Species Abundance and Habitat Relations of an Insular Montane Avifauna

Condor, 80:1-14 @ The Cooper Ornithological Society 1978

SPECIES ABUNDANCE AND HABITAT RELATIONS OF AN INSULAR MONTANE AVIFAUNA

LARRY S. THOMPSON

The impoverishment of avifaunas inhabiting visit the higher elevations, and noted “no oceanic islands has been noted for many avian specialties” in the area. decades, but it was not until the formulation of the equilibrium theory of MacArthur and STUDY AREA AND METHODS Wilson (1963, 1967) that reasons for the ob- The Great Plains region of north-central Montana served impoverishment could satisfactorily be and adjacent Canada (Fig. 1) is largely characterized postulated and, more importantly, tested. Re- by monotonous expanses of semi-arid steppe and dry cently, equilibrium theory has been tested cropland. Several isolated mountain ranges rise as using data from continental island systems, “islands” above the nrairie “sea” in this region. form- inoluding the insular alpine avifaunas of the ing a fringing archipelago of montane habitat off the Cordilleran “mainland.” These montane islands Andes (Vuilleumier 1970) and the insular (Fig. 1) support coniferous forests and would appear montane avifaunas of the Great Basin (John- to provide suitable habitat for breeding bird com- son 1975, Behle 1977). These data have shown munities similar to those of the Rocky Mountains. that, in addition to the “classical” deter- Coniferous forests of river breaks are not uplifted and, hence, not truly montane. minants of insular species number (namely, I studied the Sweetgrass Hills, which are a group island area and width of barrier), habitat of three intrusive buttes approximately 140 km E of diversity is also an important determinant of the Rocky Mountains. West Butte attains an eleva- bird species number in systems of inland tion of 2,130 m, which is over 1,000 m above that of the surrounding plains. East Butte, while not quite habitat islands. as high, has a larger uplifted area. Gold Butte, a Several studies of insular oceanic avifaunas minor uplifted area between East and West buttes, have revealed that species impoverishment is is much smaller and will not be discussed further. intimately associated with community struc- Vegetation and physiography of the Sweetgrass ture, evidenced in part by density compensa- Hills studv area have been described bv Thomnson and Kuijt (1976), who recognized 18 major com- tion (Grant 1966a, MacArthur et al. 1972), munities-5 characteristic of the plains surrounding abundance compensation ( Crowell 1962)) the buttes, 12 restricted to the montane regions, and niche shifts (Diamond 1970a), and even com- 1 (riparian cottonwood forest) which occurs widely petitive exclusion (MacArthur et al. 1972). in the plains but also extends into the montane region. These community types, which are representative of Although a few investigators have provided an major avian habitats in the study area, are grouped indication that this may be true of insular in Table 1, along with their approximate altitudinal montane avifaunas as well (Miller 1955, John- range and dominant plant species. son 1965, 1975), others have no,ted no obvious Vegetation analysis and casual bird observations differences in community structure between began on East Butte in the summer of 1972. In- tensive field investigation of the montane avifaunas mainland and insular montane avifaunas of West and East buttes began on 12 June 1973 (Hubbard 1965, Tatschl 1967, Wauer 1971). and continued through 24 August 1973, with parts However, studies dealing specifically with of 45 days spent in the study area. Data were re- the species abundance and habitat relation- corded for all birds observed in the montane community types during 29 counts (51.0 h in West Butte ships of insular montane avifaunas in the and 63.5 h in East Butte ). These data included northern Great Plains region are lacking. species, number of individuals, time, location, eleva- I investigated sample abundances, habitat tion and habitat. The total number of birds of each use, and altitudinal range of summer birds of species observed provides a measure of observed or “sample” abundance (see Preston 1948:245). Counts the Sweetgrass Hills, an isolated mountain were made during all times of day; of the 114.5 range in the northern Great Plains of Mon- total hours, 34% were spent between 04:OO and tana. The avifauna of this area has no,t pre- 09:00, 27% between 09:OO and 12:00, 18% between viously been described, although the hills 12:00 and 15:00, and 13% after 18:00 (all DST). occupy a key position as the northernmost of A typical count consisted of a 3-6 h, early morning or late evening walk from a base camp at the lower the high intrusive mountain ranges of the edge of the montane region to one of the peaks and area, and contain a surprising variety of a return to camp by a different route. Only birds montane habitats. Coues (1878) reported seen clearly enough to identify were tallied during several species of birds he observed in the these counts; birds heard but not seen, as well as foothills and plains surrounding the Sweet- birds observed outside the counting periods, were noted but were not included in the sample abun- grass Hills during the U.S. Boundary Com- dance totals. I chose this method because of the mission expedition of 1874, but he did not difficulties in assigning numbers to calls while walk-

