Caspian Terns were calling as we walked through Harriman State Park

A deep, raspy “aaayayaum” sound interrupted my wife and I as we walked along the shore of Silver Lake in Harriman State Park this week.

“What is that?” she asked as we moved forward.

“She’s a Caspian tern who brags about the fish she caught, like most anglers.”

We saw eight of the world’s largest terns flying along the shallow waters of the lake, looking for small fish swimming near the surface. Serious dives were 30 to 40 feet above the water, which would completely overwhelm large birds that were aiming for the biggest prize. Each successful capture was followed by a boastful vocalization.

“Don’t talk with your mouth full” does not apply to these birds.

Whenever I deliver fishing flies to Drift Lodge near Lake Henrys, I will stop in Harriman to try and catch these hunting birds. During their hunting periods, they ignore everything except look for something to eat. Also, I will often stop at Last Chance as well to watch them hunt the Henrys Fork of the Snake River and they can sometimes be seen perched on a rock in the middle of the water. The west shore of Lake Henrys is also a good place to observe them.

Caspian terns are found on all five continents and were named after the Caspian Sea. They enjoy both freshwater and saltwater habitats and feed almost exclusively on fish, but occasionally feed on frogs and other amphibians. They are “kleptoparasites”, meaning they will hunt and steal fish from smaller species for terns. This often occurs in the Market Lake Wildlife Management Area, where smaller Forster’s terns outnumber Caspian terns.

A Caspian tern dives and catches a fish at Market Lake. | Bill Schiess,

After capturing a fish, they usually swallow it whole, head first, in flight before another kleptoparasite, a bald eagle, can steal it from them. They have been known to catch fish too big to swallow and have suffocated to death.

Food is also an important part of the dating scene for Caspians. Once they arrive at their breeding grounds, a male catches a fish and flies over a group of terns perched on the shore, boasting of his catch. Several females may follow him and when the pursuers are down to one, he will offer her the fish, nodding his head in agreement. The pair have bonded and started elaborate aerial displays as they build their nest.

Together they dig a small depression in the sand, usually on an island where there is very little vegetation. They often pick up stones and/or sticks to make a border around the nest to mark it as they often belong to a colony of other Caspians.

Their relationship is open, as the two fool around breeding with others in the colony, but they are good parents. Both are busy defending the nest and the chicks once they hatch. They will nurture the children for almost a year – even after their relationship ends – with the children accompanied by one or both parents.

It takes three years for the young to become mature, and during adolescence they congregate in colonies of non-breeding groups. Most Caspians that summer in Idaho are these loose groups with only three areas known to produce viable nesting. This year, I observed a pair in Market Lake that went through their courtesy behavior. They may have moved or they may be nesting as I haven’t seen them, in pairs, in the last month. We will make sure they show up with children before they migrate south.

It is an interesting species to observe, but it is difficult to follow because it is nomadic and moves quickly to the best fishing grounds. It will be fun for me to watch the band in Silver Lake.

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