Sterna identification, particularly separation of Common (S. hirundo) and Forster's (S. forsteri) terns, is certainly a challenging, even intimidating, Louisiana identification problem, especially for beginning and intermediate-level birders. Anyone visiting Louisiana beaches during April through July has probably noted the similarity of non-definitive plumages of Common and Forster's terns. This is also the best time to search for a stray Arctic (S. paradisaea) or Roseate (S. dougallii) tern. But before we can take on the rarities, we need to thoroughly understand the range of plumage variation of our two common species.

Status

Forster's Tern is a year-round Louisiana resident, with numbers augmented during migration by north- and south-bound migrants. Forster's Terns may be encountered commonly during the non-breeding season along the entire coast and less commonly on inland bodies of water. Birds move during early April to Louisiana breeding colonies located in the coastal marshes and on barrier islands. As early as late June, post-breeding adults disperse to coastal beaches, often accompanied by dependent juveniles. During the late spring and summer, varying numbers of non-breeders, primarily one- and two-year-olds, remain along Louisiana's beaches.
 
Common Tern is a common migrant. Large numbers of north-bound migrants appear on Louisiana's beaches during April through early June. Fall migrants appear as early as mid-August and numbers peak in early September. Juveniles appear as early as late August. Spring and fall migrants are regularly encountered well offshore. Although certainly regular, Common Tern is rarely detected as a migrant in the interior. Numbers of summering non-breeders vary from year to year, and during a "good year" hundreds of individuals may be present on a single stretch of beach. Summering birds consist primarily of one- and two-year-olds that have only migrated as far north as the Gulf Coast. Many of these birds remain on our beaches through the summer. A few Common Terns remain as late as mid-December and some of these occasionally remain through the entire winter. Most late fall or winter records involve sub-adults (most records are from the coast of Cameron Parish). There are two breeding records from the Chandeleur Islands.
 
Arctic Tern is a vagrant to Louisiana. There are five records, all from onshore in Cameron Par. between 30 May and 23 June. These records are thought to pertain to north-bound migrants that have been accidentally "trapped" in the Gulf of Mexico. This species should be a possibility on late spring pelagics or on inland bodies of water. Arctic Tern is not expected in the fall, as southbound migrants generally move south through the eastern Atlantic Ocean; there are few fall records from along the Atlantic coast south of breeding areas.
 
There are no accepted Louisiana records of Roseate Tern. The closest colonies with any substantial numbers are in the West Indies. Roseate Terns also breed on the north Atlantic coast of North America. Both Caribbean and western N. Atlantic birds are presumed to winter along the coast of South America from Colombia south to Brazil. Be prepared to document this species' occurrence with a multitude of photographs or voice-recordings!
 

Some identification pitfalls:

Missing information
Unfortunately, field guides often underestimate the difficulty of identifying this species group. Space constraints usually limit the discussion to "obvious field marks" of birds in definitive plumages. Reliance on "obvious" marks, though certainly important for identification of birds in definitive plumages, can also lead you astray. Some important identification information is simply missing from guides. For example, none of the basic field guides mention that some Sandwich Terns, like Roseate Terns, can develop a fairly bright pink "blush" during the breeding season. Some Common Terns can also have a pinkish or mauve cast to their underparts.
 
Field marks that can pertain to more than one species
Forster's Tern is typically illustrated with light silvery gray primaries that are the same shade or lighter than the rest of the wing and mantle. This is an excellent field mark for Forster's in fresh plumage. But worn, the same primaries are nearly black and contrast with the lighter mantle; blackish primaries that are darker than the rest of the wing and mantle is a field mark often cited for Common Tern. Forster's Tern has an obvious dark eye patch in basic plumage that is very different than the dark-capped Common, Arctic, and Roseate terns, so a bird with an eye patch is a Forster's. But a bird with a cap (and lacking an obvious eye patch) is not necessarily one of the other three species; Forster's Terns with head molt or very worn feathers (feather bases are dark gray) may appear similarly capped (even in early to mid-winter). Many reports of winter "Common" terns are probably actually Forster's wrongly identified by this single character or in combination with black primaries. Common and Arctic terns have gray underparts. Although never as dark as the previous two species, many Forster's Terns also show gray underparts. Adult Arctic Terns have a well-defined white cheek patch in alternate plumage. Few references note that many Common Terns in full alternate plumage can also appear white-cheeked. The width of the blackish trailing wedge to the underside of the primaries is an excellent mark differentiating Common (also shared by Forster's) from Arctic terns, if you have experience with both species – otherwise it is a subjective character. Beware of using the wing/tail length ratio as a field mark for species recognition; the length of the tail is dependent on the bird's age and stage of molt or condition of the feathers. Bill and leg colors are often stated as being one color or another. Though often helpful, the colors indicated in some guides would certainly lead you astray.
 
