Plumage, vocal and morphometric characters support a clear split between a Central American component (from Mexico south to Costa Rica) and a mainly South American (also including Panama, part of Costa Rica and the Pearl Islands) component in this species complex. In plumage, these components are distinguished from each other, without intermediates, by the much stronger-colored brown nape of Central American birds (Figure 3). Morphometrically, there is an evident discontinuity in variation of bill and tarsus length around 10°N and 83°W, in Costa Rica, where the two components substitute each other (Figure 4). (Other measurements, when plotted against latitude and longitude, did not show any discernable pattern in variation, and these plots are therefore not shown.) Descriptive statistics for each recognized taxon are presented on Table 1.

Figure 3. 

Nape of South American (the three leftmost specimens) and Central American (the two other specimens) representatives of the Aramides cajaneus species complex. Note the much stronger color in the latter.

Figure 4. 

Length of tarsus of specimens in the Aramides cajaneus complex plotted against latitude and longitude. Green: Aramides albiventris; blue: A. cajaneus; yellow: A. avicenniae. Note the discontinuity in variation around latitude 10° N and longitude 83° W, in Costa Rica, where the distributions of A. cajaneus and A. albiventris abut each other.

The differences in song are most striking. All available recordings from South America, Panama, and the Caribbean side of Costa Rica (Figure 5), corresponding to the South American component, show a song with a basic phrase consisting of two notes (Figures 6). The first note has ascending-descending-ascending-descending frequency modulation, giving it the approximate shape of an “M” in a spectrogram (PF: 1603±66.21 Hz; MaF: 1946±153.5 Hz; MiF: 939.2±96.39 Hz; BW: 1007±178.3 Hz; D: 0.154±0.0288 sec). The second note is shorter, has a lower frequency, and appears in spectrograms as a simple, slightly ascending line (PF: 1170±87.12 Hz; MaF: 1372±112.4 Hz; MiF: 924.6±75.7 Hz; BW: 447.8±83.24 Hz; D: 0.1146±0.0269 sec). In a typical song bout, performed in a duet, this two-note phrase is repeated in a quick, loud, and lengthy sequence, occasionally interrupted by a short series of lower-pitched notes.

Figure 5. 

Distribution of the analyzed song recordings of the South American (blue), and Central American (green) components of the Aramides cajaneus species complex. Their songs are strikingly different; see text for details.

Figure 6. 

Spectrogram of the typical phrase of the song of the South American component of the Aramides cajaneus species complex (LNS 51765). Note that this spectrogram is not in the same scale as the spectrogram in Figure 7.

Figure 7. 

Spectrogram of the typical phrase of the song of the Central American component of the Aramides cajaneus species complex (LNS 23152). Note that this spectrogram is not in the same scale as the spectrogram in Figure 6.

Songs from Belize and southern Mexico (Figure 5), in the range of the second component, are strikingly different (Figure 7). The basic phrase consists of three to four short introductory notes of ascending-descending modulation, followed by three pairs of notes, with similar frequency modulation. In each pair, the first note (PF: 1246±293.92 Hz; MaF: 1436±259.7 Hz; MiF: 878±427.23 Hz; BW: 558±167.5 Hz; D: 0.067±0.014 sec) is shorter than the second (PF: 2037±368.2 Hz; MaF: 2294±358.42 Hz; MiF: 1189±322 Hz; BW: 1104±36.42 Hz; D: 0.111±0.033 sec). Although the sample size is limited, it seems that in a typical session of vocalizations, phrases are delivered in much longer intervals than in the first song type.

Even though only five recordings of the Central American component were available, the difference between its song and the song of the South American component is striking and consistent. There are neither intermediates nor any elements in each component’s vocal repertoires that are even remotely similar to the other’s song. In fact, the songs are so distinct that it is impossible even to draw correspondences or hypotheses of homology between their constituent notes. The difference is comparable to that observed between the songs of A. cajaneus and other species in the genus, such as A. saracura or A. ypecaha. Together with the plumage and morphometric differences, this substantiates the recognition of the Central American and South American components as distinct species-level taxa.

The two components are segregated by the Costa Rican mountain ranges, part of the Chorotega Volcanic Front (CVF) that divides lower Central America into Caribbean and Pacific catchments. This is congruent with the identification of the CVF as the location of a major phylogeographic break for several animal taxa in lower Central America (Bagley and Johnson 2014). In addition, the Costa Rican mountains are known to segregate several sister taxa of birds, such as Amazilia decora and Amazilia amabilis (Trochilidae), Pteroglossus torquatus and Pteroglossus frantzii (Ramphastidae), Carpodectes nitidus and Carpodectes antoniae (Cotingidae), among others (Zeledón 1892, Stiles and Skutch 1994).

