Long-billed Gnatwren Ramphocaenus melanurus
Version: 2.0 — Published July 16, 2020
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So far as is known, both sexes participate in all aspects of nesting, from construction to caring for the young. Nests are relatively bulky, open cups, built low to the ground and composed of a variety of vegetative materials. A full clutch consists of two white eggs, variably spotted and flecked with shades of brown and lavender. See below for details.
As would be expected for a species with such a broad range, there is no time of the year when individuals are not breeding within some portion of its range. Regardless, the following published breeding records are so few and are so scattered geographically that they provide only a rough idea of when breeding may be concentrated across various populations. Unfortunately, no single population has yet been studied in any detail.
- Mexico. On the Yucatan Peninsula (ardeleo) a male collected 17 May had slightly enlarged testes and two others taken 27 May and 1 June had fully enlarged gonads (29). There are a fair number of breeding records for the widely distributed rufiventris, here grouped by country and presented north-to-south.
- Belize. Russell (26) reported three active nests, one in the early stages of construction on 11 May, another under construction on 4 July, and a third with incubation underway on 12 June.
- Nicaragua. A male with moderately enlarged testes and an immature were collected 11 August, and a second immature was captured 1 July (363). Howell also found a nest with eggs on 1 July.
- Costa Rica: The first egg of a clutch was laid 14 April (7); more broadly, breeding is from April-June (6).
- Panama. On Barro Colorado Island an adult was observed sitting on a nest 23 April and another nest was under construction on 6 July (256). A young fledgling was being fed by adults on 24 July at 1000 m on Cerro Jefe (Ridgely in Wetmore et al. 16). A nest in central Panama held recently hatched nestlings in mid-July (367, 250). In addition, a female with enlarged ovaries was taken at Garachine on 8 May and a male in Veraguas was in full breeding condition on 25 May (368, 16). Together, these records give a breeding season in Panama that begins late March or early April and extends through at least early August.
- Colombia. Hilty and Brown (2) mention 12 Carriker specimens labeled as being in breeding condition from December to July in “nw Colombia,” without reference to which of the three subspecies these data might apply to (most likely sanctaemarthae, but possibly rufiventris or griseodorsalis). A September-collected specimen in breeding condition from Bolívar (2) is most likely representing griseodorsalis. In central Colombia (griseodorsalis), three individuals in reproductive condition were captured in March (36).
- Ecuador. A nest with incubation under way was found in eastern Ecuador (duidae) in early January (146).
- Peru. No data.
- Venezuela. A male collected in March in southwestern Venezuela (duidae) had moderately enlarged testes and no signs of molt (38).
- Surinam. At one nest of albiventris, the clutch was initiated on 26 October (369).
- French Guiana. Egg dates in French Guiana extend from early July to September, and what was possibly courtship feeding was observed in mid-December (294).
- Trinidad. Breeding records for subspecies trinitatis extend from February to August. These include nests where clutch initiation occurred in February (1 nest), April (1 nest), June (1 nest), and August (3 nests) (114, 20, 17, 31). Breeding activities may begin earlier in high evergreen forest than in drier monsoon scrub (114).
- Brazil. In east-central Brazil, Sick (157) reported specimens of amazonum with well-developed gonads in June and November, moderately development in October, December, and January. In southeastern Brazil (austerus), there are several records of reproduction. Nests with eggs were collected 7 June, 27 July, and 27 October (167). A juvenile was collected 23 December in Pernambuco (25). Sick (157) mentions a nest of the nominate subspecies under construction in December in Espírito Santo.
Tostain et al. (294) suggest that the preferred nesting locations are the tangled second growth surrounding openings in the canopy such as those around wind-blown trees. Jung and Mees (20) found a nest built in a fairly open portion of the forest understory, but noted that there were some patches of dense foliage close to the nest itself, as well as tangled vine thickets covering nearby saplings. Two described nests have been noted as occurring along small streams (250, 7). A nest of rufiventris in Nicaragua was built in young, roadside second growth, only 5 m from the road (363), and one in Belize was in mid-height second growth dominated by 30-foot cohune palms (26). Skutch (109) found two nests of pallidus in Venezuela, one beside an infrequently traveled path through low second-growth on a slope above a stream, and one in low (but dense) second-growth woods with little ground cover beneath the crowded saplings.
There are very few data concerning the nest construction process, though multiple authors have observed that both sexes participate in the process. Russell (26) found a nest that was “an amorphous mass of small twigs, fine bark, lichens, and moss” and, only ten days later, was well formed and ready to hold eggs (or close to it).
