In most of Central America, the above question does not come up very often, because the two species show no range overlap: Gray Hawk (Buteo plagiatus) occurs from the southwestern US to much of Costa Rica, while Gray-lined Hawk (Buteo nitidus) is found from southern Costa Rica to northern Argentina. In other words, practically the only country in the world where this question could come up is Costa Rica, because both species occur there, albeit in different parts of the country. [Update: As Jan Axel Cubilla pointed out to me, there is a well-documented January 2016 record of Gray Hawk from Bocas del Toro (Panama), a first country record. While Gray-lined Hawk does not occur in that part of Panama, birders there are advised to be careful separating the two.]
Despite the fact that there is no range overlap, at eBird Costa Rica we’ve seen many reports of Gray-lined Hawks from areas where only Gray Hawk is expected, and also some reports of Gray Hawks where presumably only Gray-lined Hawk occurs. Until 2012, these two species were considered one. Since the split is fairly recent, most field guides do not differentiate between the two, and observers may not have easy access to information pertinent to the split. (Note that the second edition of the Garrigues & Dean field guide to the birds of Costa Rica covers the two species well.) This article specifically aims to aid birders in Costa Rica, many of whom may be visiting and perhaps not as familiar with the local avifauna, to sort out the differences.
When Gray-lined Hawk was split from Gray Hawk in 2012, based on morphological, vocal and genetic differences described in a 2011 article in The Condor, the northern population retained the common name of Gray Hawk, but received a new scientific name – Buteo plagiatus – while the southern population received a new common name – Gray-lined Hawk – keeping the scientific name Buteo nitidus. That may seem confusing, but is actually not as crazy as it sounds. In fact, it happens all the time: consider for example Mangrove Rail, which received the scientific name earlier associated with Clapper Rail – Rallus longirostris – while Clapper Rail continues to exist, but under a new scientific name Rallus crepitans. The reason is simple: there is a basic rule in taxonomy which states that in the case of a split, the population that was first described retains the scientific name. In the case of the ‘Gray Hawk’, the holotype was described in 1790 by Latham from a specimen collected in French Guiana. Thus, the southern population ‘had rights’ to the name nitidus, while the common name Gray Hawk was retained for northern populations. Prior to 2012, it was okay to report Buteo nitidus in Guanacaste (northwestern Costa Rica) because back then, the name referred to Gray Hawk. Reporting Buteo nitidus from the same location after 2012 will trigger the eBird filter asking for more details.
Knowing all this, and given the fact that the two forms do not overlap geographically, couldn’t we simply identify them based on range? That is, Gray-lined Hawk on the Osa Peninsula and the Coto Colorado Valley in southern Costa Rica, and Gray Hawk nearly everywhere else in Costa Rica? Well yes, that would probably work 99% of the time, but let’s take a closer look anyway at these two field-identifiable species.
Table 1: Plumage differences between Gray Hawk and Gray-lined Hawk (based on Millsap et al. 2011).
|Plumage characters||Gray Hawk||Gray-lined Hawk|
|Crown, nape, upperparts||Gray; no barring on head and nape, faint barring on upperwings||Pale gray; narrow dark barring on crown, nape and upperparts|
|Upper tail coverts||White||Dark gray, thin white tips|
|Juvenile||Gray Hawk||Gray-lined Hawk|
|Malar stripe||Strong||Small to absent|
|Crown||Brown (no streaks)||White, or streaked with white|
|Underparts||Even streaking, no blobs||Four blobs and some streaking|
|Tail (upperside)||Narrow bands||Wide bands|
|Underside adult in flight||Gray Hawk||Gray-lined Hawk|
|Wings||Dark trailing edge||No dark trailing edge|
|Primary tips||Dark, contrasting||Barred|
|Tail||White subterminal band (nearly) same width as other white tail bands||White subterminal band much wider than other white tail bands|
There are also vocal differences, particularly in the alarm call, between the two species.
Gray Hawk is sedentary in most of its range, although northern populations (from the US and northern Mexico) migrate south during the northern winter. Exactly how far south they go, and whether some may even reach the Gray-lined Hawk range, is unknown. Gray-lined Hawk is sedentary throughout the range, although probably migratory at the southern edge of its range in Argentina. Millsap et al. (2011) noted that there is a 50 km wide buffer zone where neither species occurs, but believe that future contact is likely as deforestation in central and southern Costa Rica has created seemingly suitable habitat for Gray Hawk in those areas. Sandoval (2009) noted range expansion of Gray Hawk on the Atlantic Slope, and the eBird map shows this as well.
I encourage birders in Costa Rica and western Panama to pay close attention to these fairly common raptors of half-open landscapes, and to learn well the subtle ID differences, as well as their different ranges. Bird distributions are often plastic, and at eBird we like to be at the cutting edge of bird distribution information. Given the short distance between the two species and the ever-changing landscape, it seems plausible that these two taxa, having been separated for approximately 4.5 million years, will at some point meet. We hope to be there – with your help – to document well these changing distribution patterns.