This paper presents the results of the first ornithological exploration of the uplands of New Guinea's Van Rees Mountains. The mountains of New Guinea consist of the Central Range rising to almost 5,000 m, plus ten lower outlying ranges along New Guinea's north and north-west coast (Fig. 1). Of the ten outliers, the Van Rees Mts. are the lowest, the last to be explored ornithologically, and (as we found) home to the smallest number of upland bird species (37 species). They are close to the richer outlying Foja Mts. to the east, and to the Central Range to the west and south, with which they share all of their bird species. Conversely, only a fraction of the upland species of the Foja Mts. and Central Range occurs in the Van Rees Mts.; most are absent. The absentees consist not only of high-elevation species whose absence in the low Van Rees Mts. is unsurprising, but also some conspicuous species that are common in the Foja Mts. and Central Range at elevations considerably below the Van Rees summits, and hence might be expected in the Van Rees Mts.

Why are those species absent in the Van Rees Mts. despite the availability of much apparently suitable habitat? This question exemplifies a general problem of biogeography: to understand why different patches of similar habitat in the same faunal region support different subsets of that fauna. The Van Rees Mts. offer a good opportunity to study variation in upland species richness among mountain ranges differing in elevation, area and connectivity. They also prove to be a suitable location for studying other phenomena, including bird distributions in a large area uninhabited and apparently not hunted by humans; and the origin of non-forest species in a forested environment. A companion paper (Diamond & Bishop submitted) will examine how avian elevational ranges differ between the Van Rees Mts., Foja Mts., and other New Guinea sites.

Figure 1.

Mountains of New Guinea. The Central Range is cross-hatched. In solid black and named are ten outlying mountain ranges detached from the Central Range, and rising from the lowlands along New Guinea's north and north-west coasts. Also solid black and named are the mountains of Yapen, which today is an island separated by a shallow strait, but which at Pleistocene times of low sea level was part of the New Guinea mainland. This paper compares the Van Rees outlier with the nearby Foja outlier.

Background

Physical environment.—Lowland north-west New Guinea's interior is dominated by the Lakes Plains (Dutch name: Meervlakte), a nearly flat mountain-ringed basin at an elevation below 100 m and approximately 10,000 km² in area. The western and eastern halves of the Lakes Plains consist of the basins of the Rouffaer River (Indonesian name: Tariku) and Idenburg River (Taritatu), respectively, which join to form the Mamberamo River, western New Guinea's largest river, draining to the north coast. The mountain walls of the Lakes Plains consist of the Central Range on the south side, the Van Rees Mts. on the north-west side, and the Foja Mts. on the north-east side. At the west end of the Lakes Plains, the Van Rees Mts. and the Central Range are joined by a saddle c.350 m high (Fig. 2; I. Woxvold pers. comm.). The Van Rees Mts. and Foja Mts. are separated by the Mamberamo River gorge. Because the gorge is narrow and steep-walled for much of its length, the Van Rees Mts. and Foja Mts. are separated by as little as 1.3 or 3.0 km at the 300-m or 500-m contours, respectively (I. Woxvold pers. comm.).

We conducted 22 aerial surveys of the Van Rees Mts. by helicopter and small fixed-wing aircraft. Thirteen surveys were incidental to flights en route to and from the airfield at Danau Bira in the Van Rees Mts. (see below). The other nine were dedicated surveys over the mountains; six of the nine involved close scrutiny of the highest peaks, whose locations we confirmed using Google Earth. We thereby found that the highest Van Rees peak lies at c.02°63′S, 137°19′E, with an elevation of c.1,262 m. The elevation of 1,430 m shown on current maps is inaccurate and too high. The Van Rees Mts. are thus much lower than the Foja Mts. (summit 2,218 m).

Figure 2.

Elevational terrain of north-west New Guinea, kindly provided by Iain Woxvold; the Van Rees Mts. are separated from the Foja Mts. to the east by the narrow Mamberamo River gorge, whose bottom is at an elevation of <100 m. The west end of the Van Rees Mts. is joined to the Central Range by a saddle lying at c.350 m. VR = Van Rees Mts., F = Foja Mts., CR = Central Range, LP = Lakes Plains. Field sites: 1 = Pionierbivak on the Mamberamo River; 2 = Danau Bira; 3 = Doufo; 4 = our Van Rees mountain camp; 5 = Biri; 6 = JD′s 1979 Foja camp at the base of his Foja coastal transect; 7 = JD′s camp on his 1981 Foja inland-facing transect; 8 = the Kwerba transect of Beehler et al. (2012), rising from the lowlands of the Foja inland watershed.

