The Aleutian tern (Onychoprion aleuticus) and common tern (Sterna hirundo) are two similar-looking seabird species that can be challenging to differentiate. However, with some key identification tips, it is possible to confidently tell these two terns apart.
In this article, we will cover the key differences in appearance, behavior, habitat and range that enable positive identification of Aleutian and common terns. We will look at their size, plumage characteristics, bill color, leg color, flight style, diet, breeding sites and geographic distribution. With the help of descriptive comparisons, photos, videos and distribution maps, you will learn how to confidently separate these two species.
Some quick facts to get us started:
– The Aleutian tern is larger with broader wings and a thicker bill compared to the common tern.
– Aleutian terns have a distinctive black “mask” around the eye and forehead in breeding plumage. Common terns lack this bold facial pattern.
– Aleutian terns feeds more on fish, while common terns take more insects.
– Aleutian terns nest in large colonies on remote offshore islands in Alaska. Common terns nest along coasts and inland across North America.
Size and Shape
The Aleutian tern is noticeably larger than the common tern in direct comparison. Aleutian terns measure 14-16 inches in length with a wingspan of 33-36 inches. Common terns are smaller at 11-13 inches in length with a wingspan of 24-29 inches.
The Aleutian tern has broader wings and a stockier body shape compared to the common tern’s more slender and delicate build. Aleutian terns have a longer, thicker bill which can appear slightly heavier at the tip. Common terns have a thinner, more tapered bill.
In flight, Aleutian terns have slower, less buoyant wingbeats compared to common terns. Aleutian terns appear heavier in flight with broad wings and a blunt-tipped tail. Common terns are more graceful and lighter in flight with long, pointed wings and a deeply-forked tail.
Plumage
Aleutian and common terns share the same basic plumage pattern – gray above and white below with a black cap on the head. However, the extent of black on the head and neck is different in breeding birds.
Adult Aleutian terns develop a distinctive black “mask” extending from behind the eye to above the forehead. This thick black mask contrasts strongly with the gray plumage. The mask may extend slightly onto the nape and sides of neck.
Aleutian tern | Thick black “mask” on side of head and forehead |
Common tern | Black cap stops at eye with no mask |
Adult common terns lack the black mask and the black on the head is restricted to just a cap ending cleanly above and behind the eye. The sides of the head and neck are white in common terns, contrasting with the black cap.
The bill color also differs. Aleutian terns have dark upper mandible that fades to a dull orange-yellow at the base. Common terns have an all-orange or reddish-orange bill. Legs are orange-red in Aleutian and common terns.
In non-breeding plumage, the black head patterns becomes less distinct making identification more challenging. But structural size and bill differences still enable identification. Juvenile birds resemble non-breeding adults.
Flight Style and Behavior
The flight style and foraging behavior of Aleutian and common terns also show some differences.
Aleutian terns beat their wings slower and have more labored flight compared to common terns. Common terns are very buoyant and graceful fliers, almost tern-like. Aleutian terns make long glides when foraging.
Aleutian terns plunge dive from greater heights compared to common terns when hunting for fish. Common terns tend to dip and hover lower over the water when plunge diving for fish.
When feeding, Aleutian terns are more likely to be seen alone or in smaller groups. Common terns often feed in larger flocks with safety in numbers.
Aleutian terns tend to be quieter with raspy, grunting calls. Common terns are very vocal with sharper, louder “keeyak” calls.
Diet
Aleutian terns are more piscivorous, feeding predominantly on small fish like herring, sandlance and capelin. They also take some invertebrates like mollusks, crustaceans and marine worms.
Common terns take a higher proportion of invertebrates including insects, shrimp and other small marine creatures. But they also eat small fish frequently. Common terns are more likely to feed on flying insects.
The differences in bill size and shape likely reflect these dietary preferences. Aleutian terns have stouter bills for holding fish while common terns have finer bills for plucking insects.
Breeding
Aleutian and common terns have very different breeding habitat preferences and nesting behaviors. This provides a very reliable way to identify the species.
Aleutian terns nest in extremely large colonies of up to 100,000 pairs on remote, predator-free islands in Alaska. They nest in treeless habitats on cliffs, slopes, boulder fields or grassy areas on islands.
In contrast, common terns nest along ocean coasts or inland lakes across most of North America. Colonies are much smaller, usually fewer than 2000 pairs. Common terns nest on sandy beaches, shell flats, marshes, rocky islets or dredge spoils.
Nesting in the open, common terns are more aggressive in dive-bombing intruders that approach the colony. Aleutian terns have fewer terrestrial predators to contend with on their remote islands.
Aleutian tern | Large colonies on remote Alaskan islands |
Common tern | Smaller colonies along coasts and inland |
Range and Distribution
The breeding and non-breeding ranges of Aleutian and common terns are very distinct, providing a reliable way to separate the two species geographically.
Aleutian terns have an extremely limited breeding range concentrated in a few treeless island groups in the Aleutian Islands of Alaska. Just 4-5 islands host the entire global population.
In contrast, common terns have an enormous Holarctic breeding distribution across North America, Europe and Asia. In North America, they breeding from Alaska to California on both coasts.
In the non-breeding season, Aleutian terns migrate offshore remaining pelagic over deep waters of the northeast Pacific. Common terns migrate down the Atlantic and Pacific coasts of the Americas.
The range maps below depict the vastly different distributions of these species through the year:
Aleutian tern range map | Common tern range map |
As the maps illustrate, range and geography provide quick clues for identification. Only in the non-breeding season is there some offshore overlap in the Pacific Ocean.
Conclusion
While Aleutian and common terns are similar-looking tern species, careful observation reveals many differences that enable confident identification.
Key points to look for are:
– Size – Aleutian larger and bulkier
– Plumage – Thick black “mask” on side of head points to Aleutian
– Bill – Thicker on Aleutian, more tapered on common
– Flight – Slower, more labored in Aleutian vs. graceful in common
– Diet – Aleutian eats more fish, common more insects
– Nesting – Aleutian in huge colonies on Alaskan islands
– Range – Aleutian only along Aleutians, common widespread
By understanding these differences and doing some geographic range filtering, birders can reliably tell an Aleutian tern from a common tern. With a bit of practice, identification becomes quick and certain. Consult field guides, range maps and these tips for best results.