Wednesday, July 5, 2017

North Brother Tern Colony Collapse

This isn't all bad news - in fact colony desertion is entirely normal for terns. Read on.

Roseate Terns on the Brother Islands
North Brother Island has consistently been the best location in Nova Scotia to see Roseate Terns for decades. It was home to Canada's largest colony of this rare tern and was one of only a few known breeding locations in Nova Scotia.

Roseate Terns among the Arctic Terns and Common Terns on the Brother Islands off West Pubnico were first identified in 1982 when the Roseates numbered 55 to 60 pairs. The Brothers are two small islands off West Pubnico, Yarmouth County, known locally as "les Îles à Vert". Ted d'Eon of West Pubnico has been monitoring and managing the tern colony since 1983 in cooperation with the Canadian Wildlife Service and the Nova Scotia Department of Natural Resources. Ted's website (www.ted.ca) contains years worth of data and photos about his efforts. Due to the Roseate Tern's precarious foothold in Canada, the species depends on management for its survival (Erksine 1992).

The terns used to nest on the two Brother Islands, but since 2003, they only nest on North Brother. The number of Roseate Tern nests fluctuates from year to year with a high of 90 nests in 2002 and a low of 20 in 1991. Fifty Roseate Tern nests were found in 2016.

Roseate Terns are listed as Endangered in Canada under the Species at Risk Act. Its reproductive rate is limited by age of first breeding at age 3, small clutch size (2 for experienced birds, 1 for first-time breeders), and a relatively low survival rate of adults and young birds. One of the main threats to Canadian Roseate Tern populations is predation and displacement of colonies by Herring Gulls and Great Black-backed Gulls. The restricted distribution of this tern makes it vulnerable to localized threats such as human disturbance and weather events. (Environment Canada 2010)

Roseate Tern at North Brother on June 17, 2016. Photo by Alix d'Entremont.

Julie McKnight (Canadian Wildlife Service) holding a Roseate Tern with tiny GPS receiver at North Brother on June 17, 2016. Photo by Alix d'Entremont.

The Brother Islands have been owned by the Province of Nova Scotia since 1990 and have been a Department of Natural Resources (DNR) wildlife management area since 2007. Access to the islands between March 31 and September 1 is not permitted without written authorization of the Director of Wildlife at DNR.

Here is a video that I made in 2011 that shows the boat ride between Abbotts Harbour and North Brother Island, the Roseate Tern management area on North Brother, the eroding island edges and the terns flying above the island. The tern festival mentioned in the video is no longer running.



2017 Breeding Season on North Brother
It all began with low numbers of terns arriving at the start of breeding season. By May 8, 2017, there were only 15 terns above the island in comparison to 100 birds on May 6 of the previous year. It was noted that during the 2016 breeding season, the terns appeared to be more tightly packed than previous years, a product of the yearly erosion to the already tiny island.

A high tide event previous to a visit to the island on May 28, 2017 caused a lot of damage to the "management zone", an area with imported substrate and nesting shelters for the Roseate Terns. By June 4, 2017, it was obvious that the number of terns on the island was drastically lower than in previous years. The nest count of all three species was completed on June 12, 2017, and the results were bad; only 141 nests were found compared to 661 the previous year. This is a loss of more than 500 pairs of terns and the lowest nest count based on data from Ted's website which goes back to 1990.

Roseate Terns nesting in shelters in the "management zone" on North Brother on June 17, 2016. Photo by Alix d'Entremont.

During the June 21, 2017 visit, the number of terns above the island had shrunk to about 30 and most Roseate nests that previously had eggs now did not or were missing an egg and the majority of the remaining eggs were cold. Most of the Common Tern nests were empty and the few Arctic Tern nests that were there, were gone. Four trail cameras were set on North Brother the following day and the photos showed that the predators were a group of 4 American Crows. The colony was now abandoned and for the first time in at least 35 years, there would be no breeding success at all.


Tern Breeding Site Fidelity
Fidelity to natal colony is variable, but usually high. Predation and other causes of breeding failure are known to cause colonies to be abandoned when birds move en masse to other sites, sometimes settling synchronously in contiguous groups. (Nisbet 2002)


The Search for the Missing Terns
On the heels of this year's low nest count, Bertin d'Eon and I searched the Tusket Islands area on June 15, 2017, for the initial missing 500 pairs. A few Common and Arctic were found above Murder Island, Peases Island had 11 nests and Green Island (south of Wedgeport) had 82 nests. The birds found nesting on the Tusket Islands were likely the same that were nesting there in 2016, but a majority simply moved around between islands.

Arctic Tern nest on Peases Island on June 15, 2017. Photo by Alix d'Entremont.
Common Tern nest on Peases Island on June 15, 2017. Photo by Alix d'Entremont.
The newly expanded tern colony on Green Island, June 15, 2017. Photo by Alix d'Entremont.
On June 23, 2017, the Department of Natural Resources flew a helicopter along the coast from Yarmouth to Lockeport in search of the missing birds. Three groups of terns were identified by Pam Mills: 150 individuals in Yarmouth Harbour, 120 in Pubnico Harbour and 30 at Green Island near Cape Sable Island.

Paul Gould, Alec d'Entremont, Florian Schmitt and I searched the islands west of Cape Sable Island on June 25, 2017. A small group of about 32 terns were found on an unnamed island in the Bear Point Thrums group. Our next destination was Green Island west of The Hawk on Cape Sable Island. Four Roseates were seen at Green Island in May 2016, but they were not present during my June 2016 nest survey when I counted 85 nests. Eight Roseates were at Green Island in May 2017, but there were only a few Common Terns left by June 25, 2017. The tern nesting site appeared to have been taken over by Herring Gulls. Later that day, Bertin d'Eon and I counted 27 nests on Île Ferré and none on Île Chespêque, both in Pubnico Harbour. In 2016, there were 30 nests on Île Chespêque, so those birds may have moved 3 km south to Île Ferré.