Climate.—We are aware of rainfall data for just one location in the Van Rees Mts.; Kasonaweja (near the former Dutch Pionierbivak), at an elevation of 40 m on the Mamberamo River in the mountains' north-east corner. Mean annual rainfall there is 3.3 m distributed rather evenly or irregularly through the year, with no month receiving <21 cm mean rainfall (Brookfield & Hart 1966). The nearest other rainfall records are for seven stations on the coast west and north-west of the mountains, 30–130 km away, where annual rainfall is 2.7–3.8 m, distributed rather evenly year-round or with a modest peak in December–March and a minimum in May–October. All of these records are for lowland stations; rainfall in the Van Rees Mts. presumably increases with elevation, as elsewhere in New Guinea.

Habitats.—On our overflights, we saw that almost the entire area of the Van Rees Mts. is covered by forest. Anthropogenic non-forest habitats are very limited, because the human population is so sparse and subsists mainly by hunting and gathering (see next section). We observed five types of natural non-forest environments, whose importance for the presence and evolution of non-forest New Guinea birds we will discuss: rivers, landslides, lakes, mud volcanoes, and bare cliff faces.

Rivers range from the Mamberamo, its western arm the Rouffaer, and their small tributaries, down to mountain streams. As habitats for birds, these rivers provide the water itself, riverbanks and bars, and riparian vegetation.

Figure 3.

A mud volcano near Danau Bira, photographed from the air. It was formed by mud extruded from a hydrothermal vent. The grey area of bare mud is surrounded by concentric rings of progressively taller regenerating vegetation (© Bruce Beehler, reproduced with permission from New Guinea: nature and culture of Earth's grandest island; Beehler 2020)

Numerous landslides, probably provoked by earthquakes or by heavy rains, are recognisable in overflights as areas either of bare mud, dead trees, or non-forest successional vegetation. One large landslide clearing with an estimated area of 1.5 km² lay 2.8 km from our mountain camp. We chose our camp's location because of its proximity to a small landslide of bare ground that permitted our helicopter to land near the highest Van Rees peak, in an otherwise-forested environment where a helicopter could not land.

The largest lake in the Van Rees Mts., Lake Holmes (Danau Bira), was created by a mudslide that dammed a river valley c.100 years ago (D. Price pers. comm.). Many dead trees that formerly grew in the valley before its inundation are still standing in the shallow lake. From the air we saw two ponds at the base of the large landslide 2.8 km from our camp, also presumably created by mud dams.

Mud volcanoes, a characteristic feature of the north-west New Guinea lowlands, are circles of bare mud surrounded by concentric rings of progressively taller regenerating vegetation. They are created by extrusion of mud from a central vent. For a photograph of one of these striking structures 3 km from Danau Bira, see Fig. 3. In 1979 JD walked through forest to that mud volcano and found it dry and safe to walk on.

Finally, from our overflights we observed several vertical white cliffs, presumably limestone, at the highest Van Rees elevations, including at the summit of the tallest peak above our camp.

People.—The human population of the Van Rees Mts. is just c.4,000, divided among speakers of ten languages belonging to two Papuan language families (the Lakes Plains family and East Geelvink Bay family) (Lewis 2009). Most numerous, and occupying the largest fraction of the Van Rees Mts., are speakers of the Bauzi language, who we encountered at our Danau Bira field site. The other language group that we encountered was Biritai at Biri village. Subsistence is by hunting and gathering, fishing, sago harvesting, and recently some agriculture. First contacts of some groups by missionaries were still occurring during our field work in 1979 and 1982.

People in the Van Rees Mts. live in a few scattered villages and groups of huts at low elevations. In our overflights we saw no signs of human presence above 500 m. We also saw no such signs in the 20 days that we spent at our mountain camp at an elevation of 799 m (Diamond et al. 1999). Around our camp the presence and tameness of tree kangaroos (possibly Grizzled Tree Kangaroo Dendrolagus inustus), prized by hunters as New Guinea's largest native land mammal, testified to the absence of human hunters and visitors. The nearest village to our camp that we detected in our overflights was 41 km distant.

