Adult Forster’s terns are pale gray above and white below, with a deeply forked gray tail and black markings on the head. There are different breeding and nonbreeding colorations. The voice is a one-syllable “keerrr” that is lower toward the end.
They are easily confused with the common tern, and the following characters should help distinguish Forster’s from that species.
Breeding Forster’s terns (spring and summer):
- Head markings: black cap and nape (very similar to common tern’s)
- Bill orange with black tip; thicker than common tern’s (which also has a black tip but is reddish)
- In flight, uppersides of wings look mostly uniform bright silvery gray (common terns have darker primary feathers)
- Legs are orange and noticeably longer than the common tern’s
Nonbreeding Forster’s terns (late August through winter):
- Head markings: only a dark, elongated eye patch with varying amounts of gray on head; black does not extend around the back of the neck (the common tern retains its black nape and the black extending behind the eye; it only loses the front portion of the cap)
- Bill black; thicker than common tern’s, which is also blackish in nonbreeding coloration
- In flight, wings are bright gray with black tips on the outer tips of primary wing feathers (common terns have more black on the primary feathers)
- Resting with wings folded against the body, there is no dark horizontal bar where the wings join the body (carpal bar) (common terns in nonbreeding plumage have a dark carpal bar)
Note that Forster’s terns typically acquire their nonbreeding plumage in the second half of August (in Missouri, we usually see them from August to the end of October), while the common tern usually transitions to winter plumage after it has departed from the United States (in fall migration, we usually see common terns August through the end of September).
Similar species: About 6 species of terns have been recorded for Missouri: the Caspian, common, Forster’s, least, sooty, and black terns. Most of them, like the Forster’s tern, are usually seen as spring and fall transients. Forster’s tern is one of our more common species.
As with the similar seagulls, terns are challenging to identify because the plumage varies so much between juveniles, immatures, and breeding and nonbreeding adults. Mastering their identification often involves learning subtle differences in bill and leg shape and size, overall profile, and voice. A full account of their differences is beyond the scope of this introductory web page.
Length: 14½ inches (tip of bill to tip of tail); wingspan 31 inches.
Statewide, in appropriate habitats.
Habitat and Conservation
Usually seen as they fly around and forage over water. In Missouri, you are mostly likely to see them in May and September, when their migrating numbers are highest. A wide variety of marshy habitats are suitable during migration, including large and small wetlands, shores of lakes, and along big rivers.
Food
Small fish and insects. Terns typically fly over water and when they spy fish swimming below, they plunge down into water to capture their prey. They sometimes dive slightly underwater in their pursuit. Forster’s terns also sometimes watch for prey from perches near water. The fish they consume are usually less than 4 inches long.
Status
Common transient; uncommon winter resident.
Overall populations appear to be stable. Local colonies will decline when wetland habitat is developed or degraded. Ornithologists find it difficult to study populations of Forster’s terns because colonies often move locations over the years.
Life Cycle
Present in Missouri from mid-April to the end of May, and from early August to late October. Numbers are highest during the first half of May and during the second half of September. The breeding territory is primarily in the northern Great Plains, including Canada’s prairie provinces. Populations in eastern North America mostly spend winters in states along the Gulf and Atlantic coasts, the Caribbean, Mexico, coastal Central America.
Forster’s terns breed in colonies along the edges of large, interior wetlands with ample open water for foraging. Males and females engage in dancelike, flying courtship displays, with males presenting gifts of fishes to the female during courtship and during nesting. Nesting pairs defend their nests from other terns in the breeding colony.
Nests are constructed on the ground or upon a mound of weeds or miscellaneous dead plant materials, or atop muskrat homes. The nests may be bowl-shaped or they may simply be a shallow scrape. There is only 1 brood a year. The clutch comprises 1–4 eggs, which are incubated for about 26 days. After hatching, the young start to leave the nest within a week. Lifespan can exceed 15 years.
Human Connections
Terns are elegant, natty-looking birds that people generally appreciate more than the similar seagulls.
This species and other terns were some of the birds that were shot and used as decorations on elaborate ladies’ hats in the 1800s. As various bird species declined due to indiscriminate killing, the conservation movement began. The 1919 US Migratory Bird Treaty Act and other national and international legislation helped to bring many birds back from the brink of extinction.
This tern was named in honor of Johann Reinhold Forster (1729–1798) a European Protestant pastor, naturalist, translator, world traveler, and author. Forster was one of the first accepted authorities on the animals of North America. He was also the naturalist on the second voyage of James Cook (1772–1775), an around-the-world expedition designed to establish whether or not there was a large continent in the South Pacific, akin to the large landmasses of Eurasia and North America. Thus Forster was able to study and later describe the natural history of many of the islands and coastlines of much of the Southern Hemisphere.
Ecosystem Connections
As with many other species, habitat conservation is a key to this tern’s continued survival. Forster’s terns require large, healthy interior wetlands for breeding habitat as well as coastal beaches, estuaries, and mudflats in winter. The former is threatened by human development for agriculture and building construction; the latter can be threatened by coastal development and by sea-level rise due to climate change. Additionally, when boats speed around on lakes where these terns breed, the nests may be flooded by their wakes.
Nesting colonies of Forster’s terns may respond in groups to perceived predators, a behavior called mobbing. Once detected, a mink, hawk, or marsh rice rat will be driven away by the diving and screeching terns. Mobbing is common in many other types of birds; you might have seen a group of blackbirds flying, chasing, and screaming at a hawk or owl. In the case of Forster’s terns, other bird species, nesting in the same area, gain safety via the terns’ defense tactics, including several types of ducks and grebes.
About 350 species of birds are likely to be seen in Missouri, though nearly 400 have been recorded within our borders. Most people know a bird when they see one — it has feathers, wings, and a bill. Birds are warm-blooded, and most species can fly. Many migrate hundreds or thousands of miles. Birds lay hard-shelled eggs (often in a nest), and the parents care for the young. Many communicate with songs and calls.