of 39 trout removed by terns 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. Since then we have detected no evidence of depredation of tern eggs by the raccoon. 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, a total of 12 colony disturbances were observed at a rate of 1.4 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 also 1% 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.
This past week, Steve Hendricks, Lydia Greer, and 10 members of the Shoalwater Birders Association (Gayl West, Rose Marie Winters, Rosemary Hallin, Ann Chiller, Robert Zimmerman, Virginia Zimmerman, Terri Shields, Sandra Tellvik, David Berger and Karen DeWinter) visited East Sand Island. Gary Miller assisted with research activities in the Columbia River estuary this past week (thank you, Gary!). |
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The high and low on-colony counts for the week on East Sand Island were 11,592 terns (12 June) and 8,145 terns (18 June), respectively. The weather in the estuary this past week was mostly rainy with some clearing over the weekend. This past week, many new eggs have been laid in the northwestern region of the tern colony where terns that had nested earlier in the season had failed. 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 12 June - 18 June, 17% 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 (47%) and surfperch (26%). Year to date, 47% of the identifiable fish delivered to the East Sand Island tern colony (N = 3,506) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 32% 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 three 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 35% for the last three weeks. 
This past week, brown pelicans were not observed on the East Sand Island Caspian tern colony, as was reported last week when as many as 84 brown pelicans were observed on the tern colony interacting with Caspian terns. That was the first time we have observed brown pelicans entering the tern colony area while terns are still nesting at the colony. The week before last, 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 being billed by brown pelicans, and we suspect most of these chicks died as a result of this pelican behavior. In addition, at least one tern egg was taken by a pelican.