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Chestnut-backed Antbird Poliocrania exsul

  • Order: Passeriformes
  • Family: Thamnophilidae
  • Polytypic: 5 subspecies
  • Authors: Woltmann, Stefan, Ryan S. Terrill, Matthew J. Miller and Matthew L. Brady
Sections

Appearance

Distinguishing Characteristics

The Chestnut-backed Antbird is a medium-sized (14 cm, 29 g) understory antbird with a slate black head and conspicuous bare blue skin around the eye, chestnut brown upper parts (including the tail), and underparts that are dark gray (male) or various shades of brown (female; Figure 1). The flanks and undertail coverts are chestnut or Figure 1. Adult male (left) and adult female (right) Chestnut-backed Antbirds (nominate exsul) captured in Heredía, Costa Rica. The male was captured 24 March 2008, image by RST; the female was captured 27 March 2009, image by MLB.tawny brown in both sexes. South American subspecies have distinct white or buffy spotting on the tips of the greater and lesser coverts. Plumage of juveniles generally duller than adults. When agitated (as in territorial interactions, or in response to playback), birds characteristically cock the tail vertically and rapidly pump it downwards. The species typically is found in lowland humid forests, forest edges, overgrown treefall gaps, and second growth. It is not infrequently found in small forest patches in highly degraded landscapes.

Similar Species

Visually: Throughout much of the range the combination of medium size, blue orbital skin and dark chestnut brown upperparts should separate most similar species. Five species in Central America could be visually misidentified as Chestnut-backed Antbird: Black-throated Wren (Thryothorus atrogularis), Dull-mantled Antbird (Myrmeciza laemosticta), Immaculate Antbird (M. immaculata), female Bare-crowned Antbird (Gymnocichla nudiceps), and Song Wren (Cyphorhinus phaeocephalus). Black-throated Wren and Dull-mantled Antbird lack bare blue skin on the face. Female Bare-crowned Antbird has noticeable wing bars, and typically occurs in less forested habitats. Immaculate Antbird is generally rare, is considerably larger than Chestnut-backed Antbird, and males are entirely black. Song Wren is paler overall, has barred wings and tail, and typically occurs in vocal groups. In western Colombia and Ecuador, female Esmeraldas Antbird (Myrmeciza nigricauda) has white interscapular patch (may be concealed), has a white-spotted throat, and lacks blue orbital skin.

Vocally: In southern Central America Black-faced Antthrush (Formicarius analis) has a two- or 3-note song that is often confused with Chestnut-backed Antbird song, especially at a distance. The second note of Black-faced Antthrush is less emphasized than the first and maintains a steady pitch through the note; the second note of Chestnut-backed Antbird song is equally or slightly more emphasized than the first, and tends to drop in pitch. When three notes are given, the first two are often identical (second note may be upslurred) in Chestnut-backed Antbird, while it is the second two notes that are always identical in Black-faced Antthrush. The antthrush’s song, although often louder and more powerful, seems less decisive: the song of the Chestnut-backed Antbird is an imperative statement, whereas the antthrush is less committed to a statement of fact. Black-throated Trogon (Trogon rufus) song usually has three or more notes, and is a sadder and more drawn-out song, also not an emphatic statement. The intonation of the song of Black-throated Trogon is hollow and throaty, as opposed to the more clear whistle of Chestnut-backed Antbird. Some vocalizations of White-breasted Wood-Wren (Henicorhina leucosticta; no example of 2-note song found at Macaulay Library or on xeno-canto) also may resemble Chestnut-backed Antbird sounds, but the wren typically gives a faster delivery, and will change song type frequently – something Chestnut-backed Antbirds never do.