Use of "gizz" requires experience and good viewing conditions
Leg length is a good mark separating Forster's from Common and Common from Arctic terns. But beware, young birds have proportionately shorter legs than adults and there is also some sexual size dimorphism; males are somewhat larger and taller than females. Occasionally encountered are injured, or even one-footed birds; these individuals can appear really short-legged as they lean to compensate for the injury. Flight and resting "gizz," often cited for use in species recognition, are subjective characters that can only be helpful to experienced observers. Common Tern typically has a deep red bill with varying amounts of black/dark brown along the culmen (top of the upper mandible) and on the tip (rarely all red). From a distance the black may "disappear" giving the illusion of a smaller, all red bill. Reliance on this character, a mark for Arctic, could certainly lead you astray if the bill was not closely scrutinized.
 
Voice
Beware of English translations of "distinctive" call notes. Sometimes the translations "fit" perfectly, but more often than not, if you had to come up with your own description, it would be very different! Using your field guide's translation to interpret a particular call as the "distinctive" kee-ar-r-r-r of the Common Tern versus the hoarse kyarr of the Forster's is an exercise in futility. These translations are provided to help you key into vocalization differences that will assist species identification when you have gained sufficient experience with the calls of the different species. The best way to learn vocalizations, is to develop your own translations of the various calls.
 
Distribution
Often, field guides will make generalizations with regards preferred habitat or migration route. For instance, Arctic Tern "migrates well offshore" versus "along the coast" for Common Tern. Thus far, with only limited coverage offshore Louisiana, Common Tern is a regular migrant and the expected Sterna well offshore.

Identification basics

Good basic information regarding tern topography and basic plumage information is contained in TERNS of Europe and North America by Klaus Malling Olsen and Hans Larsson. The illustrations and photos are also particularly helpful. We recommend adding this book to your birding library, as it covers all North American species and will complement the more abbreviated tern accounts contained in National Geographic Field Guide to the Birds of North America and Seabirds, an Identification Guide by Peter Harrison. Advanced Birding by Kenn Kaufann also takes a stab at ID of this group, but focuses only on adult and juvenal plumages.
 
So, what makes identification of Forster's, Common, Arctic, and Roseate terns so difficult? Besides being all "medium-sized," these terns possess extremely similar plumage types and patterns; field marks are relatively subtle. There are very few single characters that are diagnostic. Like their relatives the gulls, terns have a distinct definitive alternate (adult breeding) and definitive basic (adult winter) plumages. There is also a distinctive juvenal plumage. Further, these species do not attain their definitive alternate plumage until their third spring, so first and second alternate birds show a range of plumage characteristics between basic and definitive alternate plumage. Many field guides do not mention or caution observers about subadults or describe these plumages. There are a couple of important plumage "basics" that need review. These are not mentioned or are "glossed-over" in the standard field guides.
 
1). Feather wear affects the actual color of the flight feathers, especially the outer primaries. When tern primaries are new, the upper surface is coated with a powdery bloom. The powdery surface is silvery white or light silvery gray. The bloom is difficult to appreciate in the field, but easy to observe if a primary feather is found on the beach. As a primary ages, the powder wears off. As an example, a primary feather of a Forster's Tern may be nearly white or nearly black depending on the age and rate of wear of the feather.
 