111 2 LARRY S. THOMPSON

Sweetgrass Hills Bear Paw

J3 Rnrk\/

Hiahwood- -

mMontane areas

kilometers

FIGURE 1. Location of the Sweetgrass Hills study area and other montane islands of the northern Great Plains.

ing a continuous and meandering transect. Where counts; remaining field time was devoted to other exact counts could not be made, the lowest estimate work. of numbers of individuals was recorded. Approxi- Additional visits to the Sweetgrass Hills were mately one-third of my field time was spent on these made during the summers of 1974 and 1975, mainly

TABLE 1. Major avian habitats of the Sweetgrass Hills study area.

Altitudinal Area Habitat range (In) (km*) Dominant vegetation Plains habitats below 1,500 1,808 Steppe (ST)” below 1,500 1,788 Agropyron spp., Stipa spp., Triticum spp. Riparian shrubbery (RS ) below 1,500 8 Amelanchier alnifolia, Crataegus douglasii, Salk spp. Potholes and reservoirs (PR) below 1,450 9 Carex spp., Juncus spp., Scirpus spp., Typha latifolia Riparian woodland ( RW ) below 1,550 Populus trichocarpa, P. deltoides Montane habitats 1,420-summits Montane grassland ( MG) b 1,500-summits Festuca idahoensis, F. scabrella, Potentilla fruticosa Rubble slopes (RB ) 1,420-summits Aspen (AS) ’ 1,420-1,800 Populus tremuloides Douglas fir forest (DF) 1,420-1,600 Pseudotsuga menziesii Lodgepole pine forest (LP) 1,500-summits Pinus contorta Spruce-lodgepole forest 1,600-summits Pinus contorta, Picea glauca x engelmannii, (SL)” Abies lasiocarpa Mountain ravines (MR) 1,500-2,000 1 Subalpine forest (SF) 2,000-summits 2 Abies lasiocarpa, Pinus contorta, Pinus albicaulis