Detailed Description

Description of nominate Myrmeciza exsul exsul from Ridgway (1911: 111):

"Adult male.—Head and neck, all round, uniform slate-black; upper parts (except pileum and hindneck) plain deep chestnut; lesser wing coverts at least partially black, those along anterior margin of wing more or less extensively white, and behind this white margin often a few small dots of white; the carpo-metacarpal region also streaked with white, and outermost alula also sometimes edged with white; underparts (except chin, throat, flanks, anal region and under tail-coverts), plain blackish slate color; flanks, anal region, and under tail-coverts plain mummy or vandyke brown…
Adult female.—Upper parts as in adult male, but slate-black of pileum and hindneck slightly duller; chin and throat slate-blackish, but usually duller than in adult male; rest of underparts plain vandyke or mummy brown…"

There are no white feathers in the interscapular region in any plumage, as are present in some other antbird species. Nestlings and recently-fledged juveniles mostly dark brown or brownish-blackish all over; females and males not distinguishable in full juvenile plumage. Males soon acquire some slate-gray feathers in the underparts, however (see Molts). Young males in formative plumage similar to adult males, but gray of underparts and crown slightly paler than in basic plumage (subspecies exsul; Figure 4), but without direct comparison this difference likely is not detectable in the field. The last feathers to be replaced in formative plumage are the primary coverts, which - with practice - usually can be identified as either juvenile or formative/basic (see Molts).

Figure 4. Underparts coloration of basic male (left; 164160), formative male (center; 164161), and basic female (right; 164159) of Chestnut-backed Antbird (nominate exsul) collected in Colón, Panama, and housed in LSUMZ. All three were collected 18 March 1997.

Figure 5. A few retained juvenile (brown) feathers in the crown of a young male (left), and extensive juvenile feathers retained in the crown of a young female Chestnut-backed Antbird (nominate exsul) captured in Heredía, Costa Rica. The male was captured 02 March 2007; the female was captured 30 Mar 2009; both images by MLB.Formative female similar to adult female, but duller overall. A mix of dark brown (juvenile) and dull black or brownish black (formative/basic) feathers in the crown can often be distinguished in birds < one year old (Figure 5).

Much variation observed in degree of white spotting on lesser coverts (subspecies exsul), ranging from extensive to none among 244 individuals (15% captured more than once; SW, unpub. data). Spotting on lesser coverts not age-related (SW unpub. data), as it is in some North American Catharus spp. (Pyle 1997). Pale spotting on greater coverts rare, and restricted to indistinct buffy tips on suspected juvenile feathers.

Variation in the degree of white on the anterior wing margin also exists, particularly on the greater alula: most often there is a distinct white anterior edge to this feather (or buffy and somewhat diffuse in juveniles (Wolfe et al. 2009, SW unpub. data), but the width of the white is variable, and all-black alulae are not uncommon (SW, pers. obs.).

All commonly available guide depictions generally adequate for identification (Hilty and Brown 1986, Ridgely and Gwynne 1989, Ridgely and Tudor 1994, Ridgely and Greenfield 2001a, Zimmer and Isler 2003, Garrigues and Dean 2007); illustration in "A Guide to the Birds of Costa Rica" (Stiles and Skutch 1989) perhaps exaggerates contrast between blackish and brownish parts as seen under typical field conditions, but it is not clear which subspecies (exsul or occidentalis) is depicted. The most accurate depiction (to our eyes) of nominate exsul is in Zimmer and Isler (2003).

Molts

The Chestnut-backed Antbird has an annual Complex Basic molt strategy (CBS; S.N.G. Howell et al. 2003, Wolfe et al. 2009). Individuals in full juvenile plumage not identifiable as male or female. Males molting into formative plumage identifiable by mix of gray and brown feathers in underparts. The Preformative (PF) molt involves all body feathers and greater coverts. Primaries, secondaries, tertials and probably the rectrices are not replaced until the Definitive Prebasic (DPB) molt (Wolfe et al. 2009). The alulae are frequently completely or partially replaced (may be asymmetric) well before the 1st basic molt (thus caution is advised in the use of this tract in aging). Figure 7 illustrates the difference between juvenile and basic primary coverts.