2). The color of each primary feather is essentially two-tone. The outer web (or leading edge) and, to a varying extent (depending on the species), across the white shaft and onto the inner web is dark gray to blackish-gray. This forms a dark stripe along the leading edge of primaries 4-9; the remainder of the inner web (or trailing edge) is white or very light gray. The narrow outer web of primary 10 is nearly black on most species regardless of state of wear. The two-tone effect of inner primaries (1-3) is not so dramatic. These two-tone feathers gives the wing a very different appearance from above and below. The primaries overlap so that the dark leading edges are visible on the top of the wing. In flight, this gives the primaries an overall gray (or black, if worn) appearance. In contrast, the light inner webs are visible from the underside of the wing, making the primaries appear white or very light. The degree of translucence, a field mark mentioned in many guides, is determined by the amount of gray vs. white on the individual feather. The more gray that crosses over the shaft onto the inner web, the less light that is able to penetrate through the overlapped primaries in flight, and therefore, the less amount of translucence.
 
3). The pattern of the primary tips is another feature influencing the appearance of the underwing in flight. On some species, the dark gray of the outer web hooks around the tip of the outer primaries (usually 4-10). This feature is responsible for the dark trailing edge observed on the underwing of Forster's, Common, and Arctic terns. On Arctic Tern the gray border is crisp and narrow. On Roseate Tern, the dark gray extends out to the tip of the primary but does not hook around so there is no dark trailing edge on the underwing.
 
4). "Step" or "arrested" molt is a specific type of wing (primary and secondary) molt employed by many Sterna species. This molt differs from the more commonly observed pattern (primaries and secondaries are all replaced in a sequential pattern over a single period of time) because molt ceases for a period, then later resumes. The delay results in the presence of two (or more) sets of feathers of different ages at the same time. Primary molt begins on the breeding grounds, usually when the adults begin feeding their young . The first three to five (inner) primaries are sequentially replaced over the course of many weeks. When fall migration begins, active molt stops or is "arrested." Adult birds seen during fall migration have two obvious sets of primaries. The outer primaries (which were replaced during the previous winter) are worn and blackish-gray and contrast with the new light gray inner primaries. When the bird arrives on the wintering grounds, primary molt resumes where it left off, at which time the very worn outer primaries are replaced. When primary molt is complete or nearly complete, often the inner primaries begin a new molt replacing the first three to five primaries. This molt then ceases prior to spring migration. Thus, some of the primaries are actually molted twice/year, and adult birds observed during spring migration may have three generations of primaries. In flight, the difference in primary generation age on the upper wing surface ("pre-spring migration" inners, older "last summer's" middle set, and the "winter's" outers) creates the dark "primary wedge." Although the wedge is a field mark often cited for Common Tern, Forster's and Roseate terns also share this type of molt and this pattern. Within each species the molt schedule may be modified depending on timing of breeding or distance of breeding colony from wintering grounds (if wintering grounds are only a short distance from the breeding colonies, then primary molt may be more or less continuous as in the case of Louisiana Forster's Terns). There may be individual variation in timing and even number of primary feathers replaced. Sub-adults have a somewhat different schedule. Without the constraints imposed by migration/breeding activities, birds start or complete primary molt much earlier than breeding adults. Arctic Tern does not have a step molt. Primary molt begins and is completed on the wintering grounds; there is usually no pre-migration molt of the inner primaries. Thus, spring Arctics, compared to the other species, show consistently uniform and relatively fresh primaries.
 
5). The pattern and color of the tail feathers, if well seen, is a reliable mark for differentiating Forster's and Roseate terns from Common and Arctic terns. The tail pattern is best observed when the tail is slightly spread and viewed from below. Both Common and Arctic terns have a gray outer webs that contrast with white inner webs, creating a border to the outside of the fork. Forster's Tern has the opposing pattern and the inner webs are gray the creating a dark border to the inside of the fork. The tail feathers of Roseate Tern are wholly grayish-white or white.