Other bird surveys.—To our knowledge, prior to our work there had been only two previous studies of birds in the immediate environs of the Van Rees Mts. Both were at the Dutch base of Pionierbivak at an elevation of 40 m on the Mamberamo's west bank in the north-east corner of the Van Rees Mts., opposite the present-day Indonesian settlement of Kasonaweja (02°28'S, 138°03'E). The first of those studies was by W. C. van Heurn, who collected several hundred specimens of 95 bird species between 15 June and 26 August 1920 and 10 December 1920 to 9 January 1921 (Hartert 1932). The second was by J. P. K. van Eechoud, who collected 250 specimens of 87 species in July–November 1939 (van Bemmel 1947). The collections combined comprise 127 species, of which 55 species were obtained by both collectors. Of the 127 species, we observed all but 12 (mostly cryptic or rare species, plus two confined to the Mamberamo River and its riverine forest that we did not visit). Both collectors evidently remained in or near the Mamberamo floodplain and did not ascend the mountains, because their collections include only three species of the lower slopes: Pheasant Pigeon Otidiphaps nobilis, New Guinea Vulturine Parrot Psittrichas fulgidus and Torrent Flycatcher Monachella muelleriana.

Other studies at sites more distant but still relevant to the Van Rees Mts. are the following: Van Balen et al. (undated) surveyed birds in 2007 at five lowland sites just north of the Van Rees Mts., and at three sites on or near the coast at the mouth of the Mamberamo River.

Surveys were conducted by us in the western Lakes Plains at Doufo near the Rouffaer River (03°14'S, 137°25'E; 136 m elevation); at Kordesi on the Rouffaer River (c.03°04'S, 137°94'E; 101 m); and at Weri south of the Van Daalen River, a south-west tributary of the Rouffaer River (c.03°36'S, 137°95'E; 112 m). Surveys were conducted in the eastern Lakes Plains near the Idenburg River by W. C. van Heurn at Prauwenbivak (Hartert 1932), by A. L. Rand (1942) at Bernhard Camp, and by van Balen et al. (2002) at Dabra near Prauwenbivak.

Surveys in the Foja Mts. by JD and by Bruce Beehler and his colleagues were summarised by Beehler et al. (2012). We (Diamond & Bishop 2020) summarised studies in the mountains of Yapen Island by us and by others.

The most complete survey of a nearby section of the western Central Range was by G. Stein (Hartert et al. 1936). Mack & Alonso (2000) reported surveys of three sites near or south of the Wapoga River drainage that separates the Central Range from the Van Rees Mts.

We (Diamond et al. 2019) surveyed lowland birds at the head of Geelvink Bay and on the isthmus south of Geelvink Bay, west of the Van Rees Mts. Finally, various other authors have made smaller collections or studies on New Guinea's north coast north of the Van Rees Mts. between Geelvink Bay and Sarmi.

Our surveys.—We observed birds at three sites in the Van Rees Mts. One site is Danau Bira, the former aviation base (now closed) of the Summer Institute of Linguistics, in gently undulating terrain on Bauzi tribal land (02.47°S, 138.02°E) at 366 m in the north-east part of the Van Rees Mts. The base's habitats, besides open water, fringing marsh, and inflow and outflow streams, were fragmented forest with many tall trees still standing, second growth, the open airfield, and open land around the buildings, surrounded by intact forest. We accessed the forest by following an inlet stream in a canoe, and by a trail connecting the base to the nearby mud volcano.

We made six visits to Danau Bira in four different years and five different months of the year: JD on 4–6 and 18–22 October 1979, and 23–29 January and 13–17 February 1981; and JD & KDB on 4–6 July 1990 and 19 April 1993. At Danau Bira we observed 152 species ourselves, ranging from 91 to 125 species on each of our longer visits. An additional 13 species (denoted by [a] in column 3 of the Appendix) were convincingly identified and described to us by D. Price, a biologist and linguist resident at Danau Bira.

Our second site was Biri village at c.02°82′S, 137°83′E (143 m elevation), visited on 6–10 October 1979 by JD. Biri lies on a small northern tributary of the Rouffaer River where gentle hills drop into the Rouffaer's flatter floodplain. Near the village are primary forest up to 50 m tall, forest on the floodplain with some standing water, an old oxbow now filled with marsh 1 m tall, and successional forests on former gardens.

Our remaining site, which was our principal locality for surveying upland birds, was a camp that we made at 02°68′S, 136°88′E; 799 m elevation. We chose this site because it was a small landslide with steep mud walls upslope but with a small flat area at which our helicopter could land; in the otherwise forest-covered terrain, this was the closest possible landing site to the highest Van Rees peak for our helicopter. Whilst the landslide itself was bare mud and evidently recent, a small section of the ridge immediately upslope supported grass, second growth and low trees as evidence of an older landslide in the area. We previously described how a dozen species of parrots, pigeons and other frugivores visited the landslide daily to consume soil (‘geophagy’) with high concentrations of minerals that bind poisonous or bitter secondary compounds contained in ingested fruits (Diamond et al. 1999).