There is one specimen that could potentially falsify the parapatric pattern described above. FMNH 30363 is clearly assignable to the Central American component, having a strong brown nape, but is labeled as coming from El Pozo, Puntarenas province, in the Pacific side of Costa Rica, where only birds belonging to the South American component are supposed to be found. There is reason, however, to believe that this specimen has been mislabeled. It, as well as a typically South America component specimen (FMNH 30364), was collected, according to their labels, by M. A. Carriker in 1907. The label of FMNH 30364 has the precise day and month of collection (June 29), but the label of FMNH 30363 has only the year, which already suggests that there may have been some sort of confusion and loss of information between its collection and its final labeling at the FMNH. Adding to the suspicion that this specimen was not collected in El Pozo is the fact that in 1910 Carriker published an annotated list of the birds of Costa Rica in which he recounts having indeed collected in El Pozo in June 1907. Curiously, however, under A. albiventris plumbeicollis, where this specimencould be expected to have been listed, he lists several specimens, but none coming from El Pozo. Besides, he writes about this taxon: “Confined entirely to the Caribbean lowlands, and probably only in the northeastern part, since there are no records of its presence in southeastern Costa Rica”. If Carriker had indeed collected a specimen with characters of A. albiventris plumbeicollis in the Pacific side of Costa Rica just three years earlier, it is very unlikely that he would fail to list it, and write that the taxon is found only in the Caribbean lowlands. Therefore, the information on the label of FMNH 30363, including locality data, is under suspicion, and this specimen does not falsify the role of the Chorotega Volcanic Front in segregating the A. cajaneus species complex into two components.

Two basic plumage morphotypes can be recognized in the Central American constituent of the Aramides cajaneus species complex (Figure 8). In Morphotype 1, found from western Honduras northwestwards to the extreme of the complex’s distribution in Mexico, there never is an homogeneous, conspicuous brown mantle, even though some birds do have a dull brown mantle, fainter along the midline, and there always are white feathers in the lower chest, in variable extension. In contrast, in Morphotype 2, found from eastern Honduras southeast to the Caribbean side of Costa Rica, there always is a homogeneous brown mantle, and there never are any white feathers on the lower chest, even though some birds do have in that area paler feathers than in the mid and upper chest, but not white. The characters of Morphotype 1, as well as comparison with type specimens, reveal that it is referable to Aramides albiventris Lawrence (syntypes from Belize and Guatemala). Morphotype 2, on the other hand, agrees with the description and holotype of Aramides plumbeicollis Zeledón (type locality: Jimenez, Costa Rica).

Figure 8. 

Left: Ventral view of typical specimens of the Central American morphotypes 2 (AMNH 103264) and 1 (AMNH 776255), respectively. Right: dorsal view of same specimens.

However, in spite of the characters noted above, the distinction between the two morphotypes is doubtful and their recognition as distinct taxa is not warranted, because there are many specimens that blend characters of the two, in various combinations. Some, such as AMNH 393516, from Ocos, Guatemala, have the white chest feathers of Morphotype 1, and the full chestnut mantle of Morphotype 2. Conversely, others, such as AMNH 471954, from Mts. La Cumbre, Honduras, lack both the white lower chest feathers and the chestnut mantle. These intermediate specimens are found mainly in Honduras, Guatemala and Belize, and Quintana Roo, Campeche and Yucatán states in southwestern Mexico but also, in fewer numbers, further northwest (four specimens in Vera Cruz and Oaxaca) and south (two specimens in Costa Rica). In many cases, the intermediate specimens occur in the same localities as either “pure” morphotype, or even the two morphotypes and intermediates all together, such as in El Boquerón, in center-eastern Honduras. No particular geographic pattern of plumage variation is noticeable throughout the extensive area of intergradation (Figures 9 and 10).

Figure 9. 

Mapping of the variation in the mantle of individuals of the Aramides cajaneus complex in Central America. Yellow: the upper back has no distinct coloration in relation to the mid and lower back. Red: a faint brownish coloration is present in the sides of the upper back. Blue: a faint brownish tinge is present across the upper back. Green: a complete, conspicuous brownish-orange mantle is present. Notice the lack of any discernable pattern in variation (see text for details).

Figure 10. 

Mapping of the variation in the lower chest of individuals of the Aramides cajaneus complex in Central America. Yellow: no white or paler feathers in the lower chest. Red: paler chestnut, but not white, feathers are present in the lower chest. Blue: white feathers present in the lower chest.

