Thamnistes

Thamnistes is mainly found in foothill forests from Central America to the northern Andes, with a particular concentration in low to medium altitude forests with moist, well-developed canopies. Its range also includes nearby lowlands and is especially common in sloping ecosystems on the Pacific side.^[7] These areas are mostly tropical or subtropical forest environments rich in biodiversity and complex vegetation structure. They provide rich food sources and breeding conditions for this genus.

While Thamnistes as a genus occupies a broad tropical range, its two currently recognized species—Thamnistes anabatinus and Thamnistes aequatorialis—show some differences in their geographic distributions and ecological preferences.

Thamnistes anabatinus is widely distributed across Central America and parts of northwestern South America. Its range extends from southern Mexico, including the states of Oaxaca and Chiapas, through Guatemala, Belize, Honduras, Nicaragua, Costa Rica, and Panama. In South America, it is found in the western montane regions of Colombia and Ecuador. It is especially common in humid forest environments at elevations between 0 and 400 metres above sea level. These forests are species-rich ecosystems with distinctive vegetation, suitable for foraging and breeding. The Extent of Occurrence (EOO) of T. anabatinus is approximately 2,340,000 square kilometres. According to the IUCN assessment criteria, this value falls far short of the geographic range threshold for the‘Vulnerable’ category. Therefore, it is classified as Least Concern on the IUCN Red List. Its generation length is approximately 3.2 years.^[8]

Because of ongoing classification uncertainties within the genus, there is currently no precise global population estimate. However, the overall population is declining slowly but steadily. According to BirdLife International, populations of T. anabatinus are projected to fall by approximately 10% to 19% over the ten-year period from 2016 to 2026.^[8]

Thamnistes aequatorialis is mainly found in the eastern Andes of South America, including Colombia, Ecuador, Peru, Bolivia, and Venezuela. It inhabits subtropical and tropical humid lowland forests from 0 to 1,700 metres above sea level. It is usually active in the middle and upper strata of the forest. This species is a non-migratory bird and inhabits its range throughout the year. According to BirdLife International, its ‘Extent of Occurrence (EOO)’ is approximately 1,950,000 square kilometres. Although this range is slightly smaller than the 2,340,000 square kilometres of its close relative Thamnistes anabatinus. It still falls far short of the geographic range thresholds for the “Vulnerable” category in the IUCN assessment. As a result, the species is rated Least Concern on the IUCN Red List. Its Generation Length is approximately 3.0 years. Its global population shows a continuous downward trend.^[9] Compared with T. anabatinus, T. aequatorialis has a slightly smaller range and lower altitudinal ceiling. However, both species have similar ecological habits and conservation status. They are dependent on moist forest environments and are not currently listed as threatened.

The feeding behavior of Thamnistes reflects typical antbird traits and involves strong interactions with other animals in the ecosystem. Its primary feeding strategy of is known as "army-ant following". This strategy can be categorized into three types of followers: occasional followers, regular followers, and obligate followers. Although this strategy is considered to have ancient evolutionary origins, its specific evolutionary pathways are still remain unclear. Current study suggests that the behaviour evolved through a gradual increase in dependence. No reversal has been detected over the long evolutionary period.^[10] Such irreversible evolutionary pathway is uncommon among animals and is of particular interest in evolutionary biology.

This feeding behaviour of Thamnistes reflects the common ecological adaptive characteristics of the family Thamnophilidae. Like many other antbirds, Thamnistes preys on arthropods and small vertebrates that are flushed from the forest floor by army ants such as Eciton burchellii.^[10]

Brumfield et al. (2007) classified the ant-following behaviour of antbirds into three types: occasional followers, regular followers, and obligate followers. Thamnistes is classified as a regular follower—it takes full advantage of foraging opportunities when encountering an army ant but does not depend on it for daily survival.^[10] One of the main advantages of this behaviour is that it helps birds find more food by following army-ant swarms. These swarms flush out a variety of prey, including insects and other small invertebrates. The following Thamnistes capture the exposed prey. This flexible behavioral strategy allows individuals to make use of ant-driven feeding opportunities when available, while still being able to forage on their own when army ants are not present. Moreover, this ant-following behavior may also affect how the ant swarms move, suggesting a two-way interaction between the birds and the ants.

