When faced with a bird in the field, a birder first takes note of its key distinguishing features, or "field marks"— overall size and shape, bill structure, plumage (the markings on head and body)—and its actions. Many birds can be identified by the colors and patterns of their plumage alone, but plumages can be difficult to assess in the field. Distance, tricky lighting, individual variation, and damage from staining, oil, or simple wear and tear (old feathers often become sun-bleached and bedraggled) can lead to the absence or obscuring of field marks. Therefore, even before studying a bird’s plumage in detail, a birder takes note of its size and structure.
The structural features of the head and the size and shape of the body provide crucial information for the identification of many species. It is usually possible to "narrow down" the identity of a bird in the field to just a few possibilities by determining its overall proportions: Is the body slender or thickset? Are the neck and legs long or short? Are the wings short, broad, and rounded or long, narrow, and pointed? How many toes are on the foot? Oftentimes, the bird’s bill alone is enough to narrow down the identification: Is it thin or heavy? Long or short? Narrow or broad? Because size can be hard to judge in the field at any distance, even on a nearby bird, it can be useful to compare the size of the bird in question to that of birds around it, if possible.
Birds in Action
Experienced birders learn quickly to identify birds by their actions as well as their structure and plumage. A feeding Wilson’s Phalarope, for instance, can be picked out quickly by its habit of racing around wildly after flying insects, an action seen in few other shorebirds. Phoebes regularly dip or flare the tail more noticeably than do smaller flycatchers. Many species hold themselves in distinctive postures when feeding or resting, and these postures can be clues to identification. For example, Broad-tailed Hummingbirds often perch in a less upright position than Rufous Hummingbirds. Sometimes a change in posture can confuse an observer: a roosting owl may appear quite compact when undisturbed but stretch into an elongated "concealment posture" when an intruder approaches; Lesser and Greater Scaup usually sleek the plumage of the head when foraging, which eliminates the small corner at the rear of Lesser’s head, a key distinguishing feature of these very similar species.
Birds in flight
Flying birds, especially when seen from a distance, can be particularly difficult to identify. Although it can be relatively easy to identify large, slow-flying birds, passerines in flight are often quite challenging, especially for the beginning birder; in fact, some species of warbler and sparrow have never been photographed well in flight.
When you are beginning to identify birds in flight, first get to know the species well when they are not flying. Learn a bird’s overall shape and plumage characteristics when it is relatively still, then watch as it flies off. Once the bird is in flight, study its overall shape, including its various wing shapes, the cadence of its wingbeats, and the features of its plumage that show up clearly at a distance. Because these aspects of flying birds are difficult to convey in still photographs, they are best learned with patience and practice in the field. A few behavioral traits of flying birds are easy to spot; for example, a large, dark, crow-like bird flying in the distance can often be identified as a raven rather than a crow if it makes "somersaults" in midair.
As you observe the birds, get into the habit of taking good notes on what you see. Record sightings on your Audubon Bird Guides app, and keep a journal of notes on the app and on the Audubon Guides website. You can also upload photographs to your sightings records, and you can scan and upload field sketches. Keep notes on the AudubonGuide Bird app or on the AudubonGuide website, take and upload photographs, and refer to your sightings records often to build your knowledge. Good note-takers also find that they recall details more easily; the process of recording notes, photographing and sketching helps fix them in the memory.
Identifying Birds by Ear
One of the greatest pleasures, and challenges, of birding is learning to identify birds by their distinctive vocalizations. Birds give both songs and calls, and they also make sounds with the wing and tail feathers and the bill, particularly when courting. This program contains more than 2,200 songs and calls of a total of 550 North American Bird species.
Songs — Male birds use songs both to attract mates and to define and defend territories. They usually repeat a song often and sometimes couple it with a flight display; a few species sing mostly on the wing. In a number of species, including some wrens, cardinalids, and blackbirds, females also sing, usually a quieter or weaker version of the male’s song (although some females sing vigorously and some sing a very different song than the male’s). Most birds sing only during the breeding season and most intensely in the early morning. Songs of a species may vary both individually and regionally. Some species have enormous repertoires: Brown Thrashers sing more than 100 distinct song types. Many flycatchers and warblers have a secondary song, given mostly at predawn or dusk and presumably directed mainly at rival males.
