Common terns and Forster’s terns are two similar species of seabirds in the tern family Sternidae. While they share some similarities and often occupy the same habitats, there are key differences between these two species that birders and ornithologists can use to tell them apart.
Physical Differences
The most noticeable physical difference between common and Forster’s terns is size. Common terns are medium-sized birds with a length of 12-15 inches, while Forster’s terns are smaller at 9-11 inches long. Common terns also tend to have longer wings and tails than Forster’s terns.
In terms of plumage, common terns have lighter gray upperparts and whiter underparts compared to Forster’s terns. The common tern’s bill is orange-red with a black tip, while the Forster’s tern has an all-orange bill. Breeding common terns develop a black cap on their heads, and their tails become deeply forked. Forster’s terns retain a white forehead in breeding plumage and have slightly less forked tails.
Juvenile common and Forster’s terns are more similar in appearance than adults. Juvenile common terns have darker bills and reduced head markings compared to adults, while juvenile Forster’s terns have dusky bills and peppered heads. However, juvenile Forster’s terns still average smaller in size than juvenile common terns.
Distribution and Habitat
Both common and Forster’s terns nest in North America and migrate to coastal areas of Central and South America in the non-breeding season. However, common terns have a broader global distribution and are also found in Europe, Asia, and Africa.
In North America, common terns nest along the Atlantic and Pacific coasts, as well as around the Great Lakes. Forster’s terns are largely restricted to coastal regions of the Pacific coast and Gulf of Mexico. They overlap and intermix along the Atlantic coast.
These species use similar coastal and inland wetland habitats for nesting and foraging. Common terns are more likely to be found on rocky islands and barrier beaches, while Forster’s terns show a preference for salt marshes and lagoons. Both species feed primarily by diving for small fish in shallow waters.
Breeding and Nesting
The breeding seasons of common and Forster’s terns overlap in late spring and summer. Courtship displays are similar, involving aerial flights, fish exchanges, and calling between mates.
Nesting habitat preferences differ, with common terns nesting more often on bare or sparsely vegetated ground on islands, beaches, or floating rafts. Forster’s terns favor nesting in salt marsh vegetation or flooded reedbeds. Nests consist of scrapes or depressions in the substrate.
Clutch sizes are typically 2-3 eggs for both species. Common tern eggs tend to be paler and less marked than those of Forster’s terns. Incubation ranges from 21-28 days. Both parents share incubation and chick-rearing duties.
Common terns are very defensive of nesting territories, even attacking humans that get too close. Forster’s terns are less aggressive nest defenders. Common terns also demonstrate higher nest-site fidelity, returning to the same breeding grounds each year.
Diet and Feeding
As coastal piscivores, the diets of common and Forster’s terns consist predominantly of small fish caught through plunge diving. Common species taken include silversides, herring, anchovies, sand lance, and minnows.
Both terns are aerial foragers, scanning for fish from flight and hovering before plunge diving into the water for prey. Prey is usually swallowed whole while the tern hovers or sits on the water surface.
Common terns are able to dive from greater heights than Forster’s terns and take larger fish on average. Forster’s terns are more likely to pick small prey from near the water surface. Their smaller size allows Forster’s terns to exploit shallower water habitats.
These species occasionally supplement their fish diets with shrimp, crabs, worms, insects, and other invertebrates. This is more frequent in the non-breeding season when terrestrial prey is more readily available.
Migration
Both common and Forster’s terns undertake lengthy annual migrations, traveling between their northern breeding areas and wintering grounds in Central and South America. They migrate in flocks, mixing with other tern species along migration routes and wintering areas.
Fall migration begins in August or September after the breeding season. Wintering areas include coastal zones from Mexico to northern Chile and Argentina. Spring migration back to the breeding grounds occurs over a broad front across North America.
Satellite tracking of Forster’s terns has recorded nonstop flights of 4,000-6,500 km over 2-5 days during migration. Common terns likely make similar marathon transoceanic flights. Younger birds may take more circuitous routes and make more frequent stopovers.
Conservation Status
Common and Forster’s tern populations declined in the late 19th and early 20th centuries due to hunting for feathers and habitat loss. Populations rebounded after protective legislation was enacted.
Both species are currently abundant and widespread. The common tern has an estimated global population of 500,000-1,000,000 breeding pairs. Forster’s terns have an estimated 80,000 breeding pairs globally.
Habitat degradation, human disturbance, predation, and disturbance from gulls are ongoing threats. But neither species is considered globally threatened or endangered at present.
Key Differences
In summary, here are some of the key differences between common and Forster’s terns:
- Common terns are larger than Forster’s terns
- Common terns have orange-red bills with black tips, Forster’s terns have all-orange bills
- Common terns have a black cap and deeply forked tail in breeding plumage
- Forster’s terns are restricted to coastal North America, while common terns have a broader global range
- Common terns prefer to nest on open beaches, barriers islands, etc., Forster’s terns in marshes
- Common terns can dive from higher and take larger prey than Forster’s terns
- Common terns are more aggressive nest defenders than Forster’s terns
Paying attention to size, bill coloration, tail shape, plumage, and behavior allows observers to distinguish these often co-occurring tern species.
Conclusion
Common and Forster’s terns show a mix of similarities and differences reflecting their close evolutionary relationship. While they look somewhat alike and occupy comparable ecological niches, important distinctions in morphology, distribution, nesting habits, foraging, and migration patterns exist between them.
Understanding these differences aids birders in properly identifying these species in the field. It also highlights the diversity of the tern family and how closely related species evolve distinct traits and adaptations over time.