Calidrid

The calidrids or typical waders are a group of Arctic-breeding, strongly migratory wading birds. These birds form huge mixed flocks on coasts and estuaries in winter. They are the typical "sandpipers", small to medium-sized, long-winged and relatively short-billed.

Their bills have sensitive tips which contain numerous Corpuscles of Herbst. This enables the birds to locate buried prey items, which they typically seek with restless running and probing.

As the common name "sandpiper" is shared by some calidrids with more distantly related birds such as the Actitis species, the term stint is preferred in Britain for the smaller species of this group.

Systematics and taxonomy
The calidrids' closest relatives are the two species of turnstone, and if the calidrids were to be considered one or two tribes Calidriini and/or Arenariini, and/or subfamily Eroliinae, the turnstones would be included in it. There exists a fossil bone, a distal piece of tarsometatarsus found in the Edson Beds of Sherman County, Kansas. Dating from the mid-Blancan some 4-3 million years ago, it appears to be from a calidriid somewhat similar to a Pectoral Sandpiper, but has some traits reminiscent of turnstones. Depending on which traits are apomorphic and plesiomorphic, it may be an ancestral representative of either lineage. It might also belong to some distinct prehistoric genus, as true calidriid sandpipers seem to have been present earlier (see below).

The interrelationships of the calidrid group are not altogether well resolved. Several former genera have been included in Calidris, such as the Stilt Sandpiper (previously Micropalama himantopus), but the new placement was also not entirely satisfactory. It was suggested, for example, that the Sanderling should be placed into a monotypic genus Crocethia, and the other small Calidris species separated as Erolia. Alternatively, it was suggested that the monotypic Aphriza, Limicola and Eurynorhynchus be also merged into Calidris.

A comprehensive analysis in 2004 –, based on newly available DNA sequence data – indicated that the extended Calidris is indeed paraphyletic (or polyphyletic if all calidrids are combined in it), but found the present DNA sequence data insufficient to resolve the relationships of some more unusual taxa such as the Curlew Sandpiper. In addition, it is known that the calidriid lineages are able to hybridize to a considerable extent and in the past, this was probably even more frequent and more hybrids would have been viable; therefore studies based on mtDNA data alone can be unreliable.

Still, three groups of close relatives emerge:
 * 1) The largest contains the smaller species, including the Sanderling, and probably also the Buff-breasted Sandpiper. If it is considered a distinct genus, the name Ereunetes would apply, first published in 1811. The Curlew Sandpiper might also belong here; it is the type species of Erolia, first published in 1816.
 * 2) The genus Calidris sensu stricto contains the knots and the surfbird.
 * 3) Another small group contains somewhat aberrant species, namely the Ruff, the Broad-billed Sandpiper, and the Sharp-tailed Sandpiper, which would use the name Philomachus.

Genera and species
The species, according to updated / traditional taxonomy, are as follows:


 * Genus Calidris sensu stricto - knots
 * Great Knot, Calidris tenuirostris
 * Red Knot, Calidris canutus

Other calidrids (all at some time placed in Calidris too)
 * Genus N.N. (Ereunetes?) - stints
 * Semipalmated Sandpiper, "Calidris" pusilla or Ereunetes pusillus
 * Little Stint, "Calidris" minuta
 * Least Sandpiper, "Calidris" minutilla
 * White-rumped Sandpiper, "Calidris" fuscicollis
 * Baird's Sandpiper, "Calidris" bairdii
 * Dunlin, "Calidris" alpina
 * Red-necked Stint, "Calidris" ruficollis
 * Long-toed Stint, "Calidris" subminuta
 * Pectoral Sandpiper, "Calidris" melanotos
 * Sanderling, "Calidris" alba or Crocethia alba
 * Western Sandpiper, "Calidris" mauri or Ereunetes mauri
 * Purple Sandpiper, "Calidris" maritima
 * Rock Sandpiper, "Calidris" ptilocnemis
 * Temminck's Stint, "Calidris" temminckii
 * Buff-breasted Sandpiper, "Tryngites" subruficollis
 * Genus Philomachus
 * Ruff, Philomachus pugnax
 * Broad-billed Sandpiper, Philomachus falcinellus / Limicola falcinellus ("Erolia falcinella")
 * Sharp-tailed Sandpiper, Philomachus acuminatus / Calidris acuminata
 * Genus Micropalama (doubtfully valid; Erolius s.str.?)
 * Stilt Sandpiper, Micropalama himantopus / Calidris himantopus
 * Genus Eurynorhynchus (doubtfully valid)
 * Spoon-billed Sandpiper, Eurynorhynchus pygmeus / Calidris pygmeus
 * incertae sedis (Erolia s.str.?)
 * Curlew Sandpiper, ?Calidris ferruginea

As mentioned above, there exists some material of birds essentially identical to calidriid sandpipers from before the Pleistocene. An undescribed species is known from the Early Miocene of Dolnice (Czechia). Tringa gracilis (Early Miocene of WC Europe) and Tringa minor (= Totanus minor, Erolia ennouchii) from the Middle Miocene of Grive-Saint-Alban (France) are scolopacids of rather uncertain affiliations; they might be charadriids.