Structure and Composition
The nest of Long-billed Gnatwren is an open cup-shaped structure built of leaf strips, dried grasses, bark, and similar materials, twigs, flexible rootlets, dark fibers, and moss. The amount of moss used, and its use as an external decoration or as a structural material may vary, perhaps between populations inhabiting wetter vs. drier habitats. Alternatively, as there have been no quantified descriptions of nest composition, the perceived variation may be due, at least in part, to observer biases and differing levels of detail described between studies. Nevertheless, Skutch’s (7) Costa Rican nest with an “outer layer of green moss” must surely have had a very different appearance than Howell’s (363) Nicaraguan nest of matted bamboo leaves that includes no mention of moss, and he explicitly states that there were no lichens on the outside. Internally, most detailed descriptions of nests mention a lining of fine fibers and, in some cases, specifically black fibers (114, 363). Howell (363) describes these as "black, horse-hair like fibers," and unquestionably is referring to Marasmius sp. rhizomorphs, a potentially interesting material for nest building (371, 372, 373, 374). In fact, Howell (363) even opined that some of these fibers had been used to sew leaves together, a behavior that would be very interesting to document.
Skutch (7) provided a nice description of the nest of rufiventris. "The outer layer was of green moss, bound together with cobweb. The generous middle layer of the wall was composed of many small, dry leaves of the scrambling bamboo. A few fine fibers resting upon these suggested that a fibrous inner layer or lining was about to be added. The nest was well screened above by the flat, many branched shoots of the Selaginella and by the overarching frond of a fern."
The nest is rather large and bulky when compared with the bird, with a relatively narrow, deep cup in the middle. Most nest descriptions mention a fair amount of "extra" material, either in the form of a thick mat of leaves and twigs below the nest (114), or leaves and fibers dangling from the sides of the nest (250). In the former case, the material piled below the nest my serve to fill the gap between the nest itself and the supporting fork(s) of the substrate sapling. In the second case, the "skirt" of material hanging below the nest may help to obscure the form of the nest. Regardless of other functions of these uses of "extra" material, both no doubt serve a camouflaging function, giving the nest the overall appearance of vegetative detritus that has naturally collected in the fork of a bush or among small plant stems.
Skutch (7, 366) apparently is the only author to mention the inclusion of spider webs in the construction of gnatwren nests (as actually pointed out in his own 109paper). As Alexander Skutch's skill at observation and description can hardly be found wanting, two potential explanations seem plausible. Either other observers failed to note spider webs in the construction, or were examining museum specimens in which webs may no longer have been apparent. Alternatively, the use of spider webs may vary between regions, populations, individuals, or even nesting attempts. This last option is made all the more appealing when taking into consideration that when Skutch (109) found his second nest of Long-billed Gnatwren, in Venezuela (pallidus), he was unable to detect the use of spider webs in its construction.
Reported nest heights include: 10-30 cm (albiventris; n = ?; 294); 30, 45 cm (trinitatis; 20, 114); 15, 15, 20 cm (rufiventris; 250, 363, 7); 30 cm (duidae; 146); 30, 30, 60 cm (austerus; 167); 23 cm (pallidus; 109); 18, 46 cm (rufiventris; 26).
Substrate / Nest Support
Nests have been reported as being supported by a variety of substrates that suggest situations providing at least partial lateral and bottom support are important aspects of nest construction. Substrates/situations include: vertical fork of a sapling (trinitatis; 114); in a small shrub "built among the vertical shoots" (rufiventris; 250); in a clump of Selaginella [many small branches] (rufiventris; 7); attached to three vertical stems of a small shrub (rufiventris; 363); a vine tangle supported by "three or four stalks of vines and a few young air-roots" (trinitatis; 20); supported by the rosette of leaf petioles at the top of a small palm or Cyclanthaceae (duidae; 146); in a clump of grass ("touceira de capim") (austerus; 167); on the branch of a sapling ("feito em um galho") (austerus; 167); "leafless undershrub amid a rather open tangle of vines" (pallidus; 109); and on the slender bent stem of a shrub (pallidus; 109).
