Forests of the tropical Andes and adjacent western Amazonia harbour exceptionally high levels of biodiversity and the region also hosts concentrations of range-restricted and threatened taxa (Brooks et al. 2002, 2006, Orme et al. 2005, Rahbek et al. 2019). In this broad region, local species richness in birds is thought to be greatest in humid foothills at c.900–1,000 m (Stotz et al. 1996, Herzog et al. 2005, McCain 2009) where forested slopes of the east Andes transition between lowland and upland (‘cloud’) forests, an ecotone mediated by the local formation of ground-level cloud banks via adiabatic cooling. Chapman (1926) noted this transition from the Amazon to the Andes nearly a century ago in reviewing collections made by the professional firm Olalla & Hijos on the slopes of Volcán Sumaco in the east Andes of Ecuador together with temperature readings taken on his instruction. He observed that when ‘they reached the old town of San José on the flanks of Sumaco, the birds sent to us indicated that they had reached the lower borders of the Subtropical Zone, and this supposition was supported by the temperature record’.

Both effective conservation planning and an improved understanding of the processes that have shaped the distribution of biodiversity require detailed inventories at all spatial scales, but few Andean foothill sites are well studied. Here, we describe the results of an eight-year bird survey at the historical collecting locality San José de Sumaco first described by Chapman (1926), a humid forested site at c.950 m elevation on the lower, eastern flank of Volcán Sumaco in Orellana province, east Ecuador, and at the intersection of the Andean and Amazonian biomes (Vivanco de la Torre et al. 1962). We present a preliminary list of 477 species recorded there through February 2023 (Appendix 1) and review historical collections made by the professional firm Olalla & Hijos a century ago. Together, these findings document a rich avifauna and highlight the conservation value of a threatened region.

Study area and Methods

Study area.—Field work was based at Bigal River Biological Reserve Research Station (Fig. 1). The Bigal River Biological Reserve (BRBR) is a private conservation area protecting c.1,000 ha of forested terrain in the east Andean foothills of Ecuador at 750–1,000 m (Fig. 1; see also Freile et al. 2015). The reserve lies near the western border of Orellana province on the lower, eastern slope of Volcán Sumaco (00°32′32″S, 77°25′40″W; Fig. 1) and lies southeast of, and adjacent to, the far larger Parque Nacional Sumaco-Napo-Galeras (PNSNG), a national protected area (IUCN category II) established in 1994 that covers 2,061 km² (IUCN 2023) of mostly forested terrain at 600–3,732 m.

The region receives prevailing winds from the east and the climate is extremely wet, with low seasonal variability, a relative max. rainfall in July, and the highest regional rainfall (>4 m per annum) expected at elevations of c.900–1,000 m (Laraque et al. 2007). While rainfall may be heavy, ground-level cloud cover (mist) is infrequent. A poorly defined period of less frequent precipitation lasts from January to February, corresponding to the Northern Hemisphere dry season, and dry periods may also occur in August, during the Southern Hemisphere dry season.

This hilly region is drained by the río Suno to the south and the río Bigal to the north (Fig. 1). The dominant vegetation type is tall, humid broadleaf evergreen (terra firme) forest (‘Western Amazon Sub-Andean Forest’ sensu Báez et al. 2010) with some canopy emergents >40 m tall. This forest is rich in tree species but dominated by Iriartea deltoidea and other palms (Brokaw & Ward 2023). Numerous small streams, swamps and damp ravines are features. Stands (some >5 ha) of Guadua angustifolia bamboo occur in some areas. Higher terrain on exposed ridges with poor soil or subject to desiccating winds hosts relatively drier forest with fewer epiphytes and more open understorey. Whereas most of the study area is clothed in tall (>30 m), closed-canopy, primary forest, some successional vegetation occurs in abandoned clearings and along a disused road. We also surveyed and include observations from nearby degraded areas, including cattle pastures, crop fields, second growth and forest fragments south of the BRBR Research Station (Fig. 1).

The area is the source of historical bird specimens collected during the early 20th century by the Olalla family (Olalla & Hijos; Fig. 2) at San José de Sumaco and synonymous or nearby localities including ‘San José Abajo’, ‘San José Nuevo’ and ‘San José Viejo’ during 1923–35, with labels bearing the names ‘Olalla Y Hijos’, ‘Olalla Y Hermanos’ and ‘Carlos Olalla’ (Chapman 1926, Paynter 1993, LeCroy & Sloss 2000, Wiley 2010; see below).

