ON THE WING

One good tern gets its supper

Posted Aug. 05, 2011, at 4:18 p.m.

Living close to the ocean has many advantages. In my case, it does a lot to support my hobby of bird-watching as there is almost always something to observe; the sea supports a variety of birds and lately it has been teeming with life. About a month ago I began noticing many large schools of young herring fish.

I certainly wasn’t the only one noticing them.

Common terns have been taking good advantage of this abundant food source. Their gurgling, raucous cries — an integral part of summer for me — have captured my attention on numerous occasions. I am always mesmerized by their graceful, acrobatic maneuvers as they turn, dip and wheel through the air on tapered and graceful wings. In buoyant flight they power 10-20 feet above the surface, scanning for schools of herring. Flipping themselves into a dive, they arrow straight down into the water, disappearing completely. A scant second later they emerge, most times with a small, silver fish in their beaks which they most often promptly swallow. They then shake the water from their feathers in midflight — a move I find comical — and resume their stalking.

I occasionally noticed terns carrying a fish aloft, dropping it and swooping down to catch it again before it hit the water. Showoffs, I thought.

Common terns are a familiar sight along the Maine coast and much of the Eastern Seaboard. In Maine, they can be confused with the less numerous Arctic tern. A reliable field identification mark is the presence of a black tip on the end of the common tern’s beak, although this can be hard to discern without the aid of binoculars. Common terns also have more extensive dark margins on their outer primary feathers. Their underwings are gray, in comparison to the Arctic tern, whose underwings are gleaming white.

According to “The Birds of North America” species account, both birds historically have suffered population declines, first from the millinery trade in the late 19th century, then with the explosion of gull populations in the 20th century. Gulls outcompete terns for the best island breeding sites, so such sites must be carefully managed to reduce gull populations and predation.

As a result of such management, common tern populations have increased, although not to previous numerous historic records, according to the account.

I wondered where the terns I was watching had been nesting. Gulls do not employ the same foraging techniques as terns, so there was no competition for food between them. There was, however, some interaction going on with the double-crested cormorants that also had gathered in the area to feed on the herrings.

I at first thought the terns were competing with the cormorants, but after observing for a while I began to notice the terns were benefiting from the cormorants’ actions.

The terns definitely concentrated their foraging wherever the cormorants were feeding. As the cormorant dove beneath the surface to pursue the fish, the tern also would dive ahead of it, catching any herrings that escaped from the cormorant. The fish were facing pursuit from the sea and the air.

At times, the tern would dive at a cormorant as it rose to the surface. If it did this to surprise the cormorant into dropping its catch, it never seemed to work. I also noticed terns hovering in the air above a seemingly unconcerned cormorant, screeching, which was quite comical. I wondered whether the terns were trying to force the cormorant to dive so they could catch the conveniently distracted fish more easily.

This was an intriguing interaction to observe, and one I probably wouldn’t have seen had I not had such easy access to the sea.

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