One nest in central Panama (rufiventris; 250) measured: 10.2 cm wide by 12.6 cm tall outside, the internal egg cup measuring 7.6 cm wide by 7.6 cm deep. Skutch’s (7) Costa Rican nest measured: 10 cm wide by 10 cm tall, with an internal cup 5.4 cm wide by 4.5 cm deep. A third nest of rufiventris, in Nicaragua, was 10.2 tall outside, with an egg cup 4.5 cm wide by 4.5 cm deep (363). A single nest of pallidus was more or less similar in all respects to the nests of other subspecies, but was decidedly broader and flatter, measuring 7.6 cm wide by 5.1 cm tall outside, with an egg cup 5.1 cm in diameter and 3.8 cm deep (109).
- R. m. rufiventris – 14.3 × 18.8, 13.5 × 19.1 mm (7); 12 × 16 mm (mean of 2 eggs[?]; 355); 13 × 17 mm (mean of 2 eggs[?]; 11).
- R. m. trinitatis – 13 × 16.7 mm (n = ?; 17); 13.2 x 17.3 mm (n = 1; 20).
- R. m. austerus – 13 × 18 mm (mean of 5(?) eggs; 167167).
- R. m. melanurus – 12 x 16 mm (mean of 2 eggs[?]; 355).
- R. m. pallidus – 12.9 × 17.8, 12.9 × 17.9 mm (109).
Color and Surface Texture
Descriptions of the eggs of Long-billed Gnatwren, though scarce, appear fairly consistent. Eggs are white to creamy white, with fine reddish-brown and brown spots and flecks, overlaid with pale lavender blotches. Markings tend to be across most portions of the eggs, but are more concentrated at the larger pole, without forming a distinctive "cap" (167, 146). Skutch (7) described a clutch of rufiventris as "white, lightly sprinkled with fine, pale cinnamon spots over the whole surface, with these markings heaviest at the thick end." The eggs of this subspecies were characterized by Nehrkorn (355) as "yellowish white with minute blackish brown dots and dashes at the blunt end," and by Howell (73) as simply "white, lightly speckled with brown." Of the eggs of trinitatis, Herklots (17) stated: The eggs are "dull white, spotted and speckled irregularly over the surface with two shades of pale brown; there may be an ill-defined wreath at the larger end." Junge and Mees (375) described a single clutch of trinitatis as "white with dispersed brown little spots." The only reported clutch of duidae (upper Rio Napo) were "white with a pale reddish tinge and red-brown speckling, heaviest at the broad end" (146). Pinto (167) described six eggs of austerus as pale yellowish white with reddish brown splotches and squiggles, superimposed on paler, bluish-gray blotches, concentrated near the larger pole. Finally, Nehrkorn (355) described the eggs of the nominate subspecies as yellowish white with lavender and violet-green ("violettgranen") spots, and scattered thin lines and squiggles of the same colors.
Both sexes develop brood patches (44).
"During incubation the little bird had a strange attitude, the bill protruding almost vertically and the eye showing just above the rim of the nest. The cup of the nest was small and deep, and this evidently caused this remarkable breeding-attitude" —Junge and Mees(20).
Not surprisingly, little has been published on the details of incubation behavior. Also unsurprisingly, almost all of the available information has been provided by the careful observations of Alexander Skutch in Costa Rica (rufiventris) and Venezuela (pallidus). Skutch's (7) observations on a pair of rufiventris at the nest are reproduced here:
"Morning and afternoon, the gnatwrens kept their two eggs continuously covered, save for the few seconds occupied by the change-over. Except for the first and last sessions of the day which were taken by the male, all of the sessions that I timed lasted for an hour or more. The longest session recorded was one hour and 3.5 minutes. The male’s sessions were, respectively, 49, 89, 95, 88, and 14 minutes. Those of the female were 60, 84, 85, and 90 minutes. If I was correct in believing that the male alone sang, and I found no evidence to the contrary, then the female also took the long night session; for the male sang as he came to replace her at dawn on April 20. The male’s five sessions totaled 335 minutes and averaged 67 minutes. The female’s four diurnal sessions totaled 319 minutes and averaged 79.8 minutes. In the course of the day the two shared fairly equally in incubation. Except in the early morning, when the male gnatwren's song announced his coming, the first intimation of the absent partner’s return was usually the departure of the incubating bird from the nest. Then, looking up into the tangle of vines and bushes in front of the blind, I would see the newly arrived bird, which apparently had approached through the top of the understory of the woods. Then, by a series of hops or very short flits, often clinging to quite vertical stems, the bird would slowly and deliberately make its way down to the nest. Meanwhile, the bird which had been sitting would make its way from the nest in the same deliberate fashion, hopping and flitting from twig to twig, sometimes sidling up a slender erect stem, until it had gained the altitude at which I had first seen the other, when it would take flight and immediately be lost to view amid the foliage. As the gnatwrens hopped down or up, they slowly and loosely wagged their long, narrow tails. The change-over on the nest was effected with the utterance of a few very low chips. Once settled in the nest, the birds always sat facing the river. From time to time they would rise up to turn the eggs. They sat nearly motionless at first, but toward the end of a long session they would grow restless and move their heads a great deal. Once a lizard, about a foot in length, passed beneath the low nest and hunted among the ground litter very near it. The male gnatwren, then on duty, sat calmly, only rising up slightly in the nest as the lizard passed."
