White-flanked Antwren Myrmotherula axillaris Scientific name definitions

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The White-flanked Antwren is perhaps the most widespread and familiar of the antwrens, a group of small, short-tailed antbirds. It is the only antwren with bold white flank patches that contrast with otherwise dark plumage. These patches are particularly conspicuous because of this birds' habit of nervously flicking its wings to reveal the flanks. It occurs in the understory of lowland forest from Honduras south through northern South America to Bolivia and coastal eastern Brazil, although microhabitat preferences vary between regions. It often associates with mixed species flocks, feeding actively on arthropods obtained from vegetation with lunges or short sallies. The nest is a cup of dead leaves placed on a branch fork.

White-flanked Antwren (White-flanked)

9–10 cm; 5–9·5 g (nominate), 7·9–8·1 g (race albigula), 6·8–10·6 g (race melaena). Male nominate race is dark grey, with concealed white interscapular patch; wings and tail blackish, remiges edged grey, wing-coverts and tail tipped white; throat, breast and centre of belly black, flanks white, crissum grey with subapical black spots and pale grey tips. Female is olive-brown above, shading to reddish yellow-brown on rump, with wings and tail dark brown, edged light cinnamon-rufous, wing-coverts edged cinnamon, sides of head mottled pale olive-brown, throat and flanks white, rest of underparts rich buff, becoming olive-brown on sides, reddish brown on crissum. Juvenile male is like female, except wings and anterior upperparts mixed yellow-brown and grey, underparts mixed white and grey, tail like adult male; subadult male lacks black on throat and breast, has wings and coverts edged and spotted pale yellow-brown. Race albigula is blacker (variable), scapulars strongly edged white, female is greyer above with wing-coverts tipped pale buff, paler below, crissum buff; melaena male resembles previous, female is similar to nominate but paler; heterozyga and fresnayana males are slightly paler, black of underparts more confined, females paler and less rufescent throughout (E populations typically most pale, but variable), tips of rectrices buff.

White-flanked Antwren (Silvery-flanked)

9–10 cm. Compared to formerly conspecific M. axillaris male is somewhat paler (including on black of belly), flanks mostly pale grey (rather than white), and white tail tips larger; female  has head ashy grey, upperparts olive-tinged grey, wing-covert tips buff, rather faint, and underparts ochraceous.

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White-flanked Antwren (White-flanked)

White-flanked Antwren (Silvery-flanked)

White-flanked Antwren (Silvery-flanked)

Coastal E Brazil (SE Rio Grande do Norte (5 Olmos, F. (2003). Birds of Mata Estrela private reserve, Rio Grande do Norte, Brazil. Cotinga 20: 26–30. Close ) S to Rio de Janeiro).

White-flanked Antwren (White-flanked)

Understorey and midstorey of evergreen forest (terra firme, transitional, and várzea/igapó) and second-growth woodland, to 1400 m. In most parts of range, not obviously confined to any particular microhabitat type within forest. In some regions, however, appears to be highly restricted to one microhabitat type, such specialization sometimes varying geographically among populations currently considered as belonging to same taxon. In Brazil, nominate race appears largely restricted to stands of Guadua bamboo within forest in N Mato Grosso, but virtually confined to river-edge forest without bamboo in other locations in S Amazonia (SW Pará, SE Amazonas, NE Rondônia) and in parts of Amapá; conversely, populations of same taxon in E Venezuela and much of N Brazil show no such specialization. In Nicaragua, albigula seems to require light-gaps in its territory.

