Telling the difference between a common tern and an arctic tern can be challenging when viewing these seabirds in flight, as they share similar silhouettes and plumage characteristics. However, with careful observation and by focusing on key identification points, it is possible to distinguish these two species.
Size and Structure
The most noticeable difference between common and arctic terns is their size. Common terns measure 28–35 cm in length with a wingspan of 70–80 cm. They have a lightweight, elegant appearance in flight, with long, narrow wings and a deeply forked tail. Arctic terns are noticeably larger, measuring 33–39 cm in length with an 89–105 cm wingspan. Their wings appear broader and more angular, while the tail is shorter with a shallower fork.
In direct comparison, arctic terns look bulky and heavyset next to the more delicate common terns. However, without a size reference, judging the scale can be tricky. Focusing on the structural differences provides a more reliable way to tell them apart.
Plumage
Both common and arctic terns breed in predominantly gray plumage, which becomes whiter as they transition into winter. While the base colors are similar, arctic terns tend to look paler and cooler-toned compared to the warmer gray hues of common terns. They also differ in specific markings:
- Bill color: Common terns have an orange-red bill with a dark tip. Arctic terns’ bills are blood-red without a dark tip.
- Legs: Common terns have orange-red legs, while arctic terns’ legs are red.
- Crown: In breeding plumage, common terns have a black cap on the crown of the head. Arctic terns lack this cap.
- Underparts: Arctic terns have a whiter breast and belly compared to common terns.
These plumage differences can be subtle. Focus on the contrast between the dark bill tip and pale cheeks of common terns versus the uniform red bill and facial pattern of arctic terns. Check the legs too, as the darker orange-red of common terns stands out from the bright red legs of arctic.
Flight Style
Observing flying style provides more clues for identification. Common terns appear graceful and buoyant in flight, flapping their long wings in slow, easy strokes interspersed with brief glides. Their movements seem light and effortless.
The flight of arctic terns looks faster, stiffer, and more powerful. They beat their broad wings rapidly, reminding some observers of a butterfly’s flight. Reduced gliding gives their flight a business-like, forceful quality compared to the relaxed style of common terns.
These differences translate to variation in speed and agility. Common terns tend to loiter and drift more, changing direction easily. Arctic terns fly straight and fast on a mission, covering more distance with less maneuverability.
Range
The range and distribution of these species also aid identification:
- Common terns have a Circumpolar distribution, found widely across temperate regions of the northern hemisphere.
- Arctic terns are true Polar specialists, breeding in Arctic regions and migrating along coastal routes further south.
In most temperate regions, common terns are the expected species while arctic terns occur only as migrants or winter visitors. However, arctic terns can overlap with common terns on migration and in wintering areas. Comparing range provides helpful context, but doesn’t give a definite ID on its own.
Behavior
Habitat use and behavior offer more distinguishing features:
- Common terns regularly occur inland and utilize freshwater lakes and rivers more than arctic terns.
- Arctic terns stay closer to the coast, seldom venturing more than a few miles inland.
- Common terns are highly aerial, flying gracefully high above the water’s surface as they search for fish.
- Arctic terns fly lower over the sea, just skimming the waves and diving directly for prey.
These general trends provide good clues, but aren’t consistent enough to reliably separate the species. Use behavior together with physical differences for the most accurate identification.
Similar Species
Other tern species occur alongside common and arctic terns in some regions. These include:
- Roseate Tern: Smaller with a thinner bill, darker gray upperparts, and distinctive elongated outer tail feathers.
- Forster’s Tern: Larger with whiter wings, paler gray upperparts, and a more buoyant flight style.
- Elegant Tern: Larger with yellow-tipped black bill, heavier yellow-orange legs, and shallower wingbeats.
These species differ more obviously from common and arctic terns, but still warrant caution. Compare relative size, plumage tone, structural details, and flight mannerisms to rule out any confusion.
Key ID Tips
To summarize key points for identification in flight:
- Size – Arctic terns are noticeably larger and bulkier than common terns.
- Wings – Arctic terns have broader, straighter wings compared to common terns’ slender, elongated wings.
- Tail – Common terns have a deeply forked tail, while arctic terns’ tails are shallower-forked.
- Bill – Common terns have a two-tone bill with dark tip. Arctic terns’ bills are uniformly blood-red.
- Legs – Common terns have orange-red legs. Arctic terns’ legs are a brighter cherry-red.
- Crown – Common terns have a black cap in breeding plumage. Arctic terns lack a distinct crown patch.
- Flight manner – Arctic terns beat wings faster and more stiffly compared to common terns’ easy, buoyant flight.
- Range – Arctic terns are coastal and polar. Common terns are widespread in temperate regions.
With good views and ideal conditions, an experienced observer can identify flying terns to species accurately. However, conditions are seldom perfect, with distance, lighting, molt, and individual variation all affecting apparent plumage and structure. Leaving some terns unidentified or grouped more broadly (e.g. “common/arctic tern”) is often the best approach in challenging circumstances.
Conclusion
Distinguishing common and arctic terns in flight requires focusing on a suite of plumage, structural, behavioral and distributional differences. Key points include the larger size yet more angular wings of arctic terns, uniform red bill, faster wingbeats, and association with northern coastal habitats. Common terns appear more delicate with two-tone bills, bright orange legs, and aerial, graceful flight style. However, with overlapping variation, caution is required and leaving some unidentified is often prudent. By watching carefully and noting multiple features, identification accuracy improves with experience as flight styles and structural distinctions become more familiar.