Methods.—We visited the San José de Sumaco area, usually for 11-day periods, during each August and January in the years 2015–23 for a total of 198 observer days. Birds were recorded via field observation, photography and sound-recording of vocalisations. To establish a baseline for future monitoring, formal survey methods were adopted in 2017 and followed during 18–29 August 2017, 3–14 January 2018, 13–24 August 2018, 5–16 August 2019, 6–17 January 2020 and 1–14 December 2020. FE conducted point counts along six (1.25 km) transects, all located within 2 km of the RBBR Research Station at elevations of 900–1,000 m (Fig. 1). Five transects followed pre-established footpaths in primary forest, and the other an unpaved road extending to the south and traversing disturbed habitats. On six mornings during each 11-day sampling period, FE conducted six counts on one of six transects at fixed points separated by 200–250 m. Point counts were conducted between sunrise (05.55–06.00 h) and two hours later. During a ten-minute period stationed at each point, FE recorded all species heard or seen. The daily order of point sampling was changed in successive visits so that each point was visited at least once closest to dawn. Not all species were equally well sampled by this method. Canopy species and species that vocalise infrequently may have been overlooked, while conspicuous or persistently vocalising species may be over-represented. To help overcome reporting bias, each point count was recorded using a Zoom 5 digital recorder equipped with omni-directional microphone and FE's written notes were later compared with the recordings, and any omissions and errors corrected. FE also compiled 280 20-species ‘MacKinnon lists’ (Bibby et al. 2000, MacLeod et al. 2011) and we collected additional observational data while making non-systematic visual and auditory observations along trails and roads. Relative abundance for species detected using standardised methods was determined based on encounter frequency (Appendix 1). Daily checklists including photographs and sound-recordings are available on eBird ( https://ebird.org/home).

Figure 1.

Map (A) showing the San José de Sumaco area and location of RBBR Research Station (star) in the Bigal-Suno interfluvial. Olive shading = eastern edge of Parque Nacional Sumaco-Galeras. Red lines = BRBR trails. Dark blue = approximate survey area. Light blue = hypothesised area collected by Olalla field parties. Dotted lines = roads. Numbered localities according to Paynter (1993) are (1) San José Nuevo (00°26′S, 77°20′W), now Pato Rumi, Comuna San José de Payamino (2) Payamino (00°30′S, 77°17′W), now Centro Poblado Parroquia, San José de Payamino (3) San José Viejo (00°32′S, 77°25′W) and (4) Avila (00°38′S, 77°25′W), now Avila Viejo. Map (B) shows position of the site in western Orellana province, Ecuador. Grey shading = areas above 800 m.

Figure 2.

An Olalla & Hijos field party in an undated photograph (archives of the Dept. of Ornithology, American Museum of Natural History, New York)

We reviewed 10,312 photographs of birds captured by an array of 13 motion-activated passive infrared detection cameras (camera-traps) used to monitor terrestrial mammals. Camera-traps were deployed in all months during 2013–20 and sited mainly along trails and streamsides inside tall broadleaf forest. One was positioned at a ‘mineral-lick’ visited by both mammals and birds. We sorted the photographs into 1,169 independent events (separated by >1 hour) and calculated frequencies for each species represented (Table 1). We also sought documented observations from other observers (see Acknowledgements) and reviewed records in the citizen science database eBird (2022).

Finally, we searched the regional literature and queried natural history collection databases for records of bird specimens collected at ‘San José de Sumaco’ and synonymous localities including ‘San José Nuevo’ and ‘San José Viejo’ (Chapman 1926, Paynter 1993, LeCroy & Sloss 2000, Wiley 2010; see below).

Results

Species richness and evidence.—We compiled records of a combined total of 526 species representing 51 families at San José de Sumaco (Appendices 1–2). Of this total, 477 species are considered documented, including 452 with vouchers in the form of a specimen in a museum collection, photograph or sound-recording (Appendix 1). Thirty-five species were identified from camera-trap photographs (Table 1). Forty-nine species were reported from the site by other observers but lack a voucher and were not confirmed in our field work (Appendix 2). Minimum criteria for inclusion of undocumented reports from other observers in the main list (Appendix 1) include multiple reports of species considered reliably identified and biogeographically likely (i.e., there are documented records from similar elevations at sites near the study area). Of the 477 species considered documented, 456 are permanent breeding residents, 17 are seasonal residents or transients including 16 boreal migrants and one austral migrant.

