Ask SIB: Mating Behavior of Terns

Several years ago and for several years we used to see a mating ritual among a class of terns in mid Spring. We saw it down at the point by the beach club but they would also appear over our Sealoft villa. The ritual was something like this. A female (I assume) tern would take off and start climbing and be quickly joined by two other terns (I assume males). She would do really intricate maneuvers- barrel rolls, inside-outs, etc. and the two pursuers tried to copy her. After several minutes of this she somehow would indicate her choice and the rejected bird would fly away. Then the couple did even more extraordinary maneuvers, diving, climbing, rolling, etc. and covering a lot of air space from the beach club out to over the Sealofts and back. They were always in perfect sync with one slightly behind and to the side of the other. It was extraordinarily beautiful, better than any ballet.

Questions: Is this standard behavior for a certain class of terns? Does it go on wherever these terns mate or is it peculiar to our coast? Which terns are these? What month are they most likely to do this?

Thanks for your feedback

Andy Allen

Answer:

Without knowing which species of tern Andy saw, it is challenging to make a definitive statement. Yet, Andy did provide some useful clues with his careful observations. To start at the basics, there are five species of terns likely to breed on or near Seabrook Island: Royal Tern, Forster’s Tern, Least Tern, Sandwich Tern and Gull-billed Tern.

Two of these species, Gull-billed Tern and Least Tern, tend to make more horizontal flights during pair bonding. So, we will rule out those species.

Forster’s Terns generally nest in the marshes, so we may want to rule out the Forster’s Tern, though it is worth understanding how they interact during pair bonding. The Royal Terns and Sandwich Terns nest on Deveaux Bank.

Most species of tern perform flights called “Dreads.” This is where a whole flock lifts up and flies around, not just because of a nearby predator, but as part of pair bonding and colony cohesion. The Forster’s Tern, Royal Tern, and Sandwich Tern have courtship flights described as “High Flight.”

What I believe Andy is referring to would be the “High Flight” behavior. That leaves the three species, Royal, Forster’s and Sandwich Terns. Here is how the scientists who wrote the pages for these terns on Birds of the World describe the variation in each species “High Flight.”

Forster’s Tern: “Two terns (sometimes 1 with fish) begin circling ascent in “jerk-flying” (Baggerman et al. 1956) manner (wings angled back with pause [jerk] at bottom of downstrokes, wing beats at faster rate [3–4 beats/s] than normal flight [2–3 beats/s]) up to 200 m. At highest point, leading tern starts gliding descent in Aerial Bent Posture (beak pointed downwards, black cap tilted away from other tern, wings held above horizontal); second tern performs Pass (gliding low over first tern) and adopts Straight Posture (beak forward, head tilted so black cap points away from other tern), initially fast and steep; Pass may be repeated. High flight may begin from ground or from flight. Terns may be silent or utter calls during Ascent. Fish-carrying tern most often the individual performing Aerial Bent Posture. After landing, if first tern was fish-carrying individual, that individual feeds the other during Posturing or Parading (see below; MKM).”

Royal Tern: In High Flight, one, presumably male, spirals upward, giving Advertising-Call, pursued by ≥1, commonly 2 individuals. During ascent, leader also gives Aack Calls, as do pursuers. Where >1 pursuer, these may pair off and continue own High Flight. Fish sometimes passed between 2 flying birds that have paired off, one dropping it a short distance to other below. Third may join and replace one of 2 original participants or cause 2 to break off aerial courtship. At peak of ascent (≥100 m), leader initiates downward glide, closely followed by pursuer. Both describe spiral descent during which Pass Ceremony may occur, as in Sandwich Tern. High Flight may last up to 25 min and lead to ground courtship, but pair more often retires to fish or to loafing flock to rest, preen, or bathe. Aerial courtship diminishes at hatching time, but occurs throughout breeding season.

Sandwich Tern: Usually aerial display follows ground courtship. An unmated male flies around a flock of loafing birds, usually carrying a fish and vocalizing in advertisement. He may alight on the ground near the flock and raise head and bill while calling, with wings held away from the body and crest raised. When female approaches, male takes off in an aerial “bent posture,” with head and bill pointed toward the ground and back arched. Female may or may not follow male into “high flight” display, and other birds may join in. Aerial courtship consists of ≥ 2 birds ascending in a circle for one to several minutes; at the apex, one bird breaks into a fast downward “glide,” often after several false starts. In the glide, the birds fly close together and trailing bird may overtake lead bird.

The short answer to Andy’s questions are as follows.

1. This is a standard behavior of many tern species.
2. It goes on wherever terns congregate.
3. The above descriptions lead me to believe that the birds Andy refers to are Royal Terns (though his assumption is about sex is wrong) as the behavior he described more closely matches that of the Royal Tern, though Andy did not mention if the birds were noisy or not.
4. The Forster’s and Royals are already here. The Royal Terns are pair bonding and setting up nesting territories in early April into May. The Forster’s Tern is mid-April to mid-May. The Sandwich Tern, which should be arriving in the next couple weeks, would be displaying in late-April to early-May. I do have to add that the first Least Terns, a SC Species of Concern should be arriving on Seabrook Island in about a week. Least Tern numbers grow through May. Each year, we have high hopes that a colony will once again establish itself on North Beach. Be sure to watch the antics of this species, but stay well out of their way so as not to disturb them!

One of the joys of living on or visiting Seabrook Island is being able to watch the birds and the bird behavior. We would love to hear from people who observe any of the above behaviors.

References:

Buckley, P. A., F. G. Buckley, and S. G. Mlodinow (2021). Royal Tern (Thalasseus maximus), version 1.1. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.royter1.01.1

McNicholl, M. K., P. E. Lowther, and J. A. Hall (2020). Forster’s Tern (Sterna forsteri), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.forter.01

Shealer, D., J. S. Liechty, A. R. Pierce, P. Pyle, and M. A. Patten (2020). Sandwich Tern (Thalasseus sandvicensis), version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.santer1.01

• – Bob Mercer, SIB’s “Resident Naturalist”