White-faced Plover

Frontiers of Taxonomy: Does the White-faced Plover Qualify as a Separate Species?

The White-faced Plover presents a taxonomic conundrum.  In appearance and ecology, it is readily distinguishable from Kentish Plover, but genetic analysis indicates that their divergence is too recent to support treatment as separate species.  The question is whether their phenotypic differences outweigh their genetic similarity.  Researchers who have studied the question closely—including the ones who performed the genetic analysis—have recommended full-species status for White-faced Plover, and both the IUCN and the Handbook of the Birds of the World Alive have accepted it.  Some taxonomic authorities either disagree or await a broader consensus, but the trend is toward acceptance of the reclassification.

The Handbook bases its decision on the following features that differentiate White-faced from Kentish: “larger size, including thicker, blunter-tipped bill; white (not black) lores; more extensively white forehead, with black of crown usually reduced; brighter and paler upperparts; much-reduced black lateral breast patches; rufous-brown of crown not extending onto nape sometimes to divide hindcollar; more white in wingbar; also ecologically, occupying generally sandier, less muddy substrates.”

The story of the White-faced Plover is confused by a history of misidentification, as it belongs to a group of sand-plovers that closely resemble one another, with varying plumages that overlap in many features, and often winter in mixed flocks.  In winter, the season of most observations, White-faced occurs together with typical Kentish and another similar species, the Malaysian Plover.  Most individuals observed closely are assignable to one or the other, but there is an apparent spectrum of some potentially distinguishing features.  As a result, many of the collected specimens were mislabeled and most old records (and some recent ones) are unreliable.

The pioneer of Asian ornithology, Robert Swinhoe, originally discovered and described the White-faced Plover in 1870, naming it Aegialites dealbatus.  However, many subsequent specimens labeled as dealbatus were in fact representatives of the more-northerly-breeding Kentish Plover populations that winter with it, and thus the distinction was lost in inconsistent labeling and thereby obscured from the people who wrote the relevant modern reference books.

This situation held until the shorebird researchers Kennerley and Bakewell noticed that a few of the small plovers wintering on southeast Asian beaches seemed to have features of both Kentish and Malaysian Plovers and some that were unique to it—most noticeably the extent of white coloration on its face.  They published their observations in 2008 and brought more widespread attention to the White-faced Plover.  They concluded “that the White-faced Plover is probably a young lineage whose phenotypic traits are encoded by a limited number of genes, whereas few additional genomic differences have so far accumulated.  Its diagnostic plumage traits may additionally be governed by differences in gene expression that would be undetectable by sequence analysis.”

A broad-based analysis published in 2011 confirmed that White-faced and Kentish are too closely related to show meaningful differentiation of the genetic markers that are usually used to assess relatedness—on these markers, White-faced clusters with Kentish, separate from their other closest relatives.  But that paper went on to recommend separate-species classification on the strength of their differences in appearance.  Considering that this group of plovers generally shows little observable differentiation even between forms that unambiguously qualify as separate species, the “local standard” should be to value the outwardly obvious differentiation of White-faced Plover.