We found natural history collection database registrations or published records for 1,652 bird specimens representing 306 species (Appendix 1) collected at San José de Sumaco and synonymous localities (see below) during 1923–35, and we examined 356 specimens representing 247 species in museum collections (principally the American Museum of Natural History, New York; AMNH) or via photographs (Appendix 1; see Acknowledgements). The continued presence of most species known from historical collections is confirmed by recent reports, but 22 species are known only from specimens. Appendix 1 presents a complete list of documented species with IUCN status (2023), relative abundance, and documentary evidence. Noteworthy records are detailed below.

Species accounts

NOCTURNAL CURASSOW Nothocrax urumutum

Thought to range mainly below 400 m, but recently documented at 1,481 m (Medrano-Vizcaíno & Rueda 2018). Considered rare and poorly known but recent camera-trap studies have contributed to our understanding of this enigmatic species (Solano-Ugalde & Real-Jibaja 2010, van der Hoek et al. 2018, Link et al. 2022). N. urumutum was among the most frequently recorded bird species in camera-trap photographs at c.950 m (Table 1), always during daylight hours (06.00–17.00 h). The images also document regular visits to a mineral lick, often in groups of up to four (Fig. 3), behaviour first reported recently for the species (Griffiths et al. 2020). We twice encountered family groups with dependent young (during daylight hours) and detected N. urumutum regularly by voice at night as birds called from midstorey perches.

TABLE 1

Thirty-five bird species recorded at 13 camera trap stations in 10,312 photographs (n = 1,669) events at San José de Sumaco. Independent events are defined as photo series separated by >1 hour.

WHITE-CHINNED SWIFT Cypseloides cryptus

Rare and poorly known in Ecuador (Marín 1993, Ridgely & Greenfield 2001). Review of several thousand photographs suggests that C. cryptus is the commonest of the three Cypseloides at San José de Sumaco. As with the previous species, C. cryptus was typically seen in mixed groups of swifts that included larger numbers of White-collared Streptoprocne zonaris and Chestnut-collared Swifts S. rutilus. Cypseloides were sometimes present overhead throughout the day but were most often seen in early evening while feeding or transiting over the RBBR Research Station clearing (Fig. 4).

Figure 3.

Camera-trap photograph of four Nocturnal Curassows Nothocrax urumutum emerging from a subterranean mineral lick (A), Red-winged Wood Rail Aramides calopterus (B; Frederick Ertl), Solitary Eagle Buteogallus solitarius (C; Frederick Ertl) and Orange-breasted Falcon Falco deiroleucus (D; Andrew C. Vallely)

LITTLE WOODSTAR Chaetocercus bombus

Rare and poorly known. Considered Near Threatened by IUCN (2023) and Vulnerable in Ecuador (Freile et al. 2019). Most Ecuadorian records are from the Pacific slope, but it is known also from the east slope foothills of Morona-Santiago and Zamora-Chinchipe provinces in southern Ecuador (Chapman 1926, Collar et al. 1992, Janni 2004). Chapman (1926) mentioned two female specimens in AMNH from ‘below San José’, documenting occurrence on the east slope north to the Volcán Sumaco region (see Zimmer 1953, Ridgely & Greenfield 2001) but there have been no reports from the area since. We located just one female at AMNH (179065). The label reads ‘below San José de Sumarco [sic]. E. Ecuador. Tropical Zone’ Date: Mch. 16, 1923. On the reverse is ‘Olalla & Sons’. The original collectors tag reads ‘16 Marzo 1923 H.P. [= hembra pequeño indicating a female with small ovaries] S. José abajo. Olalla y Hijos Ecuador’. Although this specimen shows a few rufous feathers on the rump, it agrees with other female specimens of C. bombus at AMNH (n = 11) and differs from those of Gorgeted Woodstar C. heliodor cleavsi (n = 6) in its narrower-based bill, straighter (less decurved) culmen, and more extensive black on the rectrices (Fig. 4).

Figure 4.

White-chested Swift Cypseloides lemosi (A; Andrew C. Vallely), White-chinned Swift C. cryptus (B; Andrew C. Vallely), Napo Sabrewing Campylopterus villaviscensio (C; Andrew C. Vallely), specimen of Little Woodstar Chaetocercus bombus (AMNH 179065) right, with specimen of Gorgeted Woodstar C. heliodor cleavsi (AMNH 180154), left, for comparison, at the American Museum of Natural History, New York (D), Pink-throated Brilliant Heliodoxa gularis on nest (E; Chris Fischer)