a Includes agricultural land, foothills prairie, and coulee scrub types of Thompson and Kuijt (1976). t ’ Includes their montane grassland and subalpine grassland types. e Includes their aspen groveland and aspen woodland types. *Present on East Butte only; includes their spruce-lodgepole and temperate subalpine fir forest types. INSULAR MONTANE BIRDS 3 to sites not investigated in 1973. I found no new THE SPECIES-ABUNDANCE RELATION montane species, although the observed range of AND SAMPLING ADEQUACY habitats or elevations used was expanded in several cases. Several days were spent in each of the other In determining species number, presence is montane islands shown in Figure 1, but I did not more easily proven than absence. The reli- try to completely inventory their avifaunas. ability of each report of the absence of a Bird snecies were assigned to guilds (Root 1967, particular species from a given area depends Willson 1974, Johnson 575) based on major‘ food on the adequacy of sampling of that area. In source (seeds and vegetation, invertebrates, verte- brates, nectar, omnivorous), foraging stratum (bark, their studies of avifaunal diversity, Vuilleu- ground, low canopy, mid-to-high canopy, air, water), mier (1970) and Johnson (1975) compared and method of foraging (bark and bole drilling, bark the total numbers of breeding bird species and bole gleaning, ground and brush foraging, timber known from various islands. These totals foliage foraging, sallying, raptorial, dabbling, diving, water and mud probing, swooping). included certain cases where only one or two years ’ field work had been conducted and RESULTS AND ANALYSIS where certain breeding species may have been overlooked. They also included cases where THE SWEETGRASS HILLS AVIFAUNA many years of observation have been com- I encountered 100 species of breeding birds bined, and where the cumulative species total in the study area. Their sample abundances, is probably greater than the number of birds distribution among the habitat categories breeding during a given year, due to year-to- (Table l), altitudinal range, and evidence of year changes in avifaunal composition. In breeding, are presented in the Appendix. I these cases, the multi-year “universe” being considered 18 of these species to be “mon- sampled may be quite different from a single tane,” that is, species which, in this part of years’ assemblage of breeding birds. If mean- their range, prefer montane coniferous forests ingful comparisons of species number among or related montane habitats during the breed- islands are to be made, estimates should ing season. This does not include those spe- ideally be based upon comparable amounts cies that are obligatory alpine or nocturnal of field effort per unit area, or else incorporate forms (e.g., Strix spp. and Leucosticte spp.). a measure of the degree of sampling adequacy Montane species do not normally breed in the or intensity. arid steppe, agricultural land, coulees, riparian Examination of the species-abundance re- cottonwood forests, forests of river breaks, or lation provides such an indication of sampling farm groves found in the prairie between the adequacy-one that allows a more precise montane islands. Not all of the species which estimate of species number. Preston (1948, breed in montane habitats are montane by 1962) suggested plotting sample abundance this definition. data using categories of the number’ of ob- In addition to the breeding species, 8 spe- servations’ per species (octaves of O-l, 1-2, cies believed to be late-summer visitors or 24, 4-8, 8-16, etc., observations per species) migrants were observed in the study area. on the abscissa and the number of species These are the Great Blue Heron (Ardea falling into each category on the ordinate. The herodim), Prairie Falcon ( F&o mexicanus), resultant plots describe a lognormal distri- Lesser Yellowlegs ( Tringa fluvipes), Frank- bution of the form lins’ Gull (Larus pipizcan), Common Tern (Sterna hirundo), Rufous Hummingbird ( Selasphorus V&S), Northern Water-thrush where S,. is the number of species in a cate- ( Seiurus novehoracensis), and Rose-breasted gory R octaves from the mode, S, is the Grosbeak ( Pheucticus ludovicianus) . The number of species in the modal octave, and Northern Waterthrush was reported from this a is a constant. The sample abundance data area by Coues (1878). Other birds observed obtained in this study (see Appendix) yield during this study which were previously re- an excellent fit ( r2 = 0.99), where S, = 5.45 ported by Coues (1878) are indicated in the and a = 0.24. The “species curve” for these Appendix. Species reported by Coues but not data is shown in Figure 2, and a second curve, encountered in the present study are the representing the more diverse riparian avi- Mountain Plover (Charadrius montanus) , Solitary Sandpiper ( Tringa solitaria), Burrow- fauna of the Bear Paw Mountains (based on ing Owl (Athene cunicularia), and Says’ Walcheck 1969) is plotted for comparison. Phoebe (Sayornis saya). All specimens taken Although the sample abundances ob#tained in during this study have been deposited at the this and Walchecks’ (1969) studies do not Charles C. Conner Museum, Washington State represent absolute abundances or even true University, Pullman. relative abundances, they are entirely suitable 4 LARRY S. THOMPSON

rp I5 1.6 MontaneSpecies r a SWEETORADS “ILLS @I 0 _r._. 0 BEAR PAW MOUNTAINS !i I ./ .‘ R\n I . ’ 0 9 25.l 8 10 / ? \

