What do they look like?

Because the coloring of common terns changes significantly as seasons change, they are often difficult to identify from plumage alone. They are most easily identified by their black head and red bill. The tail is forked and the tail feathers are more elongated than those of most terns. The wings are pointed and the inner and outer parts of each wing are the same width. The body of common terns is whitish-gray and the underparts are much paler than the upperparts, particularly in adult terns. The female is usually smaller than the male, although only slightly. The bill is usually pointed downward when the tern flies. Other notable characteristics include an exceptionally powerful head and neck and unusually long legs, which distinguish them from other terns such as Arctic terns.

Common terns are 37 cm long, on average, and have a wingspan of 27 cm.

Where in the world do they live?

Common terns are found from northern Canada south to the Caribbean Sea, as well as throughout Europe, Northern Asia, North Africa, and the Middle East. Some populations of common terns winter in the south to Peru and Argentina.

What kind of habitat do they need?

Common terns almost always reside in colonies. The colonies tend to be along ocean coasts, although they are also found on the shores of large lakes. The two things necessary for a colony of terns, or a "ternery," are isolation from predators and a reliable source of food nearby. The birds also must be able to communicate visually and vocally with the rest of the colony from their nests. They nest among rocks and cliffs.

How do they grow?

Not enough information available.

How do they reproduce?

During courtship, which begins in April, male terns establish their territories at the colony before beginning what is called "courtship feeding," in which males bring fish to the females as a way of courting them. Premating displays are accompanied by the male tern posturing followed by the two terns circling each other. The males mount the females for one- to two-minute intervals before copulation actually takes place. Common terns are known for wildly flapping their wings during and directly after copulating. Common terns are monogamous.

Common terns, after migrating to their breeding grounds shortly after the beginning of spring, proceed to find a mate (terns tend to be monogamous); they reproduce in early to mid-summer. It is rare for a pair to produce more than one clutch per summer. They nest among rocks and cliffs. The nests are made up of shells and debris or of dead vegetation. Clutch size is 3, on average, and the chicks hatch in 3 to 4 weeks. Young fledge in 27 to 30 days. Common tern chicks are able to fly by the time they are a month old but do not reach sexual maturity for 3 years.

One of the parents attends the nest at all times after the eggs hatch; the female is often, but not always, the one standing guard. Common terns become very aggressive after their chicks learn to move on their own because of the likelihood that the chicks will be harmed or killed by predators. Both males and females bring food back to the nest, but males are usually more involved in feeding than females are. Chicks are semiprecocial. Young terns usually learn to fly when they are 27 to 30 days old.

How long do they live?

The average lifespan is about 9 to 10 years. The oldest common tern ever recorded was 25 years old.

How do they behave?

Common terns live in colonies. There is no clearly organized hierarchy among the birds; all appear to be equal. Although all the terns migrate and live together, each family unit is responsible for its own feeding and care of eggs/chicks. They often defend feeding territories. Terns nest during the breeding season and they migrate at the end of the season.

Home Range

We do not have information on home range for this species at this time.

How do they communicate with each other?

Common terns communicate mostly with their unusual, hoarse voices, and they have three different, distinct calls. During mating, communication is mainly visual and tactile.

What do they eat?

The diet of common terns is usually limited to fishes. Often, if food is particularly abundant, the terns will catch more fish than necessary. Common terns sometimes catch fish that are too big for them to swallow. This, combined with their tendency to catch as much as is available even if it is not needed, explains why it is not unusual to see fish scattered around terns' nesting grounds. At the beginning of the breeding season, terns may eat insects, annelids, and echinoderms in addition to fish. However, throughout the later parts of the breeding season, the tern's diet is much more limited.

These terns are very good at catching insects. Many have been reported to fly near the surface of the water and pick insects off the surface while in flight. It is rare for them to eat dead food. They usually fly at great heights before diving for their prey, a behavior uncommon in other terns. The terns dive into the water after a fish, come to the surface, shake the water from themselves, and fly off with the fish. When a solitary tern catches fish in the same spot repeatedly, other terns from its colony join it.

Foods eaten include: "Food fishes," such as whiting, herring, haddock; sand launces; insects; crabs, shrimp, and other crustaceans; annelids; mollusks; fish eggs; and in certain cases, echinoderms. Terns that nest near bodies of fresh water often consume minnows in place of fish like herring and whiting.

What eats them and how do they avoid being eaten?

When a predator comes too close to a tern colony, any terns that spot the predator begin to call loudly to the rest of the colony. Adult terns come over to mob the predator while the chicks take cover in the high grass or in their nests.

Also, the sheer number of terns in a colony aids in the strategy of "passive avoidance". In other words, the probability of any one tern being harmed by a predator is much less because of the number of other terns that the predator could choose instead.

A common tactic among members of colonies, and in fact among all members of tern colonies and gull colonies, is called a "panic." This means that an entire colony of terns flies up making noise, falls silent suddenly, and then swoops back down toward the ground. This can be very threatening to potential predators and often assures that the colony will be left alone, particularly by smaller predators such as blue jays or grackles.

Known predators include: red foxes (Vulpes vulpes), raccoons (Procyon lotor), striped skunks (Mephitis mephitis), minks (genus Mustela), long-tailed weasels (Mustela frenata), squirrels (subfamily Sciurinae), dogs (Canis lupus familiaris), cats (Felis silvestris), rats (Rattus norvegicus), gulls (genus Larus), herons (family Ardeidae), hawks (family Accipitridae), falcons (family Falconidae), owls (family Strigidae), blue jays (Cyanocitta cristata), grackles (genus Quiscalus), reptiles (class Reptilia) and ants (family Formicidae).

What roles do they have in the ecosystem?

Common terns have an impact on populations of the prey they eat and are an important food source for their predators. Occasionally the fish they catch and do not eat are eaten by other scavenging animals living in the same area.

Do they cause problems?

There are no known adverse affects of common terns on humans.

How do they interact with us?

In the nineteenth century, terns were exploited commercially for their eggs and feathers.

Are they endangered?

The Michigan DNR considers common terns to be threatened in Michigan. Human interference has caused health and habitat problems for the terns. Human disturbance in the form of deliberately damaging eggs, chicks, and nests has become a problem because the coastal areas where terns nest are also areas where people picnic and sunbathe. Nature photographers and bird watchers, while meaning no harm, sometimes disturb the terns when they are nesting. Humans also cause problems for the terns through environmental pollution with chemicals, which weaken the eggshells and cause birth defects. Some adults and chicks die when they become tangled in netting or plastic. In the nineteenth century, terns were removed from nearly all of their former habitats when they were exploited commercially for their eggs and their feathers.

Some more information...

None.

Contributors

Kristina Sepe, University of Michigan

Phil Myers, Museum of Zoology

Contributors

Alaine Camfield, Animal Diversity Web

References

Burger, J., M. Gochfeld. 1991. The Common Tern. New York, NY: Columbia University Press.

Malling Olsen, K., H. Larsson. 1995. Terns of Europe and North America. Princeton, NJ: Princeton University Press.

Palmer, R. 1941. Behavior of the Common Tern. Boston, MA: Boston Society of Natural History.

Seago, M. 2002. "Common Tern, Sterna hirundo" (On-line). Accessed 03/05/04 at http://www.birdsofbritain.co.uk/bird-guide/common-tern.htm.

Terres, J. 1980. The Audubon Encyclopedia of North American Birds. New York, NY: Alfred A. Knopf.

Ehrlich, P., D. Dobkin, D. Wheye. 1988. The Birder's Handbook: A Field Guide to the Natural History of North American Birds. New York: Simon and Schuster.