Occurrence of intermediates or hybrids, by itself, does not preclude recognition of two populations as separate species, as long as the variation is not clinal and specimens from outside the intergradation zone maintain their diagnosability (Helbig et al. 2002). In the present case, plumage variation does not appear to be clinal (even though tarsus and culmen measurements, when plotted against latitude and longitude, do hint at clinal variation, the length of both increasing towards north and west; Figure 4). However, the zone of intergradation is too extensive, and intergrades too numerous, to allow recognition of two evolutionarily units. Also due to these intermediates, diagnosability between Morphotypes 1 and 2 is not absolute anywhere in Central America. Unfortunately, no songs from within the range of Morphotype 2 were available, so vocal data cannot be used to inform a decision on the taxonomic status of these populations. Therefore, based on the data we currently have at hand, it appears that the two morphotypes are deeply connected, and cannot be considered distinct evolutionary nor taxonomic units. Thus, we propose that the Central American component of the A. cajaneus species complex be recognized as a single species, Aramides albiventris Lawrence, 1868, with Aramides plumbeicollis Zeledón, 1892 (see ahead for a discussion on the date of its publication) as a junior synonym. At the same time, we also emphasize the importance of further study of these populations, in order to better understand the genetic and historical processes underlying this very complex scenario of phenotypic variation.

Regarding the other subspecies currently recognized in Central America, Miller and Griscom (1921) described A. plumbeicollis pacificus, based on a single specimen (AMNH 143684) from western Nicaragua. This specimen presents a slight indication of a chestnut mantle, but it is notably more tenuous along the midline, and it has no pure white feathers on the lower chest. It is one of the intermediate specimens between the two A. albiventris morphotypes, and thus A. plumbeicollis pacificus is a synonym of A. albiventris.

The characters used by Bangs (1907) and (Dickey 1929) to describe, respectively, A. albiventris mexicanus and A. vanrossemi do not support the recognition of these taxa when a large series of specimens is examined. Their supposed diagnostic characters in relation to albiventris vary widely throughout southern Mexico and Guatemala. For example, one of the putative diagnostic characters of A. mexicanus would be a narrower and more fulvous (instead of white) band in the lower chest. However, the extension and exact tone of the pale feathers in the lower chest are variable throughout the distribution of A. albiventris. AMNH 393517, from Ocos (Guatemala), for example, presents a wide, pure white band, while AMNH 393518, from the same locality, presents only a few pure white feathers, the rest of the band being yellowish white, and is in this respect very similar to AMNH 471952, from northern Vera Cruz (within the supposed distribution of A. mexicanus) and FMNH 110121, from northern Guatemala. Two specimens from Sarabia, Oaxaca (AMNH 776255 and 776256), also within the supposed distribution of A. mexicanus, are very different from each other in the amount of white feathers in the lower chest. Therefore, this character is too variable in southern Mexico and adjacent regions to be taxonomically informative. A similar situation is presented by the other putative diagnostic character of A. mexicanus, “all the colors darker” (Bangs 1907). In fact, the holotype of A. mexicanus (MCZ 102281) does not in any way stand out from the range of individual variation observed in A. albiventris, and they are therefore synonyms.

Dickey (1929) described Aramides vanrossemi based on a single specimen (UCLA 18750) from Barra de Santiago, Ahuachapan, El Salvador. This specimen, too, does not depart significantly from the range of individual variation seem throughout the range of A. albiventris. Contrary to the stated by Dickey, it is not “slightly paler throughout”. Also, the author’s statement that it had “lake red instead of yellow” irises is unjustified, given that all birds in the Aramides cajaneus complex have red irises, as attested by specimen labels and abundant photographs available online (Internet Bird Collection; http://ibc.lynxeds.com/). Similarly, the statement that the “terminal third of the maxilla [is] green instead of yellow” does not make sense as this too is typical of the whole complex. Thus, A. vanrossemi is also here considered a junior synonym of A. albiventris.

Two taxa can be identified in the South American component of the Aramides cajaneus species complex: Aramides cajaneus (Statius Müller, 1776), sensu stricto, found from Costa Rica south to Uruguay and northern Argentina; and Aramides avicenniae Stotz, 1992, found in a small part of the coast of southeastern Brazil.