We occupied our camp from 15 September to 4 October 1994. From there we cleared a system of trails whose highest point was 1,206 m on the summit ridge. From that highest accessible location on the ridge rose a bare vertical-walled rock tower that was impossible or imprudent, and probably ornithologically unrewarding, to climb. Its top, measured as 1,262 m using Google Earth and aircraft overflights, is the highest point in Van Rees. One of our trails descended until it reached 534 m at a river, which we followed by wading down to 494 m. Thus, the elevational range of our observations was 1,206 to 494 m. To listen for nightbirds, and to hear the dawn chorus at different elevations, we also established two further camps at 1,049 m and 1,022 m, where each of us overnighted for two nights and spent parts of three days.

All of the terrain accessible and visible from our mountain camp was sloping, with no extensive flat areas. We estimated the canopy as reaching a max. height of 45 m. There were many treefalls and areas of somewhat open forest, some of them possible results of former landslides. Only at our highest elevations just below the summit ridge did we encounter modest amounts of moss and epiphytic growth; there was no heavily mossed forest, as is common at higher elevations elsewhere in New Guinea. Recognised tall trees included casuarinas and oaks. The largest animals that we saw were tree kangaroos, and varanid lizards up to c.1.8 m in length. We detected no signs of human presence.

Methods.—Our methods of observation were similar to those described for our studies elsewhere in New Guinea (Diamond & Bishop 2015, 2020, 2021, Diamond et al. 2019). Briefly, because there were no motor roads or motor vehicles at any of our four study sites, our observations were made on foot trails, plus from canoes on Danau Bira and by wading in streams at Danau Bira and our mountain camp. We devoted much effort to recording vocalisations with Sony TCM 5000 EV tape-recorders, playing back unidentified vocalisations in the field to attract and identify singers, and re-listening to recordings in camp each day because our directional microphones often captured vocalisations that we had not noticed in the field. We paused at fruiting and flowering trees where birds gathered. We began observing by 05.00 h to detect nocturnal birds. Elevations of all significant observations were measured using Thommen altimeters or a Garmin GPS. We did no collecting.

At Biri JD was constantly accompanied by villagers, who pointed out and identified birds that we and they saw or heard together. They told us about their experience of each species, and described others that lived in the area of their village but which they did not encounter with us. These conversations were carried out in Indonesian (Bahasa Indonesia), and bird names used were in the local Biritai language—names that JD went to much effort to identify and learn. In our list of species for Biri we include three distinctive and vocal large non-passerines (denoted by [b] in column 3 of the Appendix) that Biri villagers described to JD, and that can be expected at Biri, but JD did not observe himself. At our mountain camp we were constantly accompanied by three Ketengban-speaking men who had worked with us in the previous year at their village of Okbap in New Guinea's Star Mts., and whom we brought with us to the Van Rees Mts.: Robert Uropka, Peter Kaliala and Benni Kediman. We already published the list of Ketengban-language names for 169 bird species that we learned from them (Diamond & Bishop 1999).

Our nomenclature follows that of Beehler & Pratt (2016).

Species totals

The Appendix lists all bird species recorded for the Van Rees Mts. by us at our three sites, and by van Heurn or van Eechoud at Pionierbivak at the mountains' base. A concluding section of species accounts provides significant observations for species of particular interest.

A total of 223 species has now been recorded for the Van Rees Mts. (including Pionierbivak). Table 1 summarises for each site the number of species recorded, the number of specimens collected (if any), and the number of days of observation or collecting. The number of species per site reflects mainly the circumstances of field work, rather than intrinsic differences between sites. The lowest number (119 species) is for Biri, at which JD spent only five days. At each of our three sites we observed more species in 5–24 days than van Heurn or van Eechoud collected in three or six months respectively at Pionierbivak, because we devoted almost all of our time to observations and had the advantage of tape-recorders, whereas van Heurn and van Eechoud's records were based only on specimens they collected; they did not report observations. Our high species total for Danau Bira may be real: its habitats included not only forest but also the lake, marshes, and a large area of anthropogenic clearing lacking at other sites.