Their study concluded that army-ant-following to be a phylogenetically conserved foraging behavior with a long evolutionary history.Its evolutionary pathway is characterized by a gradual progression from occasional to obligate following. Brumfield et al. (2007) found that this behavior evolved in a consistent sequence across multiple antbird lineages. It begins with occasional following, advancing to regular following, and eventually developing into obligate dependence.^[11] Brumfield et al. (2007) specifically noted: "We did not observe evolutionary reversals from the most specialized obligate state," ^[11]indicating that reversals from high to lower levels of dependence were not detected in their phylogenetic reconstruction. This kind of irreversible evolutionary pattern is relatively rare in behavioral studies and suggests that once established. It tends to remain stable within specific lineages. Although the study documented one exception—a case in which Myrmeciza melanoceps may have shifted from regular to occasional following —this was treated as an isolated instance that does not alter the overall conclusion that ant-following behavior tends toward evolutionary stabilization.^[11]

In addition to the phylogenetic pathways, this study also integrated geographic distribution data. It found a strong relationship between ant-following strategies and local competitive pressures. For example, regular followers are more likely to evolve in regions with fewer obligate followers. In areas such as western Amazonia, obligate followers are more abundant and diverse, whereas regular followers such as Thamnistes are typically found in areas with lower colony densities and reduced competition—such as the forests of the Brazilian Atlantic coast and west of the Andes.^[12]

Furthermore, in ecosystems with a high abundance of army ants and greater species competition, there is increased evolutionary pressure toward obligate dependence. In contrast, environments with lower competitive intensity continue to provide ecological space for regular and occasional followers. This behavioural hierarchy is also observed spatially: larger birds typically dominate the front edge of a moving army-ant colony. While smaller and medium-sized birds, such as Thamnistes, are more often found feeding along the middle or rear sections..^[13]

Overall, Thamnistes, as a representative of regular followers, demonstrates a middle stage in the evolution of foraging strategies among antbirds. Its behavioural stability, ecological flexibility, and phylogenetic position make it a valuable model for understanding the behavioural ecology of tropical forest bird communities.

The vocalizations of Thamnistes play an important role in taxonomic classification and exhibit distinctive features within the family Thamnophilidae. According to Isler and Whitney (2017), vocalizations plays a key role in identifying species in this group. Since suboscine birds do not learn their songs, their vocalizations are instinctive and remain stable over time.^[14] This stability makes vocalizations useful for distinguishing species.

In their study, the vocalizations of Thamnistes were classified into two main categories: songs and calls. Songs were further subdivided into slow songs and fast songs based on rhythmic tempo. Calls were categorized as basic calls, abrupt calls, and other calls according to syllable structure and behavioural context.^[15]

Slow songs were recorded in all studied populations and were relatively simple and repetitive in structure. In contrast, fast songs were only found in two populations—aequatorialis and rufescens—and were characterized by a greater number of syllables and a faster tempo. Although the fast and slow songs shared a basic structure, they showed consistent and distinguishable differences in rhythm and acoustic parameters, and are therefore considered separate song types.^[16]

Among slow songs, all populations except rufescens demonstrated similar structures. It typically begins with a flat, low-pitched introductory note, and followed by descending elements with minimal variation between syllables. In contrast, the slow songs of rufescens showed significant differences, including a faster tempo, higher frequency peaks, and a gradual upward trend in syllable frequency.^[16] These distinctions further support its treatment as a separate taxonomic unit.

For fast songs, rufescens also displayed markedly different characteristics compared to aequatorialis. Its fast song not only had more syllables and a quicker tempo, but often included one to three highly distinctive "slipping" introductory notes—a structural feature not observed in other antbirds. This unique acoustic pattern provides a diagnostic feature for rufescens.^[16]Moreover, recordings of the same individual showed that slow and fast songs can occur in sequence within a single vocalization event. This phenomenon may provide new insights into song function and mechanisms of individual recognition.

Basic calls were found in most populations, with the exception of rufescens. These typically consist of one to three syllables. The three-syllable form includes a rising note, a lower inverted V-shaped note, and a high-pitched terminal syllable with a dipping frequency. Some individuals produce only partial versions of this call. In contrast, data on the calls of rufescens are limited, with only two high-pitched sliding syllables recorded; these may have diagnostic significance for species identification.^[17]

Another type, the abrupt call, was recorded in some populations and consists of a sharp, inverted V-shaped syllable. This call is short in duration, has a high peak frequency. However, due to limited recordings, inter-population variation could not be assessed. Additionally, several other unclassified call types were noted, but not analyzed further because of insufficient data.^[18]

In addition, study of Isler and Whitney (2017) also assessed vocal variation across geographic populations by analyzing spectrograms and evaluating twelve acoustic parameters. These included note count, tempo, frequency, duration and variation of both notes and intervals.^[19] These acoustic features provide a basis for comparing populations from different regions and are often used to identify possible sound differences between species. The findings offer quantitative evidence of structural differences in vocalizations within the genus Thamnistes and serve as a useful reference for species delimitation.

This study also concluded that these vocal differences were consistent across populations and were statistically significant. Vocalizations in Thamnistes are considered an important mechanism of reproductive isolation. Such differences may play a role in mate recognition or territorial behavior, potentially contributing to the maintenance of population separation.^[20] These results provide further support for understanding evolutionary relationships and species boundaries within the genus, They are particularly helpful in groups where morphological differentiation is limited.