Terns are seabirds in the family Laridae that are found across the world. There are around 45 species of terns that range greatly in size and coloration, making identification challenging. However, there are some key features to look for when identifying terns.
What are the main physical features of terns?
Terns share some common physical traits that aid in identification:
- Slender bodies
- Long, pointed wings
- Forked or notched tails
- Sharp, pointed bills
- Webbed feet
Terns are lighter-bodied than gulls and have longer, more angular wings. Their bills are usually sharper and more stiletto-like compared to gulls. The tail shape is a good identifier – terns have forked or notched tails, while gulls have square or rounded tails.
What is the size range of terns?
Terns range greatly in size, from the very small Least Tern at 21–24 cm to the large Caspian Tern at 48–56 cm. Some common size ranges include:
- Small terns – 21–28 cm
- Medium terns – 28–38 cm
- Large terns – 38–56 cm
Larger terns like the Caspian Tern tend to have massive red-orange bills, while smaller terns have more slender all-black bills.
What are some identifying features of tern plumage?
Plumage can be very helpful in identifying terns. Some key features include:
- Gray or white upperparts
- Black caps on heads (most species)
- Black or white undersides
- Black wingtips (“window pane” pattern)
- Orange-red legs and feet (some species)
- Black-tipped orange/red bill (some species)
During breeding season, terns have a black cap and more colorful plumage on the bill and feet. In winter, the black cap recedes and the colors become more drab.
What are some common tern species and their identification tips?
Here are some brief identification notes for common North American tern species:
Species | Identification Tips |
---|---|
Common Tern | – Medium size (35–39 cm) – Black cap, red-orange legs – Red-orange bill with black tip |
Forster’s Tern | – Medium size (33–36 cm) – Long, forked tail – Orange bill with black tip |
Least Tern | – Very small (21–24 cm) – Black-tipped yellow bill – White forehead |
Caspian Tern | – Large size (48–56 cm) – Massive orange and black bill – Heavy red-orange bill |
Arctic Tern | – Medium size (32–39 cm) – Short red legs – Blood-red bill |
Black Tern | – Small size (24–29 cm) – Black undersides in summer – White cheek patch |
How can tern behavior help with identification?
Observing tern behavior can provide additional clues to the species:
- Flight style – Terns have fast, buoyant flight low over water. They hover frequently before plunge-diving.
- Diving style – Terns dive directly from flight into water to catch fish. Plunge-diving style varies by species.
- Vocalizations – Terns have a wide range of calls. Identifying species-specific calls takes practice.
- Association – Some species associate in mixed flocks, while others remain solitary.
With experience, observers can learn to identify species by behavior alone even at a distance. For example, the energetic, erratic dive style of the Black Tern is quite distinctive.
What are some tips for judging tern size?
It can be tricky gauging the size of lone birds over water. Here are some tips for size estimation:
- Compare relative size to known birds (e.g. gulls). Larger terns approach gull size.
- Consider the size of the head and bill in proportion to the body.
- Look at the length of the wings in relation to the tail during flight.
- Get photos or video for size reference if possible.
- Note whether legs extend well beyond the tail when swimming (bigger birds).
When multiple terns are near each other, size differences become much more apparent. Use flock mates as a gauge.
What about immature terns and winter/non-breeding plumage?
Identifying terns outside of breeding season can be more challenging because they lack distinctive breeding plumage. Here are some tips:
- Immature terns are browner, streakier, and lack black caps or bright bill colors.
- Non-breeding adults have duller plumage, grayer heads, and darker bills.
- Check range maps – only some terns remain in winter plumage year-round.
- Note structural features like bill shape, wings, tail forks.
- Compare relative size and proportions to other known birds.
- Observe behavior and flight mannerisms.
While harder to identify, practice with wintering and immature birds will make you a better overall tern observer.
What about similar families that may be confused with terns?
Some other seabirds share superficial similarities with terns. Compare:
- Skimmers – Larger and heavier, with massive laterally compressed bills.
- Jaegers – Raptorial seabirds, bulkier with hooked bills and tapered tails.
- Phalaropes – Smaller shorebirds that swim in circles, have needle-like bills.
Being familiar with related families will help avoid misidentification errors. When in doubt, check field guides to compare.
What about hybrid or vagrant terns species?
Rarely, vagrant individuals or hybrid birds may occur outside their expected range or with mixed traits. Some tips if you suspect an odd tern:
- Carefully check multiple field guides to rule out similar species.
- Note any anomalous physical traits that don’t match guides.
- Consult local experts and submit documentation for review.
- Do not assume hybrids even if traits are mixed – convergent evolution happens.
- Consider whether it could be an immature or non-breeding bird.
Proper documentation and expert analysis is important to confirm rare terns. Do not assume a sighting is unprecedented without thorough checking.
What field guides and resources help with tern identification?
Some recommended field guides and online resources for identifying terns include:
- National Geographic Field Guide to the Birds of North America
- Sibley’s Field Guide to Birds of Eastern North America
- Peterson’s Field Guide to Eastern Birds
- iBird Pro app – detailed guides with images, audio, and identification tips
- AllAboutBirds.org from Cornell Lab of Ornithology
- eBird.org – range maps and sightings data
- Local birding groups (for help with difficult IDs)
Using multiple guides and online resources helps give a more complete perspective for identification. Reach out to experienced birders when needed.
What equipment helps with tern watching and identification?
Having the right gear makes observing and identifying terns much easier:
- Binoculars – 8×42 models are a good balance of magnification and light gathering.
- Spotting scope – Gives better view of key field marks at a distance.
- Camera – Get diagnostic photos even at long range with zoom lens.
- Digiscoping adapter – Connects camera to spotting scope for ultra-zoom photos.
- Birding apps – Aid real-time ID by providing guides, audio, and sightings.
- Field notebook – Record field marks and details while observing to aid ID.
High quality optics allow you to view key identification features you’d never notice with the naked eye. Always bring optics when tern watching.
What techniques help in observing/identifying terns?
Some best practices for observing terns include:
- Get to know common species first before tackling harder ones.
- Study guides to familiarize yourself with key field marks.
- Look for flocks – easier to compare multiple birds.
- Practice IDing while terns are flying – good for challenging ID skills.
- Watch diving behavior -can be diagnostic even from a distance.
- Listen for calls – many terns have distinctive vocalizations.
- Photograph birds – can provide conclusive ID through analysis of shots.
- Take notes – record key field marks while observing.
- Consult experts when needed – no shame in asking for help on a tricky bird!
Patience and practice are key. Tern watching provides great enjoyment while sharpening your identification skills as a birder.
Conclusion
Identifying terns can be challenging but very rewarding. By learning the key field marks, behaviors, and spectrographic differences between species, both novice and experienced birders can hone their skills. Patience and practice are needed, as is a willingness to consult guides and experts when uncertain. With good optics and techniques, terns watching provides an enjoyable way to see fascinating seabirds while gaining better ID abilities.