Non-breeding winter visitors

Table 2 lists the 17 winter visitors recorded: nine from Australia, eight from the Palearctic. At least two and probably five species are represented by both Australian winter visitors and a resident New Guinea population. In New Guinea, Oriental Dollarbird Eurystomus orientalis is represented by the paler duller Australian winter visitor E. o. pacificus and by the darker brightly coloured resident E. o. waigiouensis, which we easily distinguished in the field under good light conditions and close views. In the case of Brush Cuckoo Cacomantis variolosus, during the Australian winter months of June–July van Heurn collected specimens at Pionierbivak that Hartert (1932) identified as Australian C. v. dumetorum (now synonymised with C. v. variolosus). We found this cuckoo species common and singing at all three of our sites during the Australian spring months of September–October, when one would expect Australian winter visitors to be back in Australia, so we assume that our observations were of north-west New Guinea resident C. v. infaustus. Two herons at Danau Bira, Intermediate Egret Ardea intermedia and Pied Heron Egretta picata, plus Rainbow Bee-eater Merops ornatus, are widespread in New Guinea as visitors from Australia but also breed in southern New Guinea (Beehler & Pratt 2016); their status at Danau Bira is unknown. The other four Australian species and the eight Palearctic species in Table 2 occur in New Guinea solely or overwhelmingly as winter visitors, not as breeders.

TABLE 1

Species totals for Van Rees sites. Numbers of species recorded at the site (column 3), and (column 5) numbers of upland species as defined in the text, taken from the Appendix. Number of winter visitor species recorded (column 4) from Table 2. The four sites are listed in order of elevation. Differences in species totals among sites partly reflect differences in survey effort (lowest for Biri), but also differences in site habitat diversity (highest at Danau Bira).

TABLE 2

Non-breeding winter visitors from Australia (Au) or the Palearctic (Pal) recorded in the Van Rees Mts. at the sites coded in the right-hand column (see Appendix for codes). Fourth column: √ = breeds locally or infrequently in New Guinea, but most New Guinea observations are believed to involve non-breeding visitors; subsp = Van Rees record of an identifiably distinct migrant subspecies at the site or sites coded in the right-hand column, and also of a distinctive New Guinea resident breeding population at other Van Rees sites (see Species Accounts for details).

As elsewhere in New Guinea, we found the winter visitors overwhelmingly in open non-forest habitats. Our sole observations of winter visitors in forested areas were of Oriental Cuckoo Cuculus optatus, Channel-billed Cuckoo Scythrops novaehollandiae and White-throated Needletail Hirundapus caudacutus, of which we had respectively three, two and one sightings of individuals flying over (but not perched in) forest at our mountain camp. We also saw Cuculus optatus repeatedly in open habitats at Danau Bira, our site with the most extensive open habitat, and correspondingly with the greatest variety of winter visitors (11 species). Other than the 17 winter visitors, all other species listed in Table 1 as recorded for the Van Rees Mts. are New Guinea residents.

Upland species

Upland species number.—As in our other recent papers (Diamond & Bishop 2015, 2020, 2021), we define ‘upland species’ as those largely confined to sloping elevated terrain, and absent from the flat lowlands at or near sea level. That definition proves to be more useful and less arbitrary than defining ‘montane species’ as those largely confined to elevations above an arbitrary figure, such as 1,000 m or 1,700 m. We have discussed in more detail elsewhere (Diamond & Bishop 2015, 2020) the advantages of this definition, and the ambiguities and practical issues in applying it. By this definition, we recognise 37 Van Rees species as upland species (= S_up), denoted by an asterisk in the Appendix.

To place this number of 37 in context, Table 3 compares it with S_up values for New Guinea's nine other outlying mountain ranges. It is apparent that S_up increases with outlier-range elevation, but the correlation is not perfect. (Especially the S_up value of only 44 species for Cyclops is lower than expected for its elevation of 2,160 m, probably due at least in part to the Cyclops' small area.) Van Rees, as the lowest outlier (1,262 m), has the smallest S_up value.

TABLE 3

Upland avifaunas of Van Rees Mts. and the other outliers. Column 1: NCR = North Coastal Range. Column 3: the number of upland species on that outlier. Values for outliers other than Van Rees are taken from Table 1 of Diamond & Bishop (2021).

Is that value of upland species likely to be an underestimate? Yes, undoubtedly, but by how much? We surveyed three Van Rees sites, and van Heurn and van Eechoud a fourth. As apparent from Table 1, column 5, which lists the number of upland species recorded at each site, that number increases with elevation. Of our 37 upland species, we recorded 17 species at two or three sites, and 20 species at just one site (in all but one case, our highest-elevation site the mountain camp). Of the 20 species recorded at only one site, 16 were recorded repeatedly or were common, but four were recorded just once or twice: Wallace's Owlet-nightjar Aegotheles wallacii, Blue-collared Parrot Geoffroyus simplex, Spotted Honeyeater Xanthotis polygrammus and Yellow-legged Flycatcher Kempiella griseoceps. That is, there were four species that we were ‘lucky’ to observe, whilst it was unlikely that we would overlook the other 33 species because we found them at multiple sites (17 species) or observed them repeatedly at a single site (16 species). These results suggest that we observed most of the upland species actually present at our sites, but that there were some species we were ‘unlucky’ not to see, along with the four species to which we must attribute our mere one or two observations to good luck. Of course, as we discuss under Outlook, additional high-elevation Van Rees sites should be surveyed; they are likely to yield additional upland species.

