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Watching terns in southern New Jersey provides some of the most interesting and exciting summer birding. An amazing 15 species have been recorded in the region, so hopes of bumping into a rarity run high on the minds of many birders as they sort through tern flocks. But noticing that rarity depends largely on your familiarity with the common species. The two most abundant in our region, Forster’s and Common Terns, present surprisingly difficult identification challenges for beginner and even expert birders. As with gulls and shorebirds, part of the problem is a substantial change in appearance from one season to another and between age categories. Two important steps will help make learning terns easier. First, get started in spring or early summer when most birds encountered will be adults in breeding plumage. Second, start by learning features that have nothing to do with plumage variation: size, structure, and voice. These characteristics are the foundation that enable you to master tern identification.

Before even seeing a tern, it is helpful to know which species to expect at a given season or location. Forster’s Terns can be seen almost year-round in southern New Jersey. They breed in back-bay marshes and, in the summer, forage in all tidal wetlands as well as interior marshes and lakes. They are seldom seen in large numbers over the ocean during the height of the breeding season (June and July). However, by late summer, migrants begin to collect along the coast where they feed commonly over the ocean, with peak numbers occurring in fall from September to mid-November and in spring from April to mid-May. They winter irregularly in small numbers along the coast, with typically only a very few birds seen during the coldest months of January and February. Common Terns are present in New Jersey for only about half the year, mostly from late April to October with peak numbers in May and August. They nest on vegetated sandbars and sand islands adjacent to ocean inlets. A few nest in back-bay marshes but they are otherwise seldom encountered away from beaches or the ocean.

Now that we have a handle on probability, let’s begin the identification process by considering size and structure. Among the tern clan, Forster’s and Common both are medium-sized, smaller than Royal or Caspian, and larger than Least or Black. Forster’s are slightly larger than Commons, but the difference is often not obvious. Structurally, Forster’s Terns have bulkier heads, thicker necks, longer, thicker bills, and longer legs than Commons. Like a lot of structural characteristics, these may not be obvious at first, but after “getting your eye in” (getting used to looking for these differences) they can become very useful identification tools. While standing, in order to balance their heavier heads and bills, Forster’s Terns tend to either hold their necks more vertically or assume a more up-tilted pose than Common Terns. Common Terns, with their slimmer, lighter-weight heads and bills, have a much sleeker look and tend to stand more horizontally with the head merging seamlessly into the body. Common Tern’s head and body proportions, combined with their shorter legs, often make them look as if they are crouching lower than Forster’s. In flight, Forster’s Terns often have a front-heavy appearance with their larger heads and bills often seeming to protrude farther in front of the wings than on a Common (analogous to the more prominent head on a Cooper’s Hawk compared to a Sharp-shinned). Common Terns look comparatively slim in front with their center of gravity usually shifted to the mid-section.

Another feature that doesn’t change much through the year is voice. Forster’s Tern’s common call is a harsh, evenly descending “keeeer” or a quick series of similar notes. Common Tern’s call has more of a musical ring and is a clearly two-parted “keee-rrrrr” with the second part distinctly lower in pitch. Often, this call is introduced with a stuttering series of hard notes, like “k-k-k-keee-rrrrr”. Both species give a variety of short “kip” notes that are very similar though slightly squeakier in Common. (Note than in late summer, begging juveniles give calls very different from those of adults.)

When making plumage-based identifications, it is important to first figure out what plumage the bird is in. Fortunately, in late spring and early summer, most birds you encounter will be adults in breeding plumage. In both species, a quick look at the head pattern will tell you what you need to know: a fully black cap means a breeding plumage adult. The most useful field marks for distinguishing birds in breeding plumage are:

1. Breast/neck pattern: Forster’s Tern may be recognized by its clean white underparts and neck contrasting strongly with its gray back. Common Tern, on the other hand, has light gray underparts that blend seamlessly with its gray back.
2. Primaries & tail: Common Tern shows dark primaries that usually extend slightly beyond the relatively short tail streamers. Forster’s Tern shows long tail streamers that extend past the wing tips and pale primaries that contrast with darker wing coverts (note, however, that as the summer wears on, Forster’s primary tips will become gradually darker before being replaced by fresh pale feathers).
3. Cap pattern: A subtle feature that is useful with practice is cap pattern. Forster’s Tern has a slightly broader wedge of white between the bill and the cap than Common. Forster’s also has slightly more extensive black both behind the eye and immediately in front of it. Now this may seem like a ridiculously fine point but on those lazy summer days when terns are dozing off, the white lower eyelid of a Forster’s Tern invades the black cap and gives it a white-eyed look that may be obvious even from a great distance. Common Tern's cap, in comparison, usually has a more clean-cut look with only limited white invading the cap when the eye is closed.
4. Leg and bill color: The legs and bill of Forster’s tend to be a pure orange color whereas those of Common Tern tend to be a darker and brighter red to orange-red. (It should be noted that on both species, these colors change to black in the nonbreeding season.)