I am contracted to do tern surveys for Environment and Climate Change Canada with a focus on Roseate Terns, so on June 28, 2017, Bertin d'Eon, Duncan Bayne (DNR Biologist) and I visited Toby Island and Coffin Island in Lunenburg County. There were two Roseates on Toby last year, so it was important to check the island again. We briefly saw one Roseate and similar to the previous year, the bird was never seen again. I assume that the Roseate Terns aren't nesting on Toby since they are not seen returning to the colony once everything calms down from a flush.

Arctic Tern habitat on Coffin Island, Lunenburg County, on June 28, 2017. Photo by Alix d'Entremont.
On July 3, 2017, Calvin d'Entremont and I checked Common Eider nesting shelters on Pumpkin Island near West Pubnico and then followed a Common Tern carrying food from around Bar Island straight to Gull Island. We anchored the boat and then walked onto the island when I heard the distinctive harsh snarl of a Roseate Tern overhead. Once the terns lifted from the colony I was able to count at least 7 Roseates among the hundreds of terns. This was what I had been searching for!

That afternoon, Ted d'Eon, Ingrid d'Eon, Gavin Maclean, Roland d'Eon, Bertin d'Eon and I returned to Gull Island with Ted's boat to do a thorough count of the nests. There were 233 viable nests and an additional 15 or so chicks running around. Most of these terns were new to Gull Island since we only counted 10 nests in 2016. The new terns on Gull Island represent about 40% of the terns that used to be on North Brother. The Gull Island colony is spread along the western and southern edge of the pond. Based on the stage of development of the chicks, it appears that nesting began earlier on the southern edge of the pond. There were many large chicks running around in that area. The northwestern group of nests only had 3 small chicks out of 60 nests. It seems plausible that the birds that never arrived at North Brother in early June nested on the southern side of the pond while another wave of birds arrived and nested on the western side of the pond after the total collapse of North Brother.

Google Maps aerial photo of Gull Island.
Duncan Bayne had told me about how docile the terns were at North Brother this year. That wasn't the case on Gull. They were attacking nearby gull chicks and would hit us in the head, so the tern colony appears healthy. Multiple Common Terns were seen arriving from the north carrying food. This is in contrast to North Brother where the terns were mostly heading south in search of prey.

I was able to get two field-readable leg bands on the Roseates on Gull Island. Both birds were banded as adults at the North Brother colony; one was banded in 2014 and the other in 2016.

Roseate Tern on Gull Island, July 3, 2017. Photo by Alix d'Entremont.

On July 5, 2017, Shawn Craik (Université Sainte-Anne), Manon Holmes (Acadia Univeristy), Nick Knutson (Université Sainte-Anne) and Ben Morton visited Gull Island and were able to get an accurate count of 24 Roseate Terns. They also confirmed two areas where the Roseates appeared to be nesting.

The closest mainland location from Gull Island is the southern tip of Surette's Island which is about 6 km away. It is about 7.5 km away from Abbotts Harbour, where Ted and I typically launch our boats. Gull is the island in Lobster Bay that is the farthest away from all of the others, so should be a good place for the terns. We saw 5 gull chicks on the island and only about 10 gulls around. There were probably more earlier in the season. If we can get a permit to get rid of the gulls early next year, that would be beneficial.

The abandonment of North Brother is the end of an era, but the terns seem to have chosen a great spot to resettle. Gull Island isn't about to become a ledge through soil erosion like North Brother is. It isn't a tall island, but should at least outlast us. We hope to acquire a permit to destroy gull nests next year before the terns arrive. As stated by Erksine (1992), local control of gulls numbers may be unavoidable if we wish to retain breeding eiders, terns or storm-petrels. We provide easy access food to gulls in the form of landfills and fish processing plants which results in unnaturally large populations.


Tern Colony Changes in Southwest Nova Scotia 2016/2017
The nests at most of the terns colonies around Lobster Bay were counted in the last two years, so I thought I'd show the numbers on two maps. The only large colony that was missed is probably the one on Flat Island which are likely mostly Arctic Terns. A majority of the North Brother terns are still not accounted for, but at least the Gull Island colony seems to be doing well. Let's hope that it gets even larger in 2018.









References:
Environment Canada. 2010. Amended Recovery Strategy for the Roseate Tern (Sterna dougallii)
in Canada. Species at Risk Act Recovery Strategy Series. Environment Canada. Ottawa.
vii + 36 pp.

Erskine, A.J. 1992. Atlas of breeding birds of the Maritime Provinces. Nimbus Publ./N.S. Museum. 270 pp.

Nisbet, Ian C. 2002.Common Tern (Sterna hirundo), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/comter
DOI: 10.2173/bna.618

4 comments:

  1. Excellent summary of Roseate Tern besting status in SW Nova Scotia.

    ReplyDelete
  2. Terrific detail as always Alix. With regard to HERGs starting to nest (more) on Green Island off CSI, could this possibly be related to the total inundation during the new moon in June of the HERG colony on the island in the inlet at Daniel's Head? The whole area was under water around10pm and I could hear the screams of the adults as the nests and chicks drowned for hours that night (also an on-shore wind making things worse). Meanwhile, happy to hear you found the relocation to Gull Island of the Roseates! Pinned it on my map.... Keep up the great work with Ted et al.

    ReplyDelete
  3. What a great read. It going to be harder for us Zodiac-less birders but I'm very glad the terns seems to have shifted to a more stable environment.

    ReplyDelete