Comparison with Foja.—The outlier to which the Van Rees Mts. are nearest is Foja, separated from Van Rees by the narrow Mamberamo River gorge. Because Foja is much higher than Van Rees (2,218 vs. 1,262 m), it supports many more upland species: 95 vs. 37 (Table 3). Unsurprisingly, of the 37 upland species of Van Rees, all except three or four have been recorded in Foja: Aegotheles wallacii, Red-fronted Lorikeet Charmosyna rubronotata and Kempiella griseoceps, and perhaps Barred Cuckooshrike Coracina lineata for which there is a single uncertain Foja record (Beehler et al. 2012: 101). Several or all of these species are likely to be present but overlooked in Foja, because Kempiella griseoceps and Charmosyna rubronotata are otherwise known from seven and eight outliers, respectively, and all four are variously cryptic and /or difficult to identify and /or uncommon.

While at least most Van Rees species occur in Foja, conversely most Foja species (58 of 95) are unrecorded from Van Rees. Especially ‘flagrant’ Van Rees absences are those of Foja species common or abundant on other outliers at elevations well below the Van Rees summit, which are noisy or behaviourally conspicuous, and occur on all nine other outliers (Fairy Lorikeet Charmosyna pulchella, Red-breasted Pygmy Parrot Micropsitta bruijnii, Black-bellied Cicadabird Edolisoma montanum, Island Leaf Warbler Seicercus poliocephalus, Black Fantail Rhipidura atra, White-faced Robin Tregellasia leucops), or on eight of the nine (Red Myzomela Myzomela cruentata, Black-winged Monarch Monarcha frater).

Table 4 (column 4) lists, for each Van Rees upland species, the number of outliers occupied by that species. The average number of outliers occupied, out of a total of ten outliers, is eight. Nine Van Rees upland species occupy all ten; 11 or six species occupy nine or eight outliers, respectively; and just four occur on fewer than six. Conversely, 35 of New Guinea's 193 upland species are absent from every outlier and confined to the Central Range, and 28 species occupy only a single outlier plus the Central Range (Diamond & Bishop 2021). Evidently, some upland species are good colonists and succeed in occupying most or all outliers, whilst others are poor colonists and fail to occupy most or all outliers.

How can these species differences be explained? Why do the Van Rees Mts. conform to these trends in some cases and violate them in others, by respectively supporting or not supporting populations of many species that exist on most or all other outliers?

Availability of habitat at elevations compatible with a species' requirements is a major factor explaining why a given upland species is or is not present on a given outlier (Diamond & Bishop 2020, 2021). The 35 Central Range species absent from all outliers are mostly species that live at high elevations on the Central Range, and for which all outliers are too low to provide suitable habitat. The 28 Central Range species present on a single outlier are confined to one of the two highest outliers, Vogelkop and Huon. Let's examine presence or absence of Foja species in the Van Rees Mts. from this perspective. Van Rees' summit is 1,262 m; Foja's summit is 2,218 m; many Foja populations have elevational floors higher than the Van Rees summit; and 15 Foja populations have floors between 1,600 and 1,637 m, much higher than the Van Rees summit. Do Foja elevational floors help to explain presence or absence of Foja species in the Van Rees Mts.?

Beehler et al. (2012) tabulated elevational distributions of Foja populations as measured by JD in 1979 and 1981, and by Beehler and his colleagues in 2005, 2007 and 2008. We divided the 95 Foja upland species into 13 sets differing by 50 or 100 m in elevational floor. For each set, we calculated the percentage of its species that do occur in Van Rees. Fig. 4 plots that percentage as a function of the set's mean elevational floor in Foja.

TABLE 4

Van Rees upland species on other outliers. The first and second columns list all upland species in the Van Rees Mts. The third column checks those Van Rees species recorded from the Foja Mts. (from Beehler et al. 2012). The last column informs on how many of New Guinea's ten outlying ranges (including Van Rees) the species has been recorded (from Diamond & Bishop 2020). Note that most Van Rees upland species have also been recorded from all or nearly all of the other outliers.