Calls — Birds of all ages and sexes give many calls, short vocalizations used to maintain contact, beg for food, solicit courtship or copulation, warn of predators, or scold intruders in their territory. Some calls are distinctive and instantly recognizable, while others sound quite similar to calls of other species and are thus less useful for field identification. Most North American passerines "learn" their songs from their parents or others of their species. The vocalizations of flycatchers, by contrast, are innate (known from birth); their songs are sometimes very similar to their call series (this is especially true in the kingbirds).
The best way to learn vocalizations and other sounds birds make is to watch them court and sing, listen carefully to the birds and to the bird sound recordings in this program, and then write down your own notations or phonetic transliterations or humorous mnemonic jingles—something that will stay with you. A fellow birder once said that he never failed to recognize a Summer Tanager singing because it says, to his ear: "Peanut butter, peanut butter, peanut butter, your mama." To supplement field study of songs, many birders record birds with a sound recorder or video camera or listen to recordings of bird vocalizations on videos, compact discs, or web sites.
Limits of Field Identification
It is not possible to identify every bird in the field. Young Allen’s and Rufous Hummingbirds, for instance, are essentially identical, only distinguishable with the bird in hand. There are also individual birds that stump even proficient birders. These include birds that have unusually pigmented plumage, bills or legs that differ in color from most of their species, or broken or deformed bills. Innately aberrant birds are usually rare, except in shorebirds and gulls, two groups in which atypical individuals are often observed.
Some bird species show remarkable variety in plumages, particularly those that have many subspecies (although most subspecies of North American birds show only minor differences from each other). The subspecies of some birds show clinal differences, meaning that a change in one characteristic or another varies gradually over a broad geographic area. No field guide can depict all possible plumages of all North American birds—besides the many subspecies (this guide depicts some of the more distinctive ones) are countless individual birds that differ noticeably from typical individuals of their species or subspecies. (Anyone who has spent a day observing thousands of Herring Gulls can confirm this point.)
Hybrids — the offspring of two different species—are often impossible to identify with certainty in the field. Hybrid birds, of course, do not belong to a single species, nor do they always show characteristics that are clearly intermediate between the parent species. Across much of the continent hybrid birds are rather rare, but there are certain well-known zones of hybridization (such as the Pacific Northwest, the Canadian Rockies, and the Great Plains) where they are more common. To further complicate matters, hybrids often breed with non-hybrids, resulting in young called backcrosses. In such situations, identification is difficult or impossible, sometimes even with a study of DNA.
Vagrancy in Birds
A species’ known, expected geographical range is usually a useful clue in an identification, but the appearance of vagrants, birds far out of their typical range, complicates matters. Most vagrants reported in North America are birds that have strayed a few hundred miles out of range but that breed within North America; all such species are depicted in this guide. A small minority of vagrants come from Eurasia, South America, or Africa, and these species are usually not depicted.
The study of bird vagrancy is relatively new. Some theories suggest that vagrants have a faulty navigation system and confuse, for example, north and south, leading them to migrate northward instead of southward in the fall. Most such strays are long-distance migrants, often passerines or shorebirds that spend the winter in tropical regions of the world. Other vagrants have been blown off course by weather systems; for example, European Golden-Plovers returning to Iceland in spring sometimes make landfall in Newfoundland because of strong easterly winds. Some vagrants are transported to North America on large ships; on rare occasions hummingbirds, rails, and even owls have been found in large shipments of plants. Seabirds—especially the tubenoses, such as petrels and shearwaters—are particularly prone to wander far out of range; there are few barriers to their travel over the open ocean, and strong weather systems such as hurricanes can also displace them great distances.
In some cases, vagrants may be pioneering individuals or flocks of birds that are testing out new breeding or wintering areas; they may remain in areas well out of range and establish breeding populations or become part of a growing wintering population.
Reporting vagrants — Birders relish the sight of a new or unusual species in their area. Anyone might discover a vagrant species, sometimes just by looking out a window and seeing something unfamiliar. Vagrants are of special interest to ornithologists because they contribute to the collective picture of a species’ natural history, including range expansion, responses to weather and climate, and migratory patterns. All states and most provinces have bird records committees that keep track of, review, and archive reports of rare birds; such committees are grateful to receive well-documented reports of unusual species or subspecies. Upon finding an unusual-looking bird, then, it is a good idea to take notes as well as a video or photographs, in order to study the bird in detail afterward and so that others can review the notes and images.