BLACKISH ANTBIRD Cercomacroides nigrescens

Chapman (1926) treated foothill populations of C. nigrescens as C. n. approximans and did not list San José de Sumaco (or any of its variant) for the species although he mentioned two males and two females taken by Olalla & Hijos at ‘Lower Sumaco’. Shortly after, Zimmer (1931) described C. n. aequatorialis, designating a female from that series taken on 9 January 1926 as the holotype (AMNH 184517). The allied lowland form C. fuscicauda described by Zimmer (1931), now treated as a separate species, Riparian Antbird, following Mayer et al. (2014), is not known from San José de Sumaco. Identification difficulties and a paucity of comparative material in museums has impeded understanding of the distributions of these forms. Two female specimens in MLZ (7639, 7688) were registered as Dusky Antbird C. tyrannina, later identified as C. fuscicauda (R. Terrill in litt. 2022), but from photographs they appear to be Black Antbird C. serva. A male (USNM 323078) taken by Olalla & Hijos on 20 April 1924, at ‘Abajo, San José’, was received by USNM in exchange from AMNH in 1930 and catalogued as ‘Cercomacra serva’. The USNM label bears the notation ‘Cercomacra nigricans A.W.’ (= Alexander Wetmore) but this determination is puzzling in view of the bird’s uniform dark tail and is perhaps best explained as a lapsus for Cercomacra (now Cercomacroides) nigrescens. We found C. nigrescens infrequently and regard it as rare at San José de Sumaco where it is presumably represented by Zimmer’s aequatorialis.

Figure 5.

Specimen (MLZ 33855) of Shrike-like Cotinga Laniisoma elegans at the Moore Laboratory of Ornithology, Los Angeles (A), Striated Antbird Drymophila devillei (B; Chris Fischer), Spectacled Redstart Myioborus melanocephalus (C; Michel Mifsud), Red-crested Finch Coryphospingus cucullatus (D; Chris Fischer), Slate-coloured Seedeater Sporophila schistacea (E; Chris Fischer)

CINNAMON MANAKIN-TYRANT Neopipo cinnamomea

Rare in western Amazonia (Ridgely & Tudor 2009). In Ecuador, known mainly from below 400 m, but recently reported at c.1,000 m in Morona-Santiago province (Pozo-Zamora et al. 2022). We encountered solitary individuals in forest understorey at c.950 m and regard the species as rare at San José de Sumaco. One was photographed by FE on 25 August 2017.

Figure 6.

Sonogram of Acadian Flycatcher Empidonax virescens call notes (A), and photographs of the same bird (B–C; Chris Fischer)

Discussion

Historical collections and the locality San José de Sumaco.—In 1923–24, the firm Olalla & Hijos (Fig. 2), professional collectors then under contract to Frank M. Chapman at AMNH, worked at a series of collecting stations along an elevational transect on the east slope of Volcán Sumaco. Their foothill station, San José de Sumaco, was worked principally during 2–31 March 1923 and 12 March–29 April 1924 (Chapman 1926, Paynter 1993, LeCroy & Sloss 2000, Wiley 2010). The resulting collections include at least 213 specimens taken in 1923 and 746 specimens in 1924. Much of this material is referenced, albeit without collection registration numbers, in Chapman (1926), and most is at AMNH. This material includes the holotypes of at least six taxa including Leucopternis princeps zimmeri Friedmann, 1935, Megascops guatemalae napensis (Chapman, 1928), Dysithamnus mentalis napensis Chapman 1925, Myrmothera campanisona signata J. T. Zimmer, 1934, Pachysylvia semibrunnea leucogastra Chapman, 1924, and Microbates cinereiventris hormotus Olson, 1980.

In the following years (1927–35), while Alfonso & Ramón Olalla worked in Peru and Brazil, Carlos Olalla remained in Ecuador (Wiley 2010) and returned to collect on the lower slopes of Volcán Sumaco during 1–18 April 1927, 1–27 January 1929, 13–25 August 1929, 9 July–28 August 1932, 18–25 September 1932, 10–20 March 1933 and 11–12 August 1935. This later material (1927–35), numbering >400 specimens, is held mainly at MLZ and appears never to have been published. Unlike the 1923–24 material, museum database registrations for specimens taken during 1927–35 appear to be incomplete and additional specimens taken in the area by Carlos Olalla are likely to be held elsewhere.

