General remarks on the identification of waders (shorebirds)
Especially to the untrained eye, but in some cases also to experts, many species of waders look alike. Differences between species can be very subtle, many species have different breeding and non-breeding plumages and thereby varying characteristics during different seasons, and birdwatchers may not have much experience with shorebirds. Because of these difficulties, and also because shorebirds are quite different from birds of bushland or urban surrounds, a specific set of criteria needs to be used to correctly identify waders.
The summary provided here is mostly based on information from the excellent "The Shorebird Guide" by M. O'Brien, R. Crossley and K. Karlson (1. edition, 2006), which provides very detailed identification advice based on photography demonstrating many different stages in the development of the birds' plumage. Although referring to shorebirds of North America, because of the extreme mobility of many species of waders, this book provides a wealth of information that also applies to birds found on other continents.
Plumage - morphology (colour pattern)
The morphology (colour pattern; overall appearance) is usually the only criterion one will need to consider to identify almost any species of, primarily land-based, bird. The number of species for which secondary criteria (such as the shape of the tail in case of birds of prey ["raptors"]) need to be employed is quite limited.
With many species of waders this will not get one very far. Here, as examples of such ambiguity, we show a number of waders that are very hard, if not impossible, to tell apart just based on their morphologies (in particular outside the breeding season, but also when taking into account that varying light conditions may make colours appear to be different).
Size, shape and colour of the bill
NB: The apparent length of a bird's bill depends on the angle at which the bird is observed ("projection effect")! Because of this we compare lateral views here.
Length compared to the size of the head
The length of a bird's bill in absolute units (e.g. mm) is not always a practical measure, because usually a bird's distance during an observation is not well known. However, the length of the bill compared to the size of the head is a good indicator that works at any distance over which one can see enough detail to note differences or make comparisons.
The practical case where this criterion can be used most effectively
is the distinction between different members of the genus Numenius
and also in the distinction between male and female birds of these
Little Curlew: ca. 1.5x (bill length in units of size of head)
Male Whimbrel: ca. 2x
Female Whimbrel: ca. 2.5x
Male Eastern Curlew: ca. 4x
Female Eastern Curlew: ca. 5x
Strength/thickness of bill
Some species have similar non-breeding plumage, bill length and
other properties. In such cases a study of the strength
(thickness) of the bill may help.
Marsh Sandpiper: needle-like bill;
Common Greenshank: strong bill
Shape/curvature of the bill
Not only the length or strength, but also the shape, in particular the
curvature, of the bill can assist in the identification of a bird
Terek Sandpiper: up-curved bill
Common Sandpiper: straight bill
Curlew Sandpiper: down-curved bill
Colour of the bill
This criterion is more often used for seabirds than waders, but it
is mentioned here because in some cases it can also help with wader
IDs. In many species of seabirds the colour of the bill changes
between breeding and non-breeding season.
Greater Crested Tern: yellow to olive-yellow bill
Lesser Crested Tern: yellow-orange to orange-red bill
Legs and feet
Length compared to the size of the body
Some waders have longer or shorter legs than others, compared to
the size of the body, which is an easy criterion for distinction
between species (it would have been more suggestive to use dorsal
views of both species, which would be very similar, but we do not
at present have suitable photos; hence frontal views, in which
differences in the
plumages can be seen):
Oriental Plover: Long (upper) legs
Greater Sand Plover: short (upper) legs
Some species have similar non-breeding plumage, bill and
other properties, but possibly differently coloured
legs and feet:
Terek Sandpiper: yellow legs and feet
Marsh Sandpiper: olive-grey legs and feet
Number and/or arrangement of toes
If all of the above criteria regarding legs and feet fail, and
if one can get a close-up view of the species in question, the
number and/or arrangement of the toes can provide a clue. In
the case presented here, there is a difference in the number
Red-necked Stint: has hind toes
Sanderling: no hind toes
Size of the body
The size of the body can also tell the difference between bird species.
If not seen simultaneously, and preferably at the same distance,
comparisons between the sizes of different birds can be challenging.
Therefore, size is often not a good distinguishing criterion. Example:
Lesser Sand Plover (left): small
Greater Sand Plover (centre): larger
Direct comparison (right): Greater Sand Plover, front left, Lesser Sand Plover, far right
While in non-breeding plumage, the distinction between these two species is usually made by the shape of the bill (see above): The Greater Sand Plover has a thinner, longer bill than the Lesser Sand Plover.
Shape/thickness of the body
The shape or thickness of the body is often better suited to tell the difference between bird species than the size. Unfortunately, we currently do not have photos to illustrate this.
Length of the neck
Two species of waders can sometimes also be separated by the length
of the neck (note that similar postures must be compared; not one
bird while it is hunched with another that is craning its neck):
Terek Sandpiper: short neck
Marsh Sandpiper: long neck
The relative length of the primary flight feathers on a bird's folded wings compared to the length of the tail is another good criterion by which species can be distinguished. For example, to distinguish between a Common Tern and an Arctic Tern in a lateral view, it is helpful to know that the Common Tern has short tail streamers, so the tips of the wings will protrude, whereas an Arctic Tern has long tail streamers, which extend beyond the primaries. At this point in time we do not have suitable photos to illustrate this.
Primary wing projection
Similarly, one can make a distinction between some species by studying the primary (wing) projection, i.e. how far the primary flight feathers stick out beyond the end of the secondaries when the wings are folded. Again, at this point in time we do not have suitable photos of waders to show this.
However, those interested can see the technique used in a different context, namely in the distinction between 3 species of "Old World Warblers" of the genus Phylloscopus seen in Europe, HERE.
Differences in flight style are usually most distinctive in some species of seabirds, such as different species of shearwaters. The way in which they skim waves alone can tell an expert the identity of a species. This criterion is very difficult to demonstrate in the form of photos, because one will normally need to see videos of different species' flight patterns for comparison.
Non-breeding adults vs. immature birds
Tidiness/crispness of plumage
When one observes young birds, in particular of species that
take only one year to mature (and thus moult into their adult
thing is for certain: They have only just grown their feathers.
Everything is crisp and new.
Adult birds in "eclipse" (or non-breeding plumage) often look similar to juvenile/immature birds. However, their feathers may already be months old, may have served during an extremely long migration, and can therefore look the worse for wear and tear. Even if wear is not readily visible, they will certainly not be as crisp and neat as a young bird's.
In the example below, apart from the browner tints of the young Sharp-tailed Sandpiper (on the right), it also very clearly has a neater appearance, with every feather (in particular the wing coverts) exactly where it is supposed to be. The non-breeding adult (on the left), on the other hand, looks much more ragged. If there was no colour difference between the two (as is the case for some species), THAT would potentially be the only distinguishing criterion:
Adult in eclipse (left): Ragged old plumage
Immature (right): Neat, tidy, crisp new plumage