I lNOlVlOUALS PER SPECIES

FIGURE 2. Species curves for the montane avifauna of the Sweetgrass Hills and the riparian avifauna of Beaver Creek in the Bear Paw Mountains I! ( based on Walcheck 1969 ). FIGURE 3. Observed distribution of bird species and breeding-season guilds among habitats. Ab- for this type of analysis (Preston 1948:245). breviations are as listed in Table 1 with two ad- The area to the right of the ordinate in ditions: TP = totals for all plains habitats; TM = totals for all montane habitats. Numbers within bars Figure 2 represents the total number of bird indicate percent contribution of montane species to species seen during counts (i.e., 36 breeding total species number for each habitat. species ), while the area to the left of the ordinate represents rare or inconspicuous species missed in sampling. The area under the species (48). This reflects bolth the greater entire species curve represents the theoretical structural diversity and the influence of ad- total number of species S, present in the uni- jacent prairie and montane habitats on the riparian woodland communities. The montanc verse being sampled, S, = S, &/a (Preston habitats together supported 46 species; of the 1948, 1962). Using the values for S, and a individual montane habitats, the predominant mentioned above, St for the universe from coniferous forests (Douglas fir, lodgepole which samples were drawn (i.e., diurnal or pine, and spruce-lodgepole forests) supported crepuscular birds inhabiting montane habi- the greatest number of species. tats of the Sweatgrass Hills during the summer of 1973) equals 40, which is fewer than As expected, the lowest proportion of montane species (O-4.5% of total species) was the 46 species actually observed. This analysis found in plains communities, where I saw suggests that all breeding bird species present only one montane bird (a Pine Siskin). in the Sweetgrass Hills in 1973 were observed Riparian woodland and montane area each at least once, and that sampling of the breed- supported 25.0% and 39.1% montane species, ing montane avifauna was complctc. respectively. The spruce-lodgepole forest (found only on East Butte) supported the HABITAT RELATIONS largest number of montane species (13) of The observed distribution of breeding bird all habitats. However, the greatest proportion species and guilds among the major avian of montane species was found in mountain habitats of the Sweetgrass Hills study area is ravine ( 75.0% ) and subalpine forest (66.7%) shown in Figure 3, along with data for all communities, which were also the most cool plains and all montane habitats collectively. and moist of the habitats studied. Habitat use by individual species is given in The number of breeding-season guilds rep- the Appendix. resented in each of the habitats is also Although I spent little time and effort in- shown in Figure 3. Note that the habitats are vestigating plains habitats, I found more much less variable in terms of guild number breeding species (63) there than in the much than species number. Although plains habitats more intensively studied and structurally di- supported mlore species than montane habi- verse montane or riparian woodland areas. tats, they supported slightly fewer guilds (16 Steppe, the most extensive individual habitat, versus 18). Thus, the absence from montane supported the most species (52, including habitats of certain guilds well-represented in waterfowl and shorebirds). Riparian wood- the prairie habitats (e.g., diving, water and land, though comprising less than 1% of the mud probing) is more than offset by the whole study area, supported nearly as many additions of guilds corresponding to the ad- INSULAR MONTANE BIRDS 5

TABLE 2. Breeding montane birds of Glacier National Park and several island mountain ranges of the northern Great Plains.”

Montane island number

Species 1 2 3 4 5 6 7 8 9 10 11 Accipiter gentiW 0 Dendragapus obscurr& 0 0 Canachites canadensis” ’ Bonasa urnbe&& 0 0 Stellulacalliope Picoides arcticus” ’ Picoides tridactylusbc Empidonax hammondii Empidonax oberholseri 0 0 X Empidonax difficilus 0 i Nuttallornis borealis 0 0 Perisoreus canadensi? ’ 0 0 Cyanocitta stelle# 0 Nucifraga columbianab” 0 X X Parus gambel? ’ 0 X X Sitta carolinensi+ 0 Sitta canadensisb 0 X X Certhia familiari? 0 Cinclus mexicanu? 0 Catharus guttata 0 0 i X Catharus ustulata 0 0 0 X Myadestes townsendib 0 0 0 X Regulus satrapab 0 Regulus calendula 0 0 X Viireo solitarius 0 Vermivora celata 0 X Dendroica coronata 0 0 X Oporornis tolmiei 0 0 X Piranga ludoviciana 0 0 X Hesperiphona vespertina 0 Carpodacus cassi& 0 Pinicola enucleato$ 0 X Spinus pin& 0 X Loria curvirostrab 0 X Loxia leucopterab 0 Junco hyemali? 0 X Zonotrichia leucophrys 0 Melospiza lincolnii 0