Chapman (1926) did not have coordinates or an elevation for San José de Sumaco and admitted some ambiguity by noting ‘the site of the town is shifted in response to the need for fresh ground for crops, the present San José being apparently lower than the preceding one (San José Viejo)' but added that ‘it should be understood that all American Museum specimens recorded from ‘San José' or ‘below San José’ are from or near San José de Sumaco’. Because Chapman did not explicitly address the locality name ‘San José Nuevo’ vs. ‘San José de Sumaco’, the precise geographic origin of specimens bearing that locality is somewhat less certain. Some with the locality ‘San José Nuevo’ may have been taken at elevations below those areas that we surveyed most intensively. Paynter’s (1993) coordinates locate San José Nuevo in the lowlands c.10 km north-east of San José Viejo (Fig. 1), whereas a hand-drawn map, prepared by W. T. Atyeo in the archives of the Dept. of Ornithology at AMNH, locates San José Nuevo in the foothills west of that position. See Fig. 1 for the present-day names of these communities and their geographic positions according to Paynter (1993). The collector’s translated itinerary in the archives of the Dept. of Ornithology at AMNH (LeCroy & Sloss 2000) records the field parties, in both 1923 and 1924, collecting at Avila (Fig. 1) then moving north, paralleling the río Suno, following the east (left) bank to reach San José de Sumaco (= San José Viejo in Paynter 1993), before turning west to ascend the volcano. Review of the itinerary followed by Carlos Olalla using collection database records suggests he followed a similar route in later years, approaching the area from either Avila or Concepción in the south. We conclude that the Olalla & Hijos collecting station ‘San José’ (and variants) is best understood as the interfluvial of the ríos Suno and Bigal, from elevations of c.1,000 m (where the two rivers approach most closely) extending several km north-east and south across a fan-shaped area of c.200 km² on the eastern flank of Volcán Sumaco to at least 500 m, but possibly as low as 400 m (Fig. 1). The upper elevation portion of this area includes the present-day RBBR Research Station and our survey transects (Fig. 1).

The common place name ‘San José’ has invited some confusion in the literature and in museum database registrations involving Olalla collecting stations on Volcán Sumaco, certain localities on the upper Pacific slope in Pichincha province (e.g., ‘Cerro San José’), and on the east slope in Morona-Santiago province (‘San José’ and ‘Río San José’). Paynter (1993), for example, listed the locality ‘San José, Ecuador’ worked by Enrico Festa in 1896 (Salvadori & Festa 1899, 1900) as ‘unlocated’, and considered it to be ‘presumably in the vicinity of Río Suno’, and ‘probably San José Nuevo’, but the itinerary for Festa given by Chapman (1926) makes clear that it is correctly traced to Morona-Santiago province in southern Ecuador.

Noteworthy elevational records.—We report high-elevation records for 80 species, and low-elevation records for nine species (Appendix 1). We define noteworthy elevational records as recent observations at c.950 m at RBBR that are outside the elevational ranges given in standard references for Ecuadorian birds (Ridgely & Greenfield 2001, Freile & Restall 2018) although, in several cases, comparable records have recently been reported from foothill sites on outlying Andean ridges in southern Ecuador (e.g., Solano-Ugalde & Real-Jibaja 2010, Freile et al. 2014, 2022, Pozo-Zamora et al. 2022). We do not include 19 species documented only by specimens because the precise elevation at which they were taken is uncertain and some may have been taken as low as 500 m (e.g., Brown Nunlet Nonnula brunnea, Chestnut-belted Gnateater Conopophaga aurita, Banded Antbird Dichrozona cincta). In a few cases, we found temperate zone species (e.g., Blue-and-back Tanager Tangara vassorii, Black-capped Tanager Stilpnia heinei) at c.950 m during periods of cold, wet weather. Other cases of presumed elevational movement in species that are generally distributed at higher elevations include a Spectacled Whitestart Myioborus melanocephalus photographed near c.950 m (Fig. 5), sight records of White-tailed Hillstar Urochroa bougueri, and specimens of Long-tailed Sylph Aglaiocercus kingii and White-booted Racket-tail Ocreatus underwoodii (Appendix 1). These low-elevation records all refer to canopy or edge-inhabiting, frugivores or nectivores that may undertake facultative elevational movements in response to weather conditions (Levey & Stiles 1992, Boyle et al. 2010). Thirty-four records concern lowland species found 400 m or more above their published elevational ranges for Ecuador in standard references (Ridgely & Greenfield 2001, Freile & Restall 2018). In contrast to the low-elevation records (involving relatively vagile species) many of these high-elevation records (n = 80) involve sedentary, forest-interior species (e.g., Striated Antthrush Chamaeza nobilis, Black-tailed Leaftosser Sclerurus caudacutus, Cinereous Mourner Laniocera hypopyrra) that we detected regularly and that we assume are breeding residents. Of these 80 high-elevation records, all pertain to species not represented in the Olalla collections from a century ago and are consistent with a pattern of upward elevational range shifts in montane forest birds also reported at other re-surveyed sites in the East Andes and attributed to climate change (Freeman et al. 2018, Neate-Clegg et al. 2021).