Scattered observations of adult behavior during incubation suggest no major departures from the above passage by Skutch (7). Howell (73), studying a nest with two eggs in Nicaragua (rufiventris), noted that incubating adults would allow observers to approach very close to the nest before departing. He does not specifically mention any type of a broken wing display, but described adults' departure from the nest as fluttering away just above ground level, wording that suggests an attempt by the adult to draw the attention of the observer away from the nest.
At the single nest watched in detail (7), the two eggs hatched about one day apart.
Condition at Hatching
At hatching, gnatwrens are devoid of natal down, with dark pinkish skin, and bright yellow mouth linings (7).
Growth and Development
The contour pinfeathers of nestling Long-billed Gnatwrens begin to break through their skin when the young are about two days old. Six days after hatching, the contour feathers began breaking their sheaths and, by day 7 or 8 the young are well-feathered (7).
In general, little has been studied concerning ontogenetic shifts in nestling behavior and correlated changes to adult behaviors during the nestling period.
From Skutch’s (7) account, it appears that brooding adults become progressively more likely to sit tight over the nestlings at the approach of an observer. Seven and eight-day-old nestlings, well feathered by that point in development, were brooded for approximately one third of the three hours that Skutch (7) observed them in the morning, in two bouts lasting 21 and 34 minutes.
Although he was not able to distinguish between the sexes, Skutch (7) noted that both parents brought food, a single item at a time, frequently arriving together to feed the young. At a nest containing two nestlings, seven and eight days old, food was brought to the nest 13 times, with the frequency of visits gradually diminishing during the morning (7). In addition to becoming more reluctant to flush from the nest as the nestlings grew older, adults also showed increased vigilance during approaches and departures from the nest. From this same observation period, Skutch (7) states: "The parents still approached the nest from above, as they had when incubating, but now they seemed to make their way to it with exaggerated caution. I would first see them in the canopy of bushes and vines twelve or fifteen feet above the ground. From this point, they seemed greatly averse to employing their wings, and proceeded downward, wherever possible, by sidling down thin vertical stems, turning from side to side as they went, and loosely wagging their long, slender tails. When they had descended as far as possible on the first stem, they would deliberately shift to another, reaching it by hopping from one to another of the intermediate twiglets. They used their wings only when other modes of progression could not profitably be used. The usual path for reaching the nest was down the slender stem of a Piper bush which formed a convenient stairway from the clustered foliage above. However, the base of this stem was still several yards from the nest, so that when they had descended to within a few feet of the ground, they hopped to a thin, obliquely ascending vine that grew close to the nest. They then hopped upward along this vine for the distance of a yard. Although by this course they ascended higher above the nest, the vine led them to some low bushes with close-set branches through which they could hop downward to the nest. They did not stand upon the rim of the nest, as most birds do while feeding the young, but clung to the slender, upright stem of one of the supporting Selaginella plants which was two or three inches above the nest. Clinging there, they stretched downward and with the tip of the long bill placed the single insect into the upraised, gaping, yellow mouth of a nestling. The parents’ departure from the nest was a reversal of their arrival. Hopping, flitting, and sidling along slender stems as they had previously, but now proceeding upward, they slowly and deliberately worked their way up to the region where the foliage of the undergrowth was most dense. From this place they took flight. Only when they removed a dropping from the nest did they leave in a more direct manner, flying away with their burden through the lowest stratum of the forest vegetation."
Behaviors not Directly Related to Care of Young
Males, apparently, continue to sing a good deal, especially in the early morning, well into the nestling period. Generally, singing was performed in the area around the nest, but not within its immediate vicinity. Skutch (7) noted that both parents were conspicuously quiet when near the nest.
Ability to get Around, Feed, and Care for Self
At fledging, the young apparently are only able to fly very short distances (7).