Helpful hints for species identification

Don't shy away from those large mixed-tern flocks – use each opportunity to study the common species. The more you know the expected species the easier it will be to spot something truly different rather than be fooled by an oddball plumage or individual. Although separating Forster's from Common terns in definitive alternate or basic plumages is relatively straightforward, sub-adults can be extremely tricky even for experienced birders. November to mid-March is an excellent time to study Forster's Terns, as it is generally the only mid-sized tern that winters in Louisiana. The full range of variation in basic plumage can be studied, and many have acquired full alternate plumage by March. Important features to note are the condition of the primaries (basic adults with gleaming silvery white primaries or sub-adults with wing molt, mixed old and new, or all old and worn primaries) and extent of darkness on the cap. It is also a good time to study the range of structural characteristics within this species (size, bill shape, leg length, etc.). The most challenging time period, but also the most productive, is during the spring and summer when there is a higher percentage of sub-adult Forster's and Common terns to sort through. During late April through July, visit the beaches to study these more challenging individuals, some of which can stump even the most experienced birders (at least temporarily)! Our best spots for summering Sterna are Fourchon Beach, and Rutherford, Holly, and Johnsons Bayou beaches in Cameron Parish.
 
It can generally be assumed that the only medium-sized terns represented in Louisiana flocks will be Common or Forster's. Abundance and ratios of individuals per species can vary during the spring and summer on a particular beach or day. It is difficult to predict which species is most expected or would be present in the greatest numbers during this period; certainly, Commons can sometimes be the dominant species. When you encounter a group of medium-sized terns on the beach during the late spring or summer, pick out one or more individuals that can be confidently identified to species. Use these birds to compare to others in the same flock. The best approach is to get as close to the flock as possible (on some beaches that are safe to drive, your vehicle provides an excellent blind) and look for individuals that possess reliable field marks. If it is not possible to drive up to the flock, slowly walk towards the group, stopping and scanning until you have reached a sufficiently close distance. Telescope views are recommended for observing detail.
 
Forster's Tern
Though it is usually best to avoid the "single field mark strategy," occasionally it does come in handy, especially for a group such as medium-sized Sterna that have subtle structural differences and overlapping characters. One reliable mark is the well-defined dark patch extending from just in front of the eye over the ear coverts of a "basic-plumaged" Forster's Tern. (Note: Gull-billed Tern (S. nilotica) may show a similar patch, but is distinguished by its much thicker bill, paler mantle, and somewhat larger size). Birds in juvenal, basic and first alternate plumages possess this feature making identification straightforward. First alternate Forster's are often present in spring-summer flocks. Although, some first alternates will appear more "capped" (and have gained a few more black feathers on the back of the crown/nape during the pre-alternate molt) the eye patch is still the dominant dfeature. Then, once definite Forster's are located, study the overall size and proportions, especially leg length, and bill shape (slightly curved culmen), length, and depth. Leg color of first alternates is usually orange, but range to brownish- or even blackish-orange . The primaries on a first alternate Forster's are typically very worn and black in appearance. Active molt may or may not be in progress, depending on the date/individual. Forster's Terns in early April may already possess fresh grayish-white inner (1-3) primaries that contrast with the worn outers. First alternates may also be in active primary molt; watch for a gap in the primaries. Occasionally, a few definitive alternate-plumaged individuals remain on the beaches during the late spring, though "field guide" perfect individuals are fairly rare. Definitive adults generally have only slightly worn primaries by late April. The primaries usually are only slightly darker than the mantle; adults generally retain the fairly light frosty look typical of fresh basic plumage. Most difficult are second alternate-plumaged individuals that have fully or nearly black caps, bi-colored bills, and blackish-gray primaries; these birds are most easily confused with Common Tern. Look for overall size, bill shape and leg length. The bill and leg color of second alternates is generally orange or rarely orange-red.
 