White-flanked Antwren (Silvery-flanked)

White-flanked Antwren (White-flanked)

White-flanked Antwren (Silvery-flanked)

White-flanked Antwren (White-flanked)

Feeds mostly on insects and spiders. Stomach contents of specimens included following: from Surinam, spiders, dipterans (Tipulidae), lepidopteran larvae; from Amazonian Brazil, cockroaches (Blattidae), true bugs (Hemiptera), beetles (Coleoptera), Hymenoptera, Formicidae and orthopterans; from SW Venezuela, beetles, beetle larvae and ants, and from Peru/Bolivia (21 specimens), in decreasing order of proportion, orthopterans, beetles, spiders, true bugs (Heteroptera), larvae, cockroaches (Blattidae), ants (Formicidae), dipterans. Also catches adult lepidopterans (moths), but clips and removes the wings before consuming body. In extensive study in Peru/Bolivia, orthopterans, beetles and larvae were selected more than expected, and many fewer ants, flies and wasps (Hymenoptera) were taken than would be predicted, on basis solely of their respective availability; in same prey-selection experiment, but using birds in outdoor cages, nearly all ants, flies and wasps and most beetles and heteropterans were ignored or rejected, but the birds readily consumed cockroaches, spiders, crickets (Gryllidae), katydids (Tettigoniidae), some grasshoppers (Acrididae), butterflies (Lepidoptera) and dragonflies (Odonata), as well as a 5-cm lizard, which was captured, beaten on branch, and swallowed whole. In most regions, forages in pairs, singly or in family groups, mostly 2–12 m above ground in relatively open area between lower dense midstorey and upper parts of dense understorey. Mean foraging height in 181 observations at Cocha Cashu (Peru) was 5·4 m, and in 150 observations in Manaus (Brazil) 7·1 m; forages lower in regions of restricted habitat usage, e.g. elsewhere in Brazil mean of 3·1 m in Rondônia (206 observations). Very active, constantly hopping and changing direction, scanning foliage between movements but seldom pausing for > 2 seconds, and always flicking the wings to reveal white flank patches. Obtains prey primarily from upper and lower surfaces of live leaves (mostly), stems and vines by perch-gleans; also with acrobatic hangs from branchlets or leaf petioles, short lunging stabs, and short sallies (these often fluttering upwards to underside of overhanging vegetation); also inspects suspended in dead leaves in Amazonia. Travels alone in some regions, but in most occurs typically with mixed-species flocks, exact composition of which varies greatly over extensive range; in some areas, e.g. French Guiana, is considered an occasional constituent of such flocks. Where a core member of understorey flocks (e.g. around Manaus), it shares a common territory with other thamnophilids in the flock but appears not to be a flock-leader. Where occurring in same mixed-species flocks as M. longipennis, tends to spend much more time in the viny tangles along trunks and in leafier, more densely foliaged portions of understorey trees; also sallies or hover-gleans much less than that species. At a study site in Nicaragua (albigula), tended not to occur in same mixed flocks as Microrhopias quixensis, but unclear whether this reflected interspecific competition or micro-habitat preferences. Occasional records from Panama, Colombia, Trinidad, Guyana, Surinam and Brazil of attending army ants (mostly Eciton burchelli, in Panama occasionally also Labidus praedator), although most visits less than 5 minutes in duration; other than a tendency to descend lower, little evidence that it modifies its foraging behaviour in presence of ants.

White-flanked Antwren (Silvery-flanked)

Probably feeds mostly on insects  and spiders, but no detailed investigation of diet. Forages in pairs, singly or in family groups, often with mixed-species flocks of other small-bodied insectivores, mostly 2–12 m above ground in relatively open area between lower dense midstorey and upper parts of dense understorey. Mean foraging height in 573 observations in Espírito Santo was 3·8 m and about one quarter of all foraging manoeuvres were aimed at dead-leaf clusters; rather than probing these as do congeneric dead-leaf specialists, it strikes them with the bill to dislodge insects. Very active , constantly hopping and changing direction, scanning foliage between movements but seldom pausing for > 2 seconds, and always flicking the wings to reveal white flank patches.

White-flanked Antwren (White-flanked)

Loudsong of nominate race , heterozyga and fresnayana a rapid uncountable series of abrupt notes, somewhat low-pitched (e.g. 3–2·2 kHz) compared with congeners, dropping sharply in pitch and gaining in intensity at beginning, then more or less levelling, pace varies geographically (slowest in SE Amazonia); of <em>albigula</em> and melaena a countable, evenly paced series of whistles (e.g. nine notes, 2·8 seconds), dropping in pitch, but gaining and then dropping in intensity. Common call (most races) typically of two notes, first shorter and higher-pitched, sometimes extended to 3–4 notes, other calls include sharp upslurred “wheep”, sometimes doubled or tripled, also very short “pip”, much longer “chirr”, and a rattle (possibly not all calls delivered by all races).