Figure 7. Juvenile (above) and basic (below) primary coverts in female Chestnut-backed Antbirds (nominate exsul) captured 25 and 28 Feb 09 in Heredía, Costa Rica. These plumage characteristics are similar in males and female, but can often be more subtle in females; not all individuals can be aged with confidence. Note that the smallest of the three feathers of the alula in the juvenile has been replaced. Images by MLB.Progression of flight feather molt appears to progress in typical manner (distally in primaries, primary coverts and rectrices, and proximally in secondaries).

Molt cycle in areas with distinct wet/dry seasons synchronized within populations (Willis and Oniki 1972, Wolfe et al. 2009), such that definitive basic molts occur shortly after the main breeding season. Timing of DPB molt less clear: At a Costa Rican site (Tortuguerro) with a pronounced dry season and a relatively long breeding season (March – October), the DPB occurs at the same time as the definitive basic molt (i.e., birds are ca. 1 calendar-year [or slightly more] old when beginning DPB; Wolfe et al. 2009).

Also unclear is the timing of molts in populations that experience less pronounced seasonality and show at least some breeding activity year-round, such as at La Selva Biological Station in Costa Rica (SW pers. obs.). In such populations, HY/AHY/ASY terminology is inappropriate, and we recommend simply naming the plumage cycle according to Howell et al. (2003).

Asymmetric flight feather, alulae and primary covert molt at SW’s study sites in northeastern Costa Rica is not infrequent. We recommend always inspecting both wings, as we have caught individuals with juvenile primary coverts on one wing and basic primary coverts on the other.

Foregoing refers exclusively to nominate exsul; no information on other subspecies.

Bare Parts

Figure 6. An unusual pale-eyed female Chestnut-backed Antbird (nominate exsul) captured 08 Mar 07 in Heredía, Costa Rica and photographed by MLB. Due to unfortunate lighting, the iris color in this image appears darker than it actually was.The legs are dark horn to blackish, and the bill is black. The skin around the eye is typically a light but brilliant blue. This blue skin covers both the forehead and throat, much as in Bare-crowned Antbird (Gymnocichla nudiceps), except that feathers cover the skin on the forecrown so that, in the field, the only blue skin visible is where the skin is bare around the eye. This blue is duller in younger birds (Figure 5, above). Whether the blue skin of adults becomes brighter with age or only with breeding condition is not known. The iris is brownish in young birds, and dull reddish-brown in adults, but there is much variation and the difference can be difficult to assess under field conditions. A rare, pale-eyed individual may be encountered (Figure 6).

Measurements

Table 1. Mean wing chord (unflattened, mm), by plumage and sex of live and preserved specimens of Chestnut-backed Antbird.
Sex/plumage 
chord(SD)  range   n Source
 live (exsul)        
Male Basic 66.2(1.54) 62-70 140 SW
Male Juvenal 64.8(1.25) 62-67 28 SW
         
Female Basic 64.7(1.41) 62-68 47 SW
Female Juvenal  63.0 60-67 24 SW
         
specimen        
exsul        
Male Basic  66.5(1.99) 63-71 21 SW (LSUMZ, AMNH) 
Male Juvenal 65.0(2.18) 61-67 9 SW (LSUMZ, AMNH)
Female Basic 65.1(1.37) 63-67 10 SW (LSUMZ, AMNH)
Female Juvenal 63.5(2.14) 60-66 8 SW (LSUMZ, AMNH)
         
Male 67.4 64-71 16 Ridgway 1911
Female 66.4 64-69.5 14 Ridgway 1911
         
Male 65.4 62.6-67.3 15 Wetmore 1972
Female 64.1 62.5-66.7 15 Wetmore 1972
         
occidentalis        
Male 68.3 65-71 10 Ridgway 1911
Female 65.7 62-69 10 Ridgway 1911
         
Male 67.2 63.5-69.2 13 Wetmore 1972
Female 64.7 62.6-68.0 17 Wetmore 1972
         
Male Basic 65.2 64-69  6 SW (LSUMZ)
         
niglarus        
Male 67.1 65.0-70.1 15 Wetmore 1972
Female 64.2 62.7-67.0 10 Wetmore 1972
         
cassini        
Male 65.2 63.0-69.4 21 Wetmore 1972
Female 63.5 62.3-64.6 10 Wetmore 1972
         
