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APPENDIX C: TAXONOMIC NOTES

Here we summarize reasons for treating various taxa as potentially separate species and mention

alternative taxonomic options for some species; these are marked in the species accounts by an asterisk

(*) preceding the English name and by use of brackets, for example, American Great Egret Ardea [alba]

egretta or Veraguas [Brown-throated] Parakeet Eupsittula [pertinax] ocularis, as explained on p. 30. The

term ‘group’ is used for one or more subspecies that form a potential species; group names refer to the

chronologically first-named taxon in a group. Further explanation of this subject was provided by Howell

& Dyer (2022).

In some cases, differences (as in plumage, morphology, vocalizations, ecology) are so clear that there

is no question different species are involved (e.g., Cayenne Lapwing, Salvadoran Flycatcher); in other

cases more work is needed, and our treatment is intended to draw attention to such cases in the hope

of promoting careful study. Our baseline taxonomy is that of IOC (Gill et al. 2021), except for oceanic

birds, for which we follow Howell & Zufelt (2019), namely for the Wedge-rumped Storm Petrel, Brown

Booby, White-tailed Tropicbird, Bridled Tern, and Brown Noddy complexes. In cases where our opinions

differ from or expand upon IOC (there are several IOC splits we find unconvincing), the following notes

summarize our reasons.

We have attempted to consider taxonomy within Middle America and between Central America and

at least adjacent South America. Taxonomy across all of South America, however, is beyond our purview,

although in some cases it is clear that Central American taxa are distinct. Thus we may break species into

Central American or Middle American taxa and ‘all the rest,’ including potentially multiple species in

South America that we have not had the resources to investigate, although groups or English names may

be suggested for some South American taxa.

Potential species-level differences for species in which all groups occur in Costa Rica are summarized

in the species accounts and not expanded upon below, namely for Black Skimmer (p. 58), White-

throated Crake (p. 92), Gray-chested Dove (p. 168), Scaly-breasted Hummingbird (p. 216), Volcano

Hummingbird (p. 224), Olive-sided Flycatcher (p. 306), Green Shrike-Vireo (p. 348), Wilson’s Warbler

(p. 366), and Ruddy-breasted Seedeater (p. 392).

Depending on available data and philosophy, our opinions vary for different taxa, grading from “suggest

that A and B may represent separate species” to “indicate that A and B are separate species.” Probably

every one of these cases represents a phylogenetic ‘species’ or lineage, according to some species concept

(Howell 2021), as do others not mentioned here. We only include taxa for which we have biological

support for possible species status (rather than simply uninformed molecular data), but undoubtedly we

have still underestimated the number of candidates.

BLACK-BELLIED WHISTLING DUCK  Dendrocygna autumnalis (p. 34). Differences in plumage and

voice suggest that the fulgens group (N America to w. Panama; Northern Black-bellied Whistling Duck)

and autumnalis group (Panama to S America, Southern Black-bellied Whistling Duck) may represent

separate species. Plumage appears to intergrade from s. Costa Rica to cen. Panama, but study is needed.

SMITHSONIAN GULL  Larus [argentatus] smithsonianus (p. 52). Molecular work and juvenile plumages

indicate that N American populations of ‘Herring Gull’ are distinct from European populations and are

more closely related to N Pacific taxa such as Glaucous-winged Gull L. glaucescens, as summarized by

Olson & Banks (2007). As proposed by those authors, the English name ‘Smithsonian Gull’ removes

misleading association with ‘the Herring Gull’ of Europe and we prefer this option over American

Herring Gull, used by other authors such as IOC.

SANDWICH TERN  Thalasseus sandvicensis (p. 54). Molecular work suggests that ‘American Sandwich

Tern’ is more closely related to Elegant Tern than to ‘European Sandwich Tern,’ and some authors

(including IOC) split N American populations as Cabot’s Tern T. acuflavidus. We find the molecular

studies weak, and unconvincing as evidence for biological species status (Howell 2021); either way,

retaining simply ‘Sandwich Tern’ for one of the taxa (as done for the European birds) is ambiguous and

needlessly confusing.

AMERICAN BLACK TERN  Chlidonias [niger] surinamensis (p. 58). Differences in plumage and

provisionally in voice suggest that the surinamensis group (New World) and niger group (Old World;

Eurasian Black Tern) represent separate species.