Figure 4.

Upland species recorded in the Foja Mts. divided into sets based on their Foja elevational floors (abscissa, in metres). For each set, the ordinate gives the percentage of the Foja upland species in that set that has also been recorded in the Van Rees Mts. For example, the point at an ordinate value of 50%, abscissa value of 850 m means that three of the six upland species with a Foja floor at 800–899 m were also recorded in Van Rees. Foja floors from Beehler et al. (2012).

Fig. 4 shows that, with one apparent exception, all Foja species whose floors exceed 1,250 m are absent from Van Rees¹. The Van Rees species with the next highest Foja floor was Hooded Pitohui Pitohui dichrous (Foja floor 1,158 m). Thereafter, with decreasing Foja floor elevation, the percentage of Foja species present in Van Rees increases until all of the lowest-elevation Foja upland species, with Foja floors averaging 132 m, do occur in Van Rees.

In light of Fig. 4, how can one explain the finding that some noisy, widespread species common on Foja and other outliers at elevations well below the Van Rees summit are nevertheless absent from Van Rees? The answer is that the relationship depicted in Fig. 4 is not a step function. It is not the case that the probability of presence in Van Rees increases in a step function from zero, for species with Foja floors above the Van Rees summit of 1,262 m, to 100% for species with floors of 1,261 m (or even of 1,000 m!). Such species with Foja floors 1 m, or even 262 m, below the Van Rees summit are not thereby guaranteed to occur in Van Rees. Instead, probability of occurring in Van Rees increases gradually with decreasing Foja floor elevation, to reach 100% only for the lowest Foja floors studied. Evidently, some additional factors besides elevational floor influence colonising success by upland populations. They will require further studies; possible factors include dispersal ability, abundance, reproductive output, and sociality.

Non-forest species in forested environments

Except for lowland savannas and marshes in low-rainfall areas, and high-elevation alpine grassland above the timberline, the climax vegetation over most of New Guinea is forest. In airplane flights over parts of New Guinea with sparse or no human population, we have often had the impression of uninterrupted forest stretching horizon to horizon. As a result, most New Guinea bird species are forest-dwellers. But in areas with dense populations of farmers, such as mid-montane grassland in the Wahgi and Baliem Valleys, there are large expanses of anthropogenic grassland supporting grassland birds, some of them endemic subspecies and species presumably of some antiquity. Those extensive anthropogenic grasslands could not have existed before the origins of New Guinea agriculture only within the last 10,000 years. Where did those species of anthropogenic non-forested habitats evolve before agricultural land clearance?

Rand (1941, Archbold & Rand 1935, Rand & Brass 1940) noted that, even in forested areas without human population, there are small open non-forest patches created by natural disturbances such as landslides and treefalls, where non-forest species could have evolved. With the post-Pleistocene development of agriculture and of anthropogenic grasslands, habitat and populations of those non-forest species expanded enormously.

The Van Rees Mts. illustrate this postulated origin of some of New Guinea's non-forest bird species. Despite the forest cover and lack of humans in most of Van Rees, we encountered natural clearings in the middle of forest, originating in four ways: landslides, mud volcanoes, streams, and lakes and marshes created by mud flows damming streams. In three of these four types of clearings we observed non-forest species absent from Van Rees forest but present in the anthropogenic clearings at Biri village and Danau Bira airfield. The details are as follows.

Our forest camp at 799 m was next to a small, treeless, evidently very recent landslide of bare mud, downslope from a small area of second growth evidently grown over an earlier landslide. In the second growth we observed daily a party of up to six White-shouldered Fairywrens Malurus alboscapulatus, a species of grassland and second growth; we also saw and repeatedly heard singing a pair of White-bellied Thicket Fantail Rhipidura leucothorax, a species characteristic of forest edge and second growth. An individual of the forest-edge Papuan Nightjar Eurostopodus papuensis sallied over the landslide clearing each evening. We encountered these three species nowhere else at our forest camp, which was otherwise surrounded by forest.

Danau Bira was a mission base at an airfield on a recently formed lake (see below), and surrounded by forest. From the mission base, on 19 October 1979 JD walked through 3 km of forest to a mud volcano, a circle of bare mud surrounded by rings of low young trees increasing in height with distance from its centre (Fig. 3). It was striking to emerge from the forest into the mud clearing, and suddenly to encounter two species of forest edge and second growth, Large-billed Gerygone Gerygone magnirostris and the cuckoo Cacomantis variolosus. Both were absent (Gerygone) or sparse (Cacomantis) in the surrounding forest. Of these five forest-edge, grassland, and second-growth species encountered at our camp's landslide and /or at the mud volcano, four—Cacomantis, Malurus, Gerygone and Rhipidura—were abundant in the large anthropogenic clearings of Danau Bira and Biri village.

