Plunge-diving Forster’s Terns

February 13^th, 2024 ~ My sunset visit to Frenchtown Road, in Port Bolivar, Texas may have ended with the calm and capable Oystercatcher, but it began with a small flock of zooming, splashing, diving Forster’s Terns.

Adult Forster’s Tern approaching breeding plumage
1/2500 sec. f/5.6 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total)

The Forster’s Tern, Sterna forsteri, is a smaller member of the Tern (sterna) Genus, which is part of the Gulls and Skimmers Family (Laridae). Terns used to be considered gulls, but were divided from them back in the 1800’s when their much more streamlined bodies, narrow pointed bills, long narrow wings and swallow-like tails made it clear they were fundamentally different from the ponderous gulls. Recently an additional differentiator has been discovered: while the rest of the Laridae Family can see ultra-violet light, the Sterna Genus cannot; Terns rely on the visible spectrum, as we do.

Forster’s Tern hovering to spy a fish
1/2500 sec. f/5.6 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total)

They are fun to photograph, because they follow a predictable flight path. They work a long narrow loop across shallow water, repeatedly using the same starting point, slowing almost to a hover, then plunging almost straight down into the water for small fish. Really small fish, roughly the length of their bright bill.

Foster’s Tern showing eye patch that will become full black cap during breeding season
1/2500 sec. f/5.6 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total)

If they didn’t spot a fish after hovering a few seconds, they looped back to the starting point, and raced down the same path again. If they came up empty after having dived on a fish, they tried again, up to three times in quick succession, like a stone skipping across the surface, before doubling back to their starting point. The quiet evening was punctuated by the repeated smacks of birds hitting the water.

They establish several elongated tracks across the water. While the one in the photo above worked a track from my left to my right, the one below worked the shallow water parallel to the marshy edge. He was successful, and has a tiny blue-brown fish… with something else, too small to identify, perhaps a dragonfly or a weed.

A successful catch
1/2500 sec. f/8 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total)

I found positive identification of the Forster’s Tern to be challenging. The Common Tern, Sterna hirundo, looks almost the same, especially when seasonal color changes are considered. The Common has orange bill and feet that turn red during the breeding season, while the Forster’s has orange bill with black tip. They both have the forked tail, with the Common’s as long as his wings when resting, and the Forster’s longer than his wings. The Common has more black in the tips of his wings than the Forster; the Forster’s wing tips may look smudgy gray from below, but they are white from above.  

 The Forster’s Terns breed along the Texas and Louisiana Gulf Coast, while the Common Tern is only here during the winter… except for first-year birds who may choose not to migrate north, staying the whole year along the Gulf Coast, and mingling confusingly with the Forster’s.  

The eBird sightings maps show that the Common Tern is much less populous here than the Forster’s Tern. So, given that the diving birds I saw were not-yet-breeding birds, with orange-and-black bill, pale wing tips, and what appeared to be very long outer tail feathers, I leaned on the statistics and concluded these were Forster’s Terns.

Watch out below!
1/2500 sec. f/5.6 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total)

The Forster’s were diving from a height of roughly 20 – 25 feet, almost straight down into the water. This is no great height for plunge-divers; Brown Pelicans can safely dive from 70 feet, and the Northern Gannet dives from heights of 100 feet or more. But this small flock of 20 or 30 Forster’s Terns, was having a fine time, coming up with prey about one out of four dives.

Forster’s Tern hits the water, target in sight
1/2500 sec. f/5.6 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total) 

With a splash, this bird appeared to belly-flop into the water, his tail held straight up, his head just beneath the surface. (I’m sure it’s not a belly flop, as a day spent doing that would be uncomfortable!)

Forster’s Tern with only his wingtips above water
1/2500 sec. f/5.6 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total)

He sank until just the tips of his upraised wings were still above the surface, then, with a powerful thrust of his wings, flew up out of the water.

Forster’s Tern zooms away with his prey
1/2500 sec. f/5.6 ISO 1250
Nikon D850 with Nikkor 600mm f/4 + 1.4x TC (850mm total)

Another fish caught! Note that the upper sides of this bird’s primaries are white where the Common Tern’s would be smudged black.

Confusion aside, I thoroughly enjoyed my evening watching Terns arrowing into the estuary off Bolivar Point. And I will be back: I’m looking forward to seeing their mating displays, strutting side by side, and doing a fish-exchange.