Total breeding montane spec:ies 37 24 23 AI 16

ditional tree layer (e.g., bark and bole glean- major types of coniferous forest again sup- ing, timber foliage foraging). The influence ported the most guilds. of a tree layer in increasing the number of MONTANE SPECIES NUMBER guilds has been noted by Willson (1974) The number of breeding montane bird spe- and Johnson ( 1975). Riparian woodland, cies (18) found in the Sweetgrass Hills in with its well-developed shrub layer and high itself conveys little information. However, degree of horizontal patchiness, supported the when examined in the context of the montane greatest number of guilds (18) of any habitat, archipelago of the northern Great Plains, it as many, in fact, as all montane habitats com- allows quantitative examination of factors af- bined. Of the montane habitats, the three fecting species number.

INSULAR MONTANE BIRDS 7

TABLE 4. Correlation coefficients (r) among dependent and independent variables.”

BE ME RE TA FA TD CT HD DB DL DM

T .62* .86** .89** .93** .95** .98** .82** .90** -.80** -.72** -.55 M .39 .71** .79** .85** .88** .92** .75** .78** -.72** -.57 -.45 W .71** .89** .90** .92** .94** .95** .81** .91** -.79** -.76** -.59* R .74** .88** .88** .85** .88** .89** .80** .87** -.83** -.88*” -.70* G .48 .72** .77** .88** .84** .85** 51 .69* -.65* -.55 -.36

“With n = 12, n significant T nt P < 0.05 (*) is 0.58 and for P < 0.01 (*“) is 0.71. Abbreviations of variables as described in the text.

