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Perseverance pay off: A good "tern" of events

Perseverance pays off:  A good “tern” of events

By Jennifer Arnold and Steve Oswald

For those of you who have been following us for a while, you know the story. For those of you who are new here, suffice to say it’s a tough life for Common Terns in this part of the world and Presqu’ile is no exception. Year after year Common Terns get off to a good start on their small nest scrapings at Gull Island, Presqu’ile Provincial Park. Then, as the summer progresses, their eggs and chicks slowly disappear as they cope with storms, predators, overgrown vegetation, and a whole long list of adversities. Despite low breeding success nearly every year, terns are drawn to the remnant of what was once the largest tern colony in the Great Lakes. 

Initially, we completed several years of study to learn about the factors influencing success and failure for Common Terns at Presqu’ile and throughout the Great Lakes region. Then in 2014, with continued support from the Friends of Presqu’ile, we worked with park staff to construct predator exclusion grids around the nesting colony. These were designed to exclude avian predators, primarily night herons, gulls, and owls, while allowing the terns to easily pass through the overhead gridlines. Since the implementation of these grids, we’ve seen significant increases in the number of young raised.

We continued to expand the grids to provide more habitat for tern nesting. However, unexpected events, such as severe storms or unusual predator activity, always seemed to cause the terns to fall short of our desired management goal. (Our aim is for all nests to raise at least one chick to fledging each year as this would allow the colony to replenish itself in the long-term). Then, the tide literally turned for the worse. The extreme flooding of Lake Ontario in 2017 tragically destroyed all the tern management grids at Presqu’ile! We quickly responded with temporary measures that went some way to preserving tern breeding success, but in late 2018 these failed: night herons penetrated these grids and killed most of the chicks before they could fledge. Only 29 of the close to 80 nearly fledged chicks survived. Once again it felt as if we were almost back to square one.

In 2019, we collaborated with park staff throughout the fall and spring to extensively re-design permanent predator exclusion grids and erect these, so they were available for arriving terns. So, you can imagine the intense disappointment when a repeat of the high-water levels and storms occurred in May and June 2019. This time not only were the grids destroyed, but the entire part of the island where the terns nest was washed over and reduced to a small sliver of land. Nesting terns persisted in this and other areas of Gull Island, but repeated storms washed out all nests, eventually causing terns to desert Gull Island in mid-June.

Our hopes for a good breeding season for terns were low. Re-nesting requires substantial energy and the likelihood of further successful re-nesting seemed low. After all, there was even less habitat for the birds to choose from and all of it seemed vulnerable to inundation from storms. So, when a small colony was observed in a swampy area of High Bluff on June 11th, we were thrilled, but not optimistic. For a colony to start this late in the summer and be successful seemed implausible. The terns had to be exhausted from their many re-nesting efforts and chicks always struggle to fledge as the season presses on and adults have to think about timing their long-distance migration to South America. Adults at late nests sometimes give up nesting at the least provocation. And what’s more, High Bluff was home to a large population of night herons, the most common predator of tern eggs and chicks at Presqu’ile.

One June 14th, another storm ravished the islands and although terns were nesting on an inland lagoon on High Bluff, many nests were washed over with water. We expected the birds to give up, but most stayed and continued incubating, even those whose nests were underwater for a day or more.  More nests were laid and soon the colony was up to 60 active nests and still growing. 

Working together with the park’s biodiversity staff, between 8th and 9th July we erected a predator exclusion grid around the nesting area and then waited and watched. Chicks continued hatching successfully and despite what we had expected, all nests seemed to be producing and raising healthy chicks! They grew big, exchanged their down for flight feathers and still they kept growing. The new grid was successfully protecting them from all the predators that called High Bluff their home: night herons, gulls and even a juvenile bald eagle. Although water levels in Lake Ontario began to recede slightly, in early August large inland ponds surrounded the Common Tern grids. Then these ponds were suddenly alive with food for hungry tern chicks, in the form of carp fry, providing an unexpected late-season feast.

Common Tern feeding Carp to its young

On August 19th, nearly a month after breeding usually wraps up in the colony, the last of 77 chicks finally left the island. The fledged chicks then gathered on beaches near and far to prepare for their long southward migration with their parents. Perhaps you saw some of them, or even marveled at their large numbers, a sight we’ve longed to see around Presqu’ile for so many years. 

And in case you are wondering… yes, we did finally reach the level of productivity required for Presqu’ile’s Common Terns to become self-sustaining. In a year in which the flood waters rose to reshape and even remove their traditional nesting grounds, against all odds, Common Terns were successful, with help from the park, help from the Friends of Presqu’ile, help from us and a lot of perseverance on all fronts.  

Thus, 2019 provided a fresh new crop of young terns to help build up the numbers of this species at Presqu’ile and throughout the Great Lakes. We hope the adaptive management of terns at Presqu’ile will allow tern populations to increase to levels that can once again resist predators, storms and the many adversities as they must have in the days of old. It will certainly take time but maybe this is the “tern-ing” point. We hope to welcome back these latest miracles as breeders at Presqu’ile in 2022 and 2023 when they finally return from South America as adults!