Terns are seabirds in the family Laridae that are closely related to gulls. There are over 40 species of terns that inhabit marine environments across the globe. While the different tern species may look similar to the untrained eye, there are key identification features that bird watchers can use to distinguish between them. Being able to properly identify tern species is important for understanding their conservation status, habitat needs, and migration patterns. This article will provide an overview of the best ways to tell terns apart using field marks, size, voice, habitat, and range.
Plumage Differences
The plumage (feathers) of terns can vary considerably between species. Looking carefully at the color pattern of the head, back, wings, tail, and belly is often the best starting point for tern identification.
Some examples of distinguishing plumage characteristics include:
- Crested terns have an obvious head crest.
- The Arctic Tern has a blood-red bill and legs.
- The Sooty Tern has a dove gray body and black cap.
- Roseate Terns have a black bill with a reddish base.
- Least Terns have a yellow bill with black tip.
During the breeding season, many terns develop intensified plumage color, so it is helpful to know what they look like in their breeding vs. non-breeding plumage. The amount of white on the forehead and undersides is a good clue as well – this varies from species to species. Taking note of more subtle differences like eyestripe shape or tail streamer length takes some practice but can further aid identification.
Size Differences
Terns range considerably in body size, with wingspans from 20 inches up to over 70 inches across species. Size is therefore a useful indicator for narrowing down species possibilities. For example:
- Least Terns are very small, about the size of a sparrow.
- Common Terns and Arctic Terns are medium-sized.
- Caspian Terns and Royal Terns are much bulkier with thick bills.
- The Sooty Tern and Bridled Tern have longer tail streamers which make them appear large.
The tern species present in a given area can help reliably indicate scale as well. If multiple terns are visible, the relative proportions will show if any individuals are exceptionally small or large.
Differences in Voice
Many terns have diagnostic vocalizations that allow identification by ear alone. With practice, one can learn the common calls to listen for:
- Arctic Terns give a loud, piercing “kirree” scream.
- Sandwich Terns make a “kerrick” call.
- Elegant Terns sound like a loud “keer-whee”.
- Common Terns have a distinct “keeyah” cry.
- Black Terns have a unique high-pitched rattle.
The pitches, rhythms, and tone qualities of tern calls vary in ways that are useful for distinguishing look-alike species. Recordings of tern vocalizations are readily available online to help new birders learn the sounds.
Habitat Clues
Terns occupy marine habitats like beaches, estuaries, and offshore waters. However, different terns frequent different areas which aids identification. Assessing the specific details of the habitat can provide clues:
- Elegant Terns prefer sandy beaches.
- Roseate Terns nest in sandy/rocky islands and barrier beaches.
- Least Terns nest on open sandy or gravelly shores.
- Caspian Terns inhabit estuaries and mudflats.
- Black Skimmers intermix with terns on wide sandy beaches.
The feeding behaviors of terns also varies by species based on prey availability. Noting whether terns are plunge diving in estuaries, picking off surface fish, or aerial flycatching insects will help indicate which species may be present.
Geographic Range
The ranges of tern species differ across continents and latitudes. Consulting a guide to species distribution can quickly narrow down identification. For example:
- Least Terns occur widely across North America.
- Caspian Terns breed inland in eastern North America.
- Roseate Terns mainly occur along the northeast Atlantic coast.
- Elegant Terns only live along the Pacific Coast.
- Bridled Terns are tropical species found in Hawaii and the Caribbean.
Range maps and checklists of regional bird species make it easy to determine which terns are expected at a given location. The range provides an additional confirming clue for difficult to distinguish species pairs.
Using a Field Guide
Field guides are essential tools for identifying terns reliably based on location, season, behavior, and diagnostic features. Good birding field guides will highlight subtle differences between similar species and note key identification criteria. They also provide useful comparison charts showing species side by side. With practice consulting a guide, tern identification becomes much easier through pattern recognition. Apps with bird calls are an even more user-friendly option. Beginner birders should carry and regularly reference a guide to build familiarity.
Taking Notes
When observing terns in the field, taking detailed notes can be very helpful for confirming IDs later on. Important notes include:
– Exact location and habitat
– Size relative to other birds present
– Distinctive markings and plumage colors
– Interesting behaviors
– Voice descriptions
– Time of year
Photographing terns of uncertain identity is also useful for enabling more leisurely at-home comparison to field guide images. Thorough field notes allow you to make useful observations over time to expand your identification skills.
Using Binoculars or a Spotting Scope
Binoculars or a spotting scope are essential gear for observing key identification features on terns. Terns can be shy, so getting close is not always possible. With 10x magnification binoculars or higher, subtle field marks can be discerned from a distance. Details like leg color, eye markings, and tail streamer differences can only be detected optically. High power binoculars also help detect diagnostic beak colors and feather patterns necessary to distinguish look-alike species. Investing in quality binoculars vastly improves tern ID abilities.
Getting Expert Help
When uncertain of an identification after consulting guides and notes, seeking expert input can help resolve the unknown. Local birding and naturalist groups have experienced members that may provide ID confirmation. Joining group outings is a good chance to get tips from veterans. Online birding forums are also useful for getting feedback by posting photos and descriptions. Experts can detect obscure clues and provide definitive IDs to build your skills.
Practicing with Common Species
When starting out, try first focusing on learning the most common and widespread tern species in your area. With repeated and varied sightings of the same species, their distinguishing traits will become familiar. Then progress to trickier and less common look-alikes. Comparing similar species side by side will sharpen the ability to spot differences. Repeated practice is key for developing expertise.
Conclusion
Telling terns apart can seem daunting at first. But armed with some basic knowledge of field marks, size, voice, habitat, and behavior – plus taking good notes and using optical gear – identification confidence will steadily improve. Consult field guides regularly, seek help from experts, and above all spend time observing terns in the field. Consistent practice over time will transform the ability to distinguish subtle species differences. Paying careful attention to detail is the key to mastering tern ID.
| Species | Key ID Tips |
|---|---|
| Arctic Tern | Blood-red bill, deeply-forked tail |
| Common Tern | Orange-red bill with black tip |
| Forster’s Tern | Orange bill with black tip, rounded wings |
| Caspian Tern | Massive red bill, short tail streamers |
| Elegant Tern | Yellow bill with black tip, elongated streamers |
| Sandwich Tern | Black cap, yellow-tipped black bill |
| Roseate Tern | Black bill with red base, long tail streamers |
| Least Tern | Very small with yellow bill, black forehead |
| Sooty Tern | Black cap and bill, white forehead |
Additional Tern Identification Tips
Here are some more useful facts to help identify terns:
- Look for black wingtips on the upperwing which may help indicate Arctic, Common, or Roseate Terns.
- Underwing pattern (dark on the leading edge or not) aids some species.
- Winter plumage is duller and has more white than summer breeding plumage.
- Immature terns can look different from adults, check guides for variations.
- Be aware of rare vagrant species outside expected range.
- If possible, observe terns flying – flight style varies.
- Check for leg and foot color differences.
- Compare the shape and density of head feathering patterns.
- Note the location terns are feeding and nesting.
- Check the base color of the beak – black or orange/red.
Conclusion
Identifying terns is a rewarding challenge that improves with experience over time. Learn the species in your area, bring optics and a guide when observing terns, take detailed notes, and request help from experts when stumped. Consistent practice will sharpen your identification skills and allow you to enjoy observing a diverse range of elegant terns.