A. avicenniae is distinguished from A. cajaneus by its gray, instead of green, back and its more greenish-gray upper wing-coverts. Throughout the distribution of A. cajaneus, back color is somewhat variable and even tends towards grayish-green in several specimens from the southwestern part of its distribution and from the northern coast of São Paulo state, not far from the range of A. avicenniae. Nevertheless, when specimens of A. avicenniae and even the grayest-backed specimens of A. cajaneus are placed side-by-side, there is a clear discontinuity in the color of their backs (Figure 11). In specimens of A. cajaneus from Ilha dos Búzios, Ilha Alcatrazes and Ubatuba, on the northern coast of São Paulo, the hindneck and upper back are clearly of different colors, even if in some of them the back is darker than the average in A. cajaneus. On the other hand, the upper back and the neck are display the same tone of gray in specimens from the São Paulo coast south of Santos (A. avicenniae). These patterns demonstrate that A. avicenniae is not merely the end of a cline, nor a variation of A. cajaneus, and it is hereby regarded as a full species.

Figure 11. 

Specimens of A. avicenniae (the rightmost specimen) and A. cajaneus (all others) from Brazil. Note the homogeneous grey coloration in the hindneck and back of A. avicenniae, while in A. cajaneus the back is always greener than the hindneck.

Both sexes of A. cajaneus (sensu stricto) have significantly smaller bill height than A. avicenniae, and males have significantly smaller bill width. In addition, the two are significantly smaller than A. albiventris in tail length, tarsus length and bill length of both sexes. (Tables 1–3).

Bangs and Penard (1918) described A. c. latens, from the islands of San Miguel and Viveros, in the Pearl Island archipelago off the Pacific coast of Panama. It was distinguished from A. cajaneus by its smaller size and overall paler plumage. A. c. morrissoni was described from the islands of San José and Pedro González, in the same archipelago, by Wetmore (1946), as being similar to latens, but told apart by its darker back and hindneck. However, all the specimens from the Pearl Islands examined, including the types of both subspecies (MCZ 114297 and USNM 376059, respectively), fall within the variation observed for A. cajaneus and these names are thus treated as junior synonyms. These synonyms of A. cajaneus, along with Rallus chiricote, A. c. venezuelensis, A. c. peruviana, A. c. salmoni and A. c. grahami, are probably the result of overemphasis on minor individual plumage variations and lack of adequate and geographically comprehensive sampling.

When the distributions of A. avicenniae, A. cajaneus and their congener A. saracura (Spix, 1825) are mapped together, it is notable they have almost perfectly parapatric distributions, a pattern never before remarked on. Contrary to what is indicated in several reference works (e. g. Ripley 1977, Taylor 1996, Taylor 1998, Erize et al. 2006, Sigrist 2009), A. cajaneus is absent from an extensive part of interior southeastern Brazil and from the Argentine province of Misiones. This area corresponds almost exactly to the distribution of A. saracura (Figure 12). Aramides saracura and A. cajaneus (or its substitute A. avicenniae) both occur on the coast of this region, but in that case A. cajaneus and A. avicenniae are mainly found in mangroves, a habitat not occupied by A. saracura (Taylor 1998). Even though A. saracura is usually considered more of a forest dweller than A. cajaneus (Taylor 1998), it is possible that their ecological preferences are not different to the point of allowing sympatry. A hypothesis derived from this distribution pattern is that A. saracura might have been the implied in the differentiation between A. avicenniae and A. cajaneus. Its presence might have acted as an ecological barrier between inland and coastal populations of A. cajaneus, leading to a process of peripatric speciation that culminated with the emergence of A. avicenniae.

Figure 12. 

Distribution of A. cajaneus (blue), A. avicenniae (yellow) and A. saracura (red) in southeastern Brazil. Note the parapatric distribution pattern.

Even though we do not recognize A. plumbeicollis as a valid taxon, a clarification is needed regarding this name, given that it is nomenclaturally available and most references have a wrong publication date for it. Hellmayr and Conover (1942) cite the description of Aramides plumbeicollis as “Anal. Mus. Nac. Costa Rica, 2, p. 3, 1888”, and a similar citation is given by Ripley (1977). Taylor (1996), Taylor (1998) and Dickinson and Remsen (2013), also have the year as 1888, but without the full reference. However, a careful examination of the relevant publications reveals that the name Aramides plumbeicollis was first published, under the rules of the Code, only in 1892.

The name Aramides plumbeicollis was first used in a catalogue of the birds of Costa Rica in tome 1 of the Anales del Museo Nacional—República de Costa Rica (Zeledón 1888: 131). In this publication the name is not associated with any definition or description of the taxon to which it refers. There is only a footnote that reads: “This species is described on page 3, Tome II of these Annals, Year 1888” (our translation from the Spanish original). According to Article 12 of the Code, names published before 1931 without a description or definition are considered available as long as they are associated with an indication of the animal they refer to. A reference to a past publication fulfills this requirement, but a reference to a future publication does not. Therefore, Aramides plumbeicollis Zeledón, 1888, as it appears in this catalogue, is a nomen nudum.