Common Tern
Definitive alternate Common Terns stand out from Forster's and have a deep dark red–based (rarely all red or red with a paler tip) bill, darker gray underparts, and proportionately shorter legs. No Forster's will ever show this combination of characters. Definitive alternate individuals are common on the beaches until mid-April, numbers begin to dwindle through the month. Occasionally, birds in this plumage remain until early June and have even been observed performing courtship displays over Cameron Parish beaches. After late April, most of the birds encountered are in first and second alternate plumage. First alternate Common Terns resemble basic plumage and are immediately distinguishable from Forster's by the dark cap/nape and a pronounced blackish gray carpal bar at the shoulder. Second summer Common Terns are probably the most difficult, especially those individuals that are not very gray below. The carpal bar is usually reduced and the their overall appearance is similar to a Forster's. These individuals tend to show a more red than orange-red (or orange)-based bill. The bill is somewhat smaller, and straighter than a Forster's. The legs also tend to be more red versus orange, and again, compared to a Forster's Tern, look relatively short-legged. The shorter, straighter bill and especially shorter legs further should resolve identification of problematic individuals. Primaries of first and second alternates may be very worn and black or relatively new and gray; some individuals are only completing molt of the outer primaries in late May through July. Remember, the blackish-gray color of the primaries will not distinguish a Common from a Forster's.
 
Once you feel comfortable with the identification of at least a few individuals, watch the birds of known ID in flight. Note body proportions. Common and Forster's have generally different flight aspects. Common Terns have "less head" projection and the wing beats are deeper. Common Terns appear whiter-rumped. The white rump is more prominent on birds with gray underparts and white wing linings. Remember, the pattern of dark on the tail feathers is a diagnostic mark to separate these two species (if all else fails) and can be observed with patience, especially when the bird is in flight above the observer.
 
Vagrants
Once you have "mastered" Forster's and Common terns, you will be better prepared to detect unusual species in the flocks– but make sure you have your camera handy because you wouldn't want to miss the opportunity to provide hard documentation of a vagrant. Obviously, the best time to look for Arctic Tern is during the date span established by the existing records. The best visual cue for spotting an Arctic among a flock of terns standing on the beach is leg length. Of course, not all short-legged terns will turn out to be Arctics (remember, there are only 5 records, so your chances of finding one is not good)! Compare proportions to other birds in the flock, Arctic Terns look almost "legless." Arctics seem to "shuffle" rather than walk. Plumage-wise, alternate Arctic Tern is most similar to Common. Compared to a Common, an Arctic Tern will also stand out to an experienced observer by its more attenuated body shape (more body "behind the shoulder than in front"), more rounded head, and smaller bill. During the spring and early summer, Arctic Tern will have uniformly fairly fresh primaries; this is in contrast to most Common Terns in corresponding ages. In all plumages, the dark border to the primaries on the underwing is more narrow and crisp in Arctic compared to Forster's or Common terns. Compared to these species in flight, Arctic appears longer-and narrower-winged and more graceful. Sub-adult Arctic Terns are more tricky but retain the distinctive underwing pattern. The secondaries of birds in juvenal plumage are paler gray than the wing coverts/mantle. Each secondary is broadly edged white, as well as the inner primaries. This results in a wing pattern suggestive of a Sabine's Gull; Common Tern lacks this pattern. First and second alternates typically show adult-type secondaries and inner primaries. Photographs should emphasize the body proportions with respect to the other species, as well as the underwing pattern in flight, if possible.
 
An alternate adult Roseate Tern should "stand out" – it is pale-mantled (even paler than a fresh Forster's) often with a pink cast to the underparts , an essentially all black bill (red-based in mid-summer to early fall), and very long all white tail. In other plumages, general structure will provide cues: slightly more slender in overall proportions compared to a Common, with proportionately shorter wings and longer, slenderer bill, and longer legs. In "basic-type" plumages, Roseate shares the dark cap with Common, but will always be lighter-mantled. The lack of a dark trailing edge to the underside of the primaries in flight and essentially all white tail feathers are diagnostic; these marks should get top priority if photographs are possible. The calls of Roseate Tern are very different from the other three species. A good tape recording should provide documentation of its occurrence in the absence of, or to complement, photographs. At a great distance, beware of Sandwich Tern which may superficially resemble Roseate in general shape and coloration, but greater size and yellow-tipped bill are obvious on closer inspection. Sandwich also shares similar rasping call. Probably best time to look for Roseate Tern is mid- to late-April.
 
Donna L. Dittmann and Steven W. Cardiff
435 Pecan Drive, St. Gabriel, LA 70776