When watching these birds in flight, many of the same characters are still useful. With its white underparts and pale primaries, a Forster’s Tern looks very white with a softly contrasting gray back and inner wing. The tail often looks exceptionally long. When going away, grayish central tail feathers may contrast subtly with a white rump. Common Tern looks gray overall with a contrasting white rump and tail. The wing tips are at least slightly darker than the back and often the central primaries are darkest of all. With its shorter tail and smaller head, Common Tern has a more compact and well-proportioned look than Forster’s.

After becoming comfortable with breeding adults in early summer, you are ready to take the plunge into the array of other plumages. A glance at any field guide will show you how different the nonbreeding head patterns are. (Although beware that some Forster’s Terns have dusky napes.) Also notice nonbreeding Common Tern’s diagnostic black “carpal bar” on the leading edge of the inner wing (juveniles show this carpal bar, too). Subadult and transitional plumages are the most difficult to learn as they require an understanding of how molt and feather wear affect the appearance of a bird. Fortunately, in all cases, size, structure, and voice are always helpful.

1) Breeding adult Common (left) and Forster’s Terns, late April, Cape May Point. Common Terns are very slightly smaller than Forster’s but the difference is seldom obvious. What is more obvious is that Forster’s has longer legs and a broader, bulkier head than Common. From this angle, notice that the Common Tern looks darker and grayer, with dark wing tips. The Forster’s look paler, with frosty wing tips.
2) Breeding adult Common (top) and Forster’s Terns, mid-July, Cape May Point. Common Tern’s light gray underparts may be difficult to discern at times but notice how the color of the breast blends seamlessly with the gray back. The clean white underparts and neck of a Forster’s contrast starkly with its gray back. Due to its bulkier head, Forster’s Terns tend to hold their heads up more than Commons, often creating a corner at the back of the neck. The Forster’s frosty wing tips are obvious here, as are its long tail streamers extending beyond the primaries. Common’s tail streamers usually fall short of their darker primaries. Note the length and color of the bill and legs.
3) Breeding adult Common Tern, early July, Stone Harbor. Common Tern has a slim head and a slim bill that is usually bright red with a variable black tip. The cap looks neat with clean edges and a relatively thin sliver of white above the bill. Notice the seamless gradation from gray back to pale gray breast.
4) Breeding adult Forster’s Terns, late April, Cape May Point. Forster’s Terns have blocky heads with relatively heavy, dagger-like bills. Bill color is orange to orange-red with a variable black tip. The cap is often not so clean-edged as that of a Common Tern. It has a wider wedge of white over the bill and a bit more extensive black before and after the eye. As a result, when the eyelid is closed (as on the left bird), Forster’s has a distinctive white-eyed look. Notice the stark contrast between the gray back and white neck and breast.
5) Breeding adult Common Tern, mid-September, Cape May Point. In flight, Common Terns look grayish overall with contrasting white rumps and tails (thin gray outer webs to their tail feathers are seldom visible). Their smaller heads and shorter tails give them a more compact look than Forster’s. The wing tips usually look darker than the back, particularly late in the season when wear darkens these feathers. This bird is just beginning its molt to nonbreeding plumage at a date when most Common Terns have this molt well underway.
6) Nonbreeding Forster’s Tern, mid-September, Cape May Point. In flight, Forster’s Terns look mostly white with a contrasting gray back and inner wing (breeding and nonbreeding adults have similar wing patterns). Their bulky heads and bills often give them a front-heavy look. When the tail is spread, a subtle contrast between the white rump and pale gray tail may be visible. This bird is just completing its molt to nonbreeding plumage. The outermost primary, the last flight feather to be molted, is not yet fully grown. Notice this bird’s distinctive black ear patch.