Was that a Gray Hawk or a Gray-lined Hawk I just saw?

By eBird Centroamérica octubre 7, 2016
Gray-lined Hawk Buteo nitidus

by John van Dort

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
 
Adult
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
Auriculars Buffy 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.

Adult Gray-lined Hawk from Brazil. Note the fine barring on the upperside of the wing, and on the nape all the way up to the crown (these parts are a shade darker and mostly unbarred on Gray Hawk). Photo © Cláudio Dias Timm

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.

The two species can be quite similar in flight. On the left an adult Gray Hawk from Honduras (photo © John van Dort). On the right, an adult Gray-lined Hawk from French Guiana (photo © Michel Giraud-Audine). Note the darker underparts and underwing coverts of the Gray Hawk, compared to the Gray-lined Hawk, and the dark trailing edge of the wings of the Gray hawk, absent in the Gray-lined Hawk. The primary tips look solid dark in Gray Hawk, barred and less contrasting in the Gray-lined Hawk. According to Millsap et al. (2011), the subterminal tail band tends to be wider in the Gray-lined Hawk, but there is some variation and overlap in this character, so it should not be used as a diagnostic field mark.

The two species can be quite similar in flight. On the left an adult Gray Hawk from Honduras; on the right, an adult Gray-lined Hawk from French Guiana. Note the darker underparts and underwing coverts of the Gray Hawk, compared to the Gray-lined Hawk, and the dark trailing edge of the wings of the Gray hawk, absent in the Gray-lined Hawk. The primary tips look solid dark in Gray Hawk, barred and less contrasting in the Gray-lined Hawk. According to Millsap et al. (2011), the subterminal tail band tends to be wider in the Gray-lined Hawk, but there is some variation and overlap in this character, so it should not be used as a diagnostic field mark. Photo Gray Hawk © John van Dort; photo Gray-lined Hawk © Michel Giraud-Audine.

 

Left: adult Gray Hawk, Arenal Volcano, Costa Rica. Right: Adult Gray-lined Hawk, Venezuela. Note the distinct barring on the upperparts of the Gray-lined Hawk on the right, and compare this to what the Gray Hawk on the left shows: some barring, but not as pronounced. Another mark: little contrast between underparts and upperparts of the Gray-lined Hawk versus darker upperparts of the Gray Hawk. Photo of Gray Hawk by Juan Zamora; photo of Gray-lined Hawk by Barloventomagico. Both images © CreativeCommons.

 

Left: juvenile Gray Hawk from Heredia, Costa Rica. Photo © Octavio Quiros. Right: juvenile Gray-lined Hawk from French Guiana. Photo © Michel Giraud-Audine. Compare head patterns: strong versus thin malar stripe; buffy versus white auriculars; dark versus pale crown. Compare also streaking on underparts: even versus concentrated in darker areas (‘blobs’). The darker areas on the upper breast are obvious on this juvenile Gray-lined Hawk, but the darker areas on the lower belly are less visible. Compare the width of the pale bands in the tail: narrower on the juvenile Gray Hawk and wider on the juvenile Gray-lined Hawk.

Left: juvenile Gray Hawk from Heredia, Costa Rica. Right: juvenile Gray-lined Hawk from French Guiana. Compare head patterns: strong versus thin malar stripe; buffy versus white auriculars; dark versus pale crown. Compare also streaking on underparts: even versus concentrated in darker areas (‘blobs’). The darker areas on the upper breast are obvious on this juvenile Gray-lined Hawk, but the darker areas on the lower belly are less visible. Compare the width of the pale bands in the tail: narrower on the juvenile Gray Hawk and wider on the juvenile Gray-lined Hawk. Photo Gray Hawk © Octavio Quiros. Photo Gray-lined Hawk © Michel Giraud-Audine.

 

Another comparison of juvenile Gray Hawk from Honduras (left) with juvenile Gray-lined Hawk from French Guiana (right). Juvenile Gray-lined Hawk has two dark blobs on the upper breast and two dark blobs on the lower belly; the streaking is more even on juvenile Gray Hawk. Photo of Gray Hawk © John van Dort; photo of Gray-lined Hawk © Michel Giraud-Audine.

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.

Cited literature:

Millsap, Brian A., Sergio H. Seipke & William S. Clark. 2011. The Gray Hawk (Buteo nitidus) is two species. The Condor 113 (2): 326–332.

Sandoval, Luis. 2009. Nuevos registros en la distribución de cuatro rapaces diurnas (Accipitridae y Falconidae: Aves) en Costa Rica. Brenesia 71-72: 79–80.

 

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