Bamboo specialists.—Bamboo specialisation in Amazonian birds was first described and is best known from south-west Amazonia, especially southern Peru (Kratter 1997, Parker et al. 1997), but is also a feature of bird communities in north-west Amazonia, where stands of Guadua bamboo are less common and tend to be smaller. We regularly detected three obligate or near-obligate bamboo specialists (sensu Kratter 1997; Large-headed Flatbill Ramphotrigon megacephalum, Black-and-white Tody-Flycatcher Poecilotriccus capitalis and Striated Antbird Drymophila devillei). The first two species were present in most stands of bamboo, including smaller areas and those near or adjacent to forest edge, whilst Drymophila devillei was found only in the largest stand covering >20 ha. Most bamboo specialists recorded are insectivores, but two granivores, Slate-coloured Seedeater Sporophila schistacea and Slaty Finch Haplospiza rustica, were also documented. The occurrence of the former appeared to be unrelated to the availability of a Guadua angustifolia seed crop, and we did not witness seeding (‘masting’) bamboo, nor did we see evidence of mass die-off during our survey. A single specimen of Slaty Finch from San José de Sumaco (AMNH 179715) may represent an unusually low occurrence but the precise elevation where the specimen was taken is uncertain. In addition to these specialists, some globally widespread species such as Scale-crested Pygmy Tyrant Lophotriccus pileatus and Ornate Antwren Epinecrophylla ornata are locally associated with, or perhaps confined to, Guadua angustifolia bamboo stands at San José de Sumaco.

Historical change.—We describe a forest avifauna generally similar to that documented by the Olallas a century ago (Appendix 1) although we note that large-bodied species (e.g., Mitu salvini, Nothocrax urumutum), now recorded frequently by camera traps, are not represented in the Olalla collections. Non-forest bird communities have undergone more profound changes with the recent creation of large clearings planted with nonnative grasses (pasture). In some cases, noteworthy elevational records (Appendix 1) involve non-forest species that are probably expanding upslope with deforestation (e.g., Ruddy Ground Dove Columbina talpacoti, Yellow-headed Caracara Milvago chimachima). Whilst the Olalla collections include some species typical of forest edge, small clearings and successional habitats (e.g., Great Antshrike Taraba major, Bluish-grey Saltator Saltator coerulescens, Orange-backed Troupial Icterus croconotus, Yellow-rumped Cacique Cacicus cela), none of the grassland species now common in non-forest areas at San José de Sumaco are represented (e.g., Yellow-browed Sparrow Ammodramus aurifrons, Blue-black Grassquit Volatinia jacarina and various Sporophila). Scant information is available to characterise ecological conditions during the time of the Olallas field work at San José de Sumaco, but a contemporaneous account by the adventurer G. M. Dyott (1929) suggests the area was then at least thinly populated. We assume that the area then consisted of a matrix of tall humid forest, secondary forest and some shifting cultivation as Chapman's remarks suggest (1926; see above). Twenty-two species represented in the Olalla collections were not confirmed during our field work, and we are not aware of any documented records from the area (Appendix 1). These species may no longer occur, may be locally rare, or may have been taken at (and are perhaps locally confined to) elevations below the areas we surveyed most intensively at c.950 m.

Species richness.—Freile et al. (2015) reported 460 species from BRBR but did not include a full list. White & Patiño (2018) reported 340 species at San José de Payamino in the nearby lowlands (Fig. 1). This study increases the number of species documented from San José de Sumaco by 202 over the total of 275 listed by Chapman (1926). The resulting total of 477 species includes 425 ‘core’ tall-forest species (Appendix 1). Whilst variation in methods, spatial extent and period of study preclude rigorous comparison (Remsen 1994, Lees et al. 2014, Robinson et al. 2018) the apparent richness of the San José de Sumaco avifauna remains notable because the site is relatively restricted in area, is dominated by a single major natural vegetation type (upland terra firme forest) and lacks the major aquatic habitats and riparian forest types (e.g., várzea) that have ‘inflated’ estimates of diversity at Amazonian lowland sites (Stotz et al. 1996, Lees et al. 2013).