Male Basic  64.3 63-66 4 SW (LSUMZ)
         
         
Table 2. Tail (mm), tarsus (mm), culmen (mm), and mass (g) measurements of various subspecies of Chestnut-backed Antbird.
Sex mean(SD) range n Source
         
exsul (live)        
Male 47(1.6) 44-50 18 SW
Female 46(1.4) 44-48 7 SW
         
exsul (specimen)        
Male 47 44-49 16 Ridgway 1911
Female 45.2 42.5-50 14 Ridgway 1911
         
Male 45.6 43.3-48.2 15 Wetmore 1972
Female 43.5 41.0-46.7 15 Wetmore 1972
         
occidentalis        
Male 48.1 47-51.5 10 Ridgway 1911
Female 46.8 44.5-49 10 Ridgway 1911
         
Male 46.7 44.0-48.8 13 Wetmore 1972
Female 44.6 41.2-46.9 17 Wetmore 1972
         
niglarus        
Male 45.7 42.5-49.7 15 Wetmore 1972
Female 43.3 40.2-47.6 10 Wetmore 1972
cassini        
Male 42  - 1 Ridgway 1911
Female 39  - 1 Ridgway 1911
         
Male 41.2 38.5-45.6 21 Wetmore 1972
Female 40.4 36.8-42.2 10 Wetmore 1972
         
         
  tarsus      
exsul (live)        
Male 29.9(1.7) 28.1-34.5 18 SW
Female 28.9(1.5) 27.5-31.0 7 SW
         
exsul (specimen)        
Male 28.2 27.0-29.5 16 Ridgway 1911
Female 28.3 27.5-31.0 14 Ridgway 1911
         
Male 28.4 27.7-29.5 15 Wetmore 1972
Female 28.3 27.5-29.3 15 Wetmore 1972
         
occidentalis        
Male 27.8 27-29.5 10 Ridgway 1911
Female 28.1 27-29.5 10 Ridgway 1911
         
Male 28.4 27.7-28.9 13 Wetmore 1972
Female 28.2 27.0-29.5 17 Wetmore 1972
         
niglarus        
Male 28.4 27.5-29.7 15 Wetmore 1972
Female 28.1 26.4-29.6 10 Wetmore 1972
         
cassini        
Male 29  - 1 Ridgway 1911
Female 26  - 1 Ridgway 1911
         
Male 27.9 26.4-28.7 21 Wetmore 1972
Female 27.3 26.7-28.5 10 Wetmore 1972
         
  culmen      
exsul        
Male 20.2 18.5-21.5 16 Ridgway 1911
Female 20 19-21 14 Ridgway 1911
         
Male 21.4 20.4-22.7 15 Wetmore 1972
Female 20.7 20.0-21.6 15 Wetmore 1972
         
occidentalis        
Male 20.7 20-22 10 Ridgway 1911
Female 20.1 18.5-21.5 10 Ridgway 1911
         
Male 22.3 21.4-23.6 13 Wetmore 1972
Female 21.4 20.2-22.5 17 Wetmore 1972
         
niglarus        
Male 21.4 20.2-22.3 15 Wetmore 1972
Female 20.8 19.9-22.2 10 Wetmore 1972
         
cassini        
Male 20.9 19.5-22.3 21 Wetmore 1972
Female 19.8 18.9-20.8 10 Wetmore 1972
         
  mass (g)      
exsul (live)        
Male 28.8(1.6) 26-31 18 SW
Female 29.0(2.5) 24-32 9 SW
         
         
         

Recommended Citation

Woltmann, Stefan, Ryan S. Terrill, Matthew J. Miller and Matthew L. Brady. 2010. Chestnut-backed Antbird (Poliocrania exsul), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, editor). Cornell Lab of Ornithology, Ithaca, New York, USA. https://doi.org/10.2173/nb.chbant1.01