Subsequent authors always gave 1888 as the date of the species’ description, probably assuming, based on Zeledón’s (1888) footnote, that Aramides plumbeicollis was indeed described in tome 2 of the Anales del Museo Nacional—República de Costa Rica. However, no exemplars of this publication could be found in the library of the Museo Nacional de Costa Rica (Adelina Jara, librarian at the Museo Nacional de Costa Rica, pers. comm.) and this volume was actually never published (Anonymous 1892, Chaves and Bolaños 2011). In fact, following tome 1, the publication was merged with the Anales del Instituto Fisico-Geografico Nacional de Costa Rica, giving rise to a new series titled Anales del Instituto Fisico-Geografico y del Museo Nacional de Costa Rica. However, the numbering of this new series continued with that of the Anales del Instituto Fisico-Geografico Nacional, and its first tome, published in 1892, but referring to 1890, is tome 3 (Anonymous 1892). It is on page 134 of this tome that the description of A. plumbeicollis is found, and this is the first time in which that name is made available under the rules of the Code.

Aramides ypecaha

This species has a seemingly disjunct distribution, being found in central Brazil, especially along the Araguaia and São Francisco river valleys, as well as, further south, in southern Brazil, Paraguay, Uruguay and northeastern Argentina, but with no records from the extensive intermediate area. Nevertheless, no morphological differentiation has been described between these two populations. Based on 66 specimens, the only difference observed was that specimens from the northern population have slightly grayer and darker backs than those from the southern population (5Y 4/3 versus 2.5Y 4/3, respectively). There is, however, considerable variation within each population, and the differences are too subtle to allow a safe, consistent diagnosis. In addition, the species’ peculiar distribution needs to be further investigated before further taxonomic or evolutionary inferences can be made.

Aramides wolfi

This species is considered Vulnerable in the IUCN Red List (BirdLife International 2012). It is also the Aramides with the most restricted distribution; only found west of the Andes from southwestern Ecuador north to the Chocó department of Colombia. From the 26 skins analyzed, we found that specimens from central and southern Ecuador are much paler than those from Colombia and the departments of Pichincha and Esmeraldas, in northern Ecuador (Figure 13). Southern specimens have pale greenish-brown backs (7.5YR 3/2 to 7.5YR 3/4), while northern ones are strong reddish-brown (5YR 2.5/2) (Figure 13). The underparts of northern specimens are also darker and redder but this is subtler than the difference in the upperparts. Where the two variants approach each other, in the region of Pichincha, Manabi and Esmeraldas, intermediates are present.

Figure 13. 

A series of Aramides wolfi in the AMNH. The six leftmost specimens, with a stronger coloration, are from northwestern Ecuador, while the four specimens to the right, from southwestern Ecuador, have a paler plumage.

This variation coincides with a notable climatic gradient; from one of the most humid regions on Earth, in southwestern Colombia, to semi-arid conditions in southwestern Ecuador. This is consistent with Gloger’s rule, according to which animal populations from humid regions tend to be darker and more pigmented than those from dry climates (Gloger 1833, Zink and Remsen 1986). The mechanisms behind Gloger’s rule are not necessarily genetic (Zink and Remsen 1986, and see Beebe 1907, Slagsvold and Lifjeld 1985), and thus we refrain from making any taxonomic or evolutionary inferences based on the variation observed in A. wolfi, and suggest that further investigations are required to determine the mechanisms responsible for it and the taxonomic implications thereof.

Aramides mangle

This species occurs along the coast of Brazil from Pará to Paraná, with some inland records in northeastern Brazil which indicate occurrence of migratory movements (Redies 2010, Marcondes et al. 2014). Two plumage variants were observed in it. The coloration pattern is the same in the two, but in one variant the whole plumage is much paler. Even though pale specimens come mainly from northeastern Brazil, there is no geographical segregation between the variants, as dark-plumaged birds also occur in that region (e. g. FMNH 403199, from Piauí, and MPEG 67808, from Maranhão). Indeed both forms have even been collected in the same locality (MNHN 1971.786 and 1971.787, from Exu, Pernambuco). Given this lack of geographical pattern, the plumage variation in Aramides mangle is considered intraspecific and taxonomically uninformative. Its exact nature remains uncertain, but we hypothesize either that (1) the pale individuals are juveniles, although there are no notes on any of the specimen labels regarding their age. (2) Dark and paler specimens represent an intraspecific polymorphism with two distinct, discrete plumage morphs or phases. Or (3) that paleness is due to feather wear, possibly related to abrasion or exposure to sunlight.