Local and global rarity.—Rarity presents a methodological impediment to survey effort but is also a general feature of tropical forest avian communities (Wallace 1878, Thiollay 1994, Jankowski & Rabenold 2007). A sample of 280 twenty-species ‘MacKinnon lists’ (Fig. 8; see Methods) captured just 73% (n = 347 species) of the total known species richness (n = 477 species, Appendix 1) and we consider more than half of the 477 documented species to be locally rare or very rare, with 110 species (22%) known from three or fewer reports. A generally positive relationship between abundance and geographic area suggests rare species are especially vulnerable to extirpation (Kattan 1992, Gaston & Blackburn 2000). Against this general pattern we note that several species recognised by Stattersfield et al. (1998) as East Andes of Ecuador endemics, and at least formerly considered Vulnerable or Near Threatened based on their restricted ranges (IUCN 2016, 2023), are among the more commonly detected species of forest interior at San José de Sumaco (e.g., Ecuadorian Piedtail Phlogophilus hemileucurus, Pink-throated Brilliant Heliodoxa gularis and Phylloscartes gualaquizae).

Endemism and biogeography.—Patterns of endemism are of interest to both conservationists and biogeographers and several general features of the San José de Sumaco assemblage are noteworthy in this respect. First, analyses of the distribution of restricted-range species reveal a global concentration at the Andean / Amazonian interface (Orme et al. 2005, Herzog & Kattan 2011, Fjeldså 2012), a pattern that invites explanation as the outcome of historical processes and identifies the region as a logical priority for conservation or a ‘hotspot’.

Second, the site lies near the western margin of the North Amazon (Napo) area (Haffer 1974, Cracraft 1985), and among the range-restricted taxa present are both Napo endemic forms with sister species confined to the adjacent lowland Inambari area (e.g., Fulvous Antshrike Frederickena fulva, Dusky Spinetail Synallaxis moesta, Golden-winged Tody-Flycatcher Poecilotriccus calopterus), and East Andean endemic taxa with sister lineages in foothill and highland areas outside Amazonia including the Pacific slope (e.g., Heliodoxa gularis, Orange-eared Tanager Chlorochrysa calliparaea, Pipreola chlorolepidota; Stattersfield et al. 1998, Hazzi et al. 2018). Sympatry in taxa representative of distantly related Andean and Amazonian centres of endemism indicates a compound history (Haffer 1974, Cracraft 1985, Stattersfield et al. 1998, Hazzi et al. 2018) and this is consistent with studies elsewhere in the East Andean foothills that have reported low phylogenetic similarity between assemblages at 900 and 1,200 m (Dehling et al. 2014), a pattern marking the interface between older Amazonian and younger Andean biota (Weir 2006, Fjeldså & Irestedt 2009).

Figure 7.

Proportional representation in the resident San José de Sumaco avifauna (n = 461 species; Appendix 1, excluding 16 long-distance migrant species) by ‘centre of abundance’ categories (median, continent-wide elevational range as given by Stotz et al. 1996). Lower tropical (n = 327, 71.1%), Hill tropical (n = 30, 6.6%), Upper tropical (n = 86, 18.9%), Middle montane (n = 12, 2.6%), Upper montane (n = 3, 0.7%).

Figure 8.

Species accumulation curve for the San José de Sumaco avifauna drawn from twenty-species MacKinnon lists (n = 280 lists, see Methods).

Third, whilst the San José de Sumaco avifauna is dominated by widespread Lower tropical species, it also includes a set of range-restricted Hill tropical or Upper tropical species (sensu Stotz et al. 1996; Fig. 7) that are confined to a narrow elevational band below the distribution of subtropical (‘cloud’) forests (e.g., Heliodoxa gularis, Pipreola chlorolepidota, Phyllomyias gualaquizae) and at least formerly considered Vulnerable or Near Threatened (IUCN 2016, 2023). The congruent foothill distributions of these taxa, each arisen from a widespread Andean clade, form a nested area within the greater East Andes area of endemism and suggest that while relationship patterns among the broad, disjunct Amazonian areas are relatively well studied, both discovery and relationships among areas in the tropical Andes remain poorly resolved and additional subcentres (nested areas) are likely to be recognised as more distributional data, taxonomic revisions and fine-scale regionalisation become available.

Finally, the San José de Sumaco avifauna includes taxa thought to have relatively extensive distributions in foothills, but considered rare (e.g., Cypseloides lemosi, Snowornis subalaris and Plain-backed Antpitta Grallaria haplonota), with some exhibiting circum-Amazonian distributions (e.g., Tinamus tao, Laniisoma elegans and Synallaxis moesta), as well as forest-dependent lowland species, in some cases with distributions reaching trans-Andean regions, but always at low densities (e.g., Buteogallus solitarius, Neomorphus geoffroyi, Nyctibius aethereus). While considered widespread, such taxa deserve attention from biogeographers and conservationists alike.

