This column usually highlights a species that can be found ‘in your backyard,’ a way of providing some information about the myriad of critters that share living space in your neighbourhood. But this week I am narrowing that scope down a bit, discussing a bird species few of you have been lucky enough to have seen.
Black terns (occasionally called by their nickname, sea swallows) are ranked in Ontario as being of special concern, which means they are neither endangered of nor threatened with extinction — yet. But their populations are in decline and scientists are watching for causes.
Terns, like their counterparts, the gulls, are a water-loving species, especially within large marshes with protected patches of open water. And therein lies their first challenge, just finding a large marsh with open water patches; few of these wetlands remain in southern Ontario.
While their cousins, the common tern and Caspian tern, tend to hang out on the rocky islands of Georgian Bay and Lake Couchiching, black terns find shelter in the calm inland waters of places like Tiny Marsh and Wye Marsh.
The reason the black terns shun the open water bays is their nests are built to be floating rafts, and are poorly constructed at the best of times, which means moderate to rough waves will upset or tear apart the nest structures, as will wake from motorboats if they are operated near a tern colony.
One of the cool things about tern eggs is they are more porous than your usual, run-of-the-mill bird egg, which means they can endure a dunking and then dry out again. Brilliant!
Another survival technique is that this species will re-nest in July if the June nesting is a failure. The challenge is to get the youngsters of this second nesting all grown up in time for the fall migration.
And migrate they do. Dave Moore of the Canadian Wildlife Service has been studying these elusive birds and has captured and outfitted some with tiny backpack radio transmitters. As the birds fly by a receiving tower, their unique radio signature is recorded and their flight path determined.
Four birds that were tagged at Tiny Marsh were tracked for both south and north migration routes and some surprising data has come forth. They left Simcoe County on July 19 and were noted to be in the Carolinas just five days later. And on the 11th day they did a stopover in Panama.
Throughout November to April, the terns flipped around the coast of Peru and Ecuador, spending almost all their time ‘at sea.’ In April they headed back up north to lay their eggs at Tiny Marsh, at first regrouping in Panama, then a short side trip to Texas, and then overland across the U.S.A. back to southern Ontario. That’s 16,000 kilometres of flight time.
Unlike songbirds that overwinter in South America, pesticide use is a very small factor in the terns’ list of perils. And unlike waterfowl that go the distance, terns are not hunted for their meat all along the way. So, why are black tern populations plummeting?
One cause is wind. With global warming, there are more wind storms more often, and those fragile floating nests are getting hammered. (As a poignant note, as I was writing this column, high winds knocked out our power for 28 hours. We recovered but I wonder the level of devastation at the local tern colony.)
And secondly there is the continuing abuse and destruction of wetlands in south and central Ontario. These terns need large and somewhat secluded areas, like the Holland Marsh; as the big wetlands are infringed upon, there will be continued loss of biodiversity. As nature goes, so go we.
Some folks have come up with an artificial nesting platform, one that is anchored in place and designed not to tip over on high winds and rough water. There seems to be only moderate acceptance of these islands by the terns, but the effort is helping.
Whether you are a dedicated birder or just a person who loves observing nature, the sighting of a black tern is a moment of note. Let’s hope those floating island platforms do for terns the same as nesting boxes did for wood ducks.