While based at our mountain camp, we reached only one stream broad enough to lie under clear sky, not overhung by trees. On the two days on which we visited that stream, we encountered a party of three Torrent Flycatchers Monachella muelleriana, an obligate specialist of fast-flowing forest-lined mountain streams, but absent in forest.

TABLE 5

Lake-associated species at Danau Bira. Non-forest species dependent on the natural creation of Danau Bira. Right-hand column: how that species uses the lake. Includes species listed in Table 1 of Diamond et al. (2019), plus additional records.

Our remaining natural non-forest site was the lake Danau Bira itself and its surrounding marsh, created by a mudslide that had dammed a stream. Table 5 lists the 23 bird species whose presence can probably be attributed to the lake's creation, although two may have been present on the pre-existing stream. Of the 23, eight (three ducks, two grebes, two cormorants, and an anhinga) swim on the lake; six herons wade in it; four (two hawks, a tern and a kingfisher) plunge into the lake to catch fish; four (a rail, two sandpipers and a warbler) live at its edges; and one (a jacana) walks on lily pads on the lake. Similar but smaller ponds created by mudslides and damming may be common: we saw two from the air at the base of the large landslide a few km from our camp. The avian colonists of Danau Bira are of course waterbirds of New Guinea's extensive lowland marshes and rivers such as the Fly, Sepik and Mamberamo, rather than landbirds of the forest edge, landslide clearings and mud volcanoes discussed above.

Outlook

Our studies provide only a first survey of the Van Rees Mts. They leave much still to be done. We suggest some priorities for further studies.

We surveyed only one transect above 494 m, although that did lie under what appears to be the highest Van Rees peak. There are several other peaks that should be surveyed. Even on our one transect, there were four upland species that we recorded only once or twice. Hence there are surely additional Van Rees upland species beyond the 37 that we observed. Candidates include Mountain Swiftlet Aerodramus hirundinaceus, Charmosyna pulchella, Micropsitta bruijnii, Masked Bowerbird Sericulus aureus, Myzomela cruentata, Mid-mountain Berrypecker Melanocharis longicauda, Great Woodswallow Artamus maximus, Torrentlark Grallina bruijnii, Monarcha frater, White-faced Robin Tregellasia leucops and Russet-tailed Thrush Zoothera heinei.

Because our focus was on upland species, we spent only brief periods at low elevation in the hills (at Biri at 143 m), and no time in the floodplain of the Mamberamo or Rouffaer (= Tariku) Rivers. Van Heurn and van Eechoud collected 127 species in the Mamberamo floodplain, but they recorded no field observations. As a result, the current list of lowland Van Rees species lacks some uncommon or local species likely to be present (e.g., Lesser Black Coucal Centropus bernsteini, Long-billed Cuckoo Rhamphomantis megarhynchus, Papuan Hawk-Owl Uroglaux dimorpha, Orange-breasted Fig Parrot Cyclopsitta gulielmitertii, Hooded Pitta Pitta sordida and Yellow-gaped Meliphaga Meliphaga flavirictus). On the Mamberamo's eastern branch, the Idenburg (= Taritatu) River, the Third Archbold Expedition sampled extensively the avifauna of marshes and swamp forests at their Bernhard Camp (Rand 1942). No such sample is available for the Van Rees Mts. lying on the Mamberamo's western branch, the Rouffaer (= Tariku) River.

A longer stay at Danau Bira, and at different seasons, will undoubtedly produce more lake-associated species than the 23 that we recorded. Indonesian New Guinea's largest lake, Lake Rombebai, on the Mamberamo's eastern bank north of the Van Rees Mts. remains completely unsurveyed ornithologically.

Why are some upland species much more successful colonists of mountains than others? Fig. 4 shows that the match between species' elevational ranges and a mountain's elevation provides much of the explanation, but only a part. Why, of those upland species much of whose elevational range falls within our Van Rees transect, are some but not others present? How can one explain ‘flagrant’ Van Rees absentees such as Edolisoma montanum, Seicercus poliocephalus and Rhipidura atra? This is a major unanswered question whose resolution awaits analysis of all of New Guinea’s outlying ranges, not just the Van Rees Mts.

Selected species accounts

We now provide brief details of significant observations of particular species.