Conservation.—Long-term studies of tropical forest bird communities have demonstrated losses of functional and taxonomic diversity in both disturbed (Palacio et al. 2019, Gómez et al. 2021, Luther et al. 2022) and undisturbed landscapes (Blake & Loiselle 2015), but the forest avifauna we describe appears largely intact, suggesting the area still presents a valuable opportunity for conservation. For example, large-bodied, terrestrial species sensitive to local extirpation from hunting pressure (Peres 2001) including Psophia crepitans, Mitu salvini and Nothocrax urumutum are regularly recorded by camera-traps (Table 1). Neotropical forest understorey species often decline following disturbance (Laurance et al. 2011, Palacio et al. 2019), including ground insectivores and obligate ant-following species, but are regularly observed at San José de Sumaco. Large raptors typically confined to extensive undisturbed areas including Buteogallus solitarius, Ornate Hawk-Eagle Spizaetus ornatus, Morphnus gujanensis and Harpia harpyja are present, and the regular occurrence of 16 species of boreal migrants, notably including the Near Threatened Contopus cooperi and Setophaga cerulea (IUCN 2023) further underscore the area's value for conservation. Twenty-two species known only from historical specimens (Appendix 1) have diverse ecologies and do not appear likely to have declined locally because of anthropogenic change. Under current IUCN treatments (2023), the San José de Sumaco avifauna includes 15 species classified as Near Threatened and seven as Vulnerable (2023; Appendix 1). We regard five of the combined 22 Near Threatened and Vulnerable species as common, fairly common or uncommon at San José de Sumaco. At the national level, the Ecuadorian Red List (Freile et al. 2019) treats one species as Critical (Buteogallus solitarius), three as Endangered (Falco deiroleucus, Military Macaw Ara militaris, Drymophila devillei), 26 as Near Threatened and 11 as Vulnerable.

Following the construction of the Archidona to Loreto road the larger Sumaco region has undergone intensive deforestation and agricultural expansion (Sierra 2000). Pastures and other degraded anthropogenic landscapes now extend along a network of new roads stretching north from the paved highway connecting the cities of Loreto and Coca (Fig. 1). Whilst subtropical forests at higher elevations on Volcán Sumaco are protected in PNSNG and are thought to remain largely pristine, the lower eastern slopes have been approached from the south by an expanding agricultural frontier, and deforestation now threatens to sever the corridor that currently connects the subtropical forests of PNSNG and the lowland forests of San José de Payamino. Ecotonal areas (i.e., gradients of temperature or moisture) may be especially sensitive to synergistic effects of climate change and deforestation (Linck et al. 2021), but also offer valuable opportunities for conservation as even small additions to protected areas in regions with high beta diversity (i.e., mountain slopes) can be expected to incorporate many new species including range-restricted taxa and narrowly adapted local populations (Bush 2002, Jankowski et al. 2009). Irrespective of this, efforts to protect the remaining forests of San José de Sumaco from human disturbance and secure a biological corridor are already warranted because local extinctions and biodiversity erosion driven by the cascading effects of fragmentation are now well documented in Neotropical forests (Lees & Peres 2006, Laurance et al. 2011). While the East Andes and Amazonia have been found to rank below critically threatened regions of the Pacific slope in national-scale prioritisation schemes (Sierra et al. 2002, Cuesta et al. 2017), the Sumaco region still emerges as a priority for biodiversity conservation when feasibility is included as a criterion (Lessmann et al. 2014).

Conclusions

The collections assembled by the Olallas on the slopes of Volcán Sumaco a century ago helped to shape Chapman's (1926) understanding of elevational zonation and avian distributions (Kattan et al. 2016). Today, the lower East Andean slopes are known to harbour some of the world's richest local bird communities, an avifauna also characterised by high irreplaceability owing to the uniqueness of its evolutionary history (Haffer 1990, Herzog & Kattan 2011). Whilst these patterns are still under investigation, few undisturbed forested transects remain available for conservation or study (Stotz 1998, Kattan et al. 2016). San José de Sumaco hosts a rich avifauna including rare, threatened and poorly known taxa. The site's geographic position in an undisturbed foothill gradient, together with its history as the source of important collections underscore its value for conservation and continued study.

We hope that the results presented here will serve as a baseline for future monitoring efforts in the face of potential anthropogenic change including elevational range shifts and biodiversity degradation. Continued field work, ideally including a sustained programme of mist-netting, could add significantly to the list presented here by documenting species hitherto known only from sight records and uncovering the presence of additional rare resident species, invasive or expanding species, and migrants. We encourage visitors to document and publish their observations from San José de Sumaco.