White-flanked Antwren (Silvery-flanked)

Loudsong begins and ends with short harsh (frequency-modulated) notes, middle ones become longer and clearer , pace slows then speeds up, intensity increases then decreases, pitch is flat and notes countable (e.g. 13 notes, 2·9 seconds). Compared to M. axillaris, loudsong is very different, differing in note shape, initial amplitude and frequency increase, and in the longer middle notes. Common a short, mostly downslurred note, singly or sometimes repeated, also calls similar to those of M. axillaris.

White-flanked Antwren (White-flanked)

Season Mar–Jul in Costa Rica, Mar–Aug in Panama, Jul–Dec and Feb–Mar in French Guiana (probably similar in Suriname), Feb–Oct in E Ecuador, Sept–Nov in E Peru; Jun–Dec in N Brazil, and juveniles fed by adults in Sept in SC (Mato Grosso), although birds in breeding condition trapped year-round at Manaus (Amazonas), especially in Oct–Feb; one record of adults feeding fledglings just out of nest in Sept in Suriname. Additional nest and egg descriptions from Brazil, Peru, Trinidad, Venezuela. Nest a deep cup composed largely of dead leaves and leaf skeletons (including leaves of bamboo and palms), often with asymmetrically placed dead leaves hanging from bottom, lined, held together and attached at rim with black fungal rhizomorphs, suspended from horizontal fork between twigs, usually with overhanging leaf or leaves, 0·2–4 m above ground in understorey sapling or shrub; nest dimensions: 6–7 cm in diameter, 8 cm deep, with cup 4·5 cm wide by 4–4·5 cm deep. Normal clutch two eggs, white or pinkish white, with reddish brown, deep purple and/or lilac-grey speckles, scrawls and blotches concentrated in wreath at larger end, size 16·7–17·5 mm × 12·3–12·7 mm (Panama), perhaps laid at c. 48-hour intervals; incubation by both parents during day, only female at night, period in C Panama 16–19 days; at two nests in Panama, parents incubated for only 76% and 50%, respectively, of time during which observed, suggesting lower levels of nest attendance than in many other thamnophilids; brooding and feeding of nestlings shared by both parents, nestling period in Panama 8–12 days. Nesting success in one year at site in C Panama was 22·2% for 18 nests, but in another year achieved 71% at five nests; and mean annual survival rate, based on marked birds that were captured in mist-nets in the same area of the country, estimated at c. 60%.

White-flanked Antwren (Silvery-flanked)

Not well known, beyond some sparse data concerning seasonality, with completed nest in early Jul and young fledglings observed in Jan, both in state of Espírito Santo; nest probably very similar in size, structure and materials to that of M. axillaris.

White-flanked Antwren (White-flanked)

Not globally threatened (Least Concern). Fairly common to common throughout its large range. Regions in which it occurs contain numerous protected areas, as well as vast contiguous areas of intact, suitable habitat that are not formally protected. Furthermore, this species has been shown to survive in secondary forest and smaller forest isolates, and is seemingly more tolerant of disturbance than are congeners, its numbers even apparently increasing in Amazonian forest exposed to fire.

White-flanked Antwren (Silvery-flanked)

Not globally threatened (Least Concern). Largely confined to Atlantic forest lowlands EBA, but also occurs in Atlantic slope of Alagoas and Pernambuco EBA. Fairly common to common and is seemingly more tolerant of disturbance than are congeners. Comparatively relatively small range and the overall regional level of deforestation argue against complacence; but, at present, the species remains fairly common in several protected areas, including Sooretama Biological Reserve, Linhares Natural Reserve, Monte Pascoal National Park, Porto Seguro/Florestas Rio Doce SA Forest, and Murici Ecological Reserve.