Breeding records of montane bird species bird species having a large minimum area or are presented in Table 2. Records are also large territory size, and on habitat diversity. given for the Pryor Mountains, a major mon- Width of barrier. Except for the number tane island approximately 160 km SSE of the of long-distance migrants and the number of Snowy Mountains outside the area shown in guilds, all dependent variables are significantly Figure 1, and for Glacier Park (footnote), a and negatively correlated with distance from representative sample of the Rocky Mountain the Cordilleran mainland, whether or not the Cordillera. Little Belt Mountains are considered to be part Data on the avifaunas and several physical of the massif. Distance effects for long-distance parameters of each sample area are sum- migrants are significant only if the Little Belt marized in Table 3. The five dependent vari- Mountains are considered to be a montane ables (pertaining to breeding montane birds) island rather than an extension of the listed for each sample area are: total number Cordillera, indicating that this range is treated of species (T); number of species which are as a “sub-continent” rather than an island by long-distance migrants (M); number of spe- these species. Distance to nearest montanc cies which are weakly migratory but which neighbor is significantly and negatively cor- winter in Montana (W); number of sedentary related only with numbers of wintering and (resident) species (R); and number of guilds sedentary species. This is to be expected, for represented by all breeding montane species these arc the species for which stepping-stone (G). The 11 independent variables are as effects on colonization are most important. follows : “base” elevation (BE), or elevation Long-distance migrants, which may cross at which the lowest montane coniferous forest hundreds of kilometers of inhospitable habitat zone begins; maximum elevation (ME) ; relief with relative ease, can presumably colonize (RE), or the difference between BE and ME; an isolated montane island as easily as one total area above base elevation (TA); forested near a source area. Note that correlations of area above base elevation (FA) ; topographic number of guilds with the three measures of diversity index (TD), which is equal to barrier width are only marginally significant RE X TA X 10d3; shortest distance to the or not’ significant. This lends support to John- Cordillera assuming the Little Belt Mountains sons’ (1975) findings that most montane is- are, first, part of the island system (DB) or, lands support a “standard” set of montane second, part of the Cordillera (DL); distance species representing a wide variety of guilds. to the nearest montane area (DM); number Should a sedentary species be absent from an of coniferous tree species exclusive of isolated island, its guild may well be Juniperrus spp. (CT); and habitat diversity represented there by a standard migratory score (HD) as defined by Johnson (1975). species. Relations among dependent and independent Effects of barrier width on habitat diversity variables were revealed by Pearson product- (Johnson 1975) are probably minor since the moment correlations (Table 4) and will be insular montane forests of the northern Great discussed below. Plains are relicts of periglacial forests Island area. All dependent variables are (Thompson and Kuijt 1976)) rather than the significantly and positively correlated with result of long-distance dispersal; the Sweet- both total area and forested area, as predicted grass Hills support nearly as many species of by island equilibrium theory (MacArthur and coniferous trees as the much larger Little Belt Wilson 1967). This reflects the effects of Mountains (Table 3). island area on colonization rate (target size Habitat effects. The remaining independent effects), on extinction rate (through total variables-base elevation, maximum elevation, population size), on distribution of montane relief, topographic diversity, number of conif- 8 LARRY S. THOMPSON erous tree species, and habitat diversity itat relations of the montane avifauna allow score-all provide rough measures of the me to evaluate possible relations between diversity of breeding habitats available to a avifaunal diversity and commun3y structure, colonizing bird species on a given island. specifically, niche shifts, density and abun- With three exceptions, the dependent vari- dance compensation, and competitive exclu- ables are significantly and positively cor- sion. A commonly described response of birlds related with these independent variables. to the release from competition on species- Species number of all four avian groups is poor oceanic islands is the expansion of variety correlated with the number of coniferous tree of habitats used, altitudinal range, and/or species, but the number of guilds is not. This foraging behavior (Crowell 1962, Sheppard suggests that the addition of a coniferous et al. 1968, Diamond 1969, 1970a, 1971) . This forest layer is sufficient to increase to near niche expansion or “ecological extension” has maximum the number of guilds represented also been noted in insular montane avifaunas, in a montane community. Adding more spe- including those of the Sierra de1 Carmen of cies of conifers seems to result in expansion Mexico (Miller 1955), the Spring and Sheep but nolt addition of guilds. The correlations’ ranges of Nevada (Johnson 1965), and of base elevations with number of migratory various high mountain ranges of New Guinea species and number of guilds are not signifi- (Diamond 1970b). There is little indication cant; this is not surprising as the fact that that characteristic grassland birds have ex- species numbers of wintering, resident, and panded into species-poor montane habitats of total montane bird species are significantly the Sweetgrass Hills. The only such species correlated with base elevation. The relation I found in montane grassland were the West- between base elevation and species number cm Meadowlark, Savannah Sparrow, and Ves- is not intuitively obvious. However, base per Sparrow. These species, although abun- elevatioa and maximum elevation were found dant in the plains, were scarce in the to be highly correlated (7 = 0.88, P < .Ol), mountains and I did not see them in non- indicating a Massenerhebung effect as de- grassland montane habita&.’ However, I noted scribed by Terborgh and Weske (1975)- apparent habitat shifts for two non-montane that is, an upward shift in vegetation zonation species, the Mourning Dove and the Common as mountain mass increases. This is particu- Flicker, both of which were found to be larly evident in the Sweetgrass Hills, where abundant at all elevations and in a wide the s&alpine forest zone occurs well below variety of habitats in bmothbuttes (Appendix). 2,000 117. For each dependent variable, the Hoffman (1960) did not find Mourning Doves highest correlation coefficients are those as- in the forests of the Little Belt Mountains, socia,ted with the topographic diversity index. but I found them in nearly all forest habitats This indicates that, in this montane archipel- of the Sweetgrass Hills. The Common Flicker ago, topographic diversity-which reflects was the only species of woodpecker in mon- such parameters as number of canyons, num- tane communities; it foraged in conifers. ber of plant species, interception of precipi- Surprisingly, the Hairy Woodpecker, normally tation, steepness of slopes, etc.-is a better a common and conspicuous member of mon- measure of habitat diversity than is the habi- tane forest communities in Montana, was tat rating system of Johnson (1975). never seen outside the riparian woodlands. An altitudinal shift was noted for the Oven- DTSCUSSION bird, which was seen in lodgepole pine forests above 1,700 m. The preceding analysis has shown that species Overall densities of all birds on oceanic is- number of the Sweetgrass Hills montane avi- lands may be greater than (Crowell 1962, fauna is determined at least in part by habitat Grant 1966a, MacArthur e,t al. 1972), similar diversity, island area, and-in the case of non-migratory or weakly migratory species- to (MacArthur et al. 1972), or less than distance from source fauna. However, the (Diamond 1970b) densities of birds on com- absence of certain species cannot readily be parable areas of mainland, depending upon explained by these determinants of species individual circumstances. However, on mon- number. For example, the absence of the tane islands, avifaunal breeding density Evening Grosbeak (Mesperiphona vespertina) appears to be generally lower than in com- or the White-crowned Sparrow (Zonotrichia parable habitat on the species-rich montane Zeucophrys) is not due to lack of suitable mainland (Diamond 197Ob, Johnson 1975). habitat or inability to colonize. This occurs in spite of increased abundances My data on the species abundance and hab- of certain species which have expanded their INSULAR MONTANE BIRDS 9 niche to include habitats not normally oc- group of montane generalists on nearly all cupied in mainland situations. islands, regardless of island size. Restricted In the Sweetgrass Hills, five species boreal or montane species are those which (Clarks’ Nutcracker, Pine &kin, Dark-eyed occur on relatively few montane islands. They Junco, Red Crossbill, and Chipping Sparrow) are typically ecological specialists and are accounted for over two-thirds of all observa- often congeneric with a standard species of tions made during counts (Appendix). Al- the same guild or at least closely duplicate though I did not estimate their breeding the foraging niche of a standard montane densilty, these species appeared to be more species. abundant than in comparable areas of the Are restricted species excluded from all but Rocky Mountain Cordillera which I have the largest and most diverse islands due to visited. This indicates a possible response to competition from the more vigorous standard lack of competition from characteristic mon- species of corresponding guilds? Many of the tane species which are absent from the Sweet- standard species may have expanded their grass Hills. niche to overlap largely with that of the Despite the apparently high densities of res#trioted specialists. In other words, is avi- these common species, my impression of the fauna1 impoverishment a consequence as well overall montane avifauna was of low individ- as a cause of niche expansion in insular ual densities of birds as well as species ecosystems? impoverishment. I made only 1,520 sightings The phenomenon of competitive exclusion of individual birds during the 114.5 h of among insular avifaunas has been discussed counts. by MacArthur et al. (1972), who found that The uneven distribution of individuals the establishment of a particular species on among species has several implications re- an island may forestall subsequent coloniza- garding the species-abundance relation dis- tion by a congener or other’ ecologically cussed earlier. In the Sweetgrass Hills, there similar species. Indeed, a “checkerboard” is a relatively high proportion of very common pattern of distribution of wrens on similar species to moderately common species as re- islands in the Panama area was observed, flected in the relatively shallow shape of the where each island contained only one of three species curve (Fig. 2). One ’ would expect possible species. The established species ap- sample abundance data from the diverse Cor- parently made it “impossibly difficult” for an dilleran avifauna to show a more equitable dis- additional species also to colonize the island. tribution of individuals among species, and On montane islands, where the extent and hence a higher species curve than that ob- variety of resources are reduced largely by tained for impoverished insular montane avi- the same factors that reduce bird species faunas. number, the intensity of competition for a Several studies (Grant 196613, 1969, 1970, mutually shared resource should be especially Lack 1969, Crowell 1973) have indicated that keen among ecologically similar species. A islands are more frequently occupied by given species may be excluded from an island ecologically dissimilar species of animals than ’ of suitable habitat not only through com- ecologically similar ones. Johnson ( 1975), in petition with a single species sharing the same his study of montane islands of the Great guild, but also by “diffuse competition from a Rasin, found that the “standard” boreal or mutually adjusted community of many spe- montane species (those which occur together cies, each member‘ of which overlaps the on nearly all montane islands) represented a potential colonists’ niche only partially” (Mac- wide variety of dissimilar foraging niches or Arthur et al. 1972:340). “guilds.” Likewise, a group of ecologically It is difficult, however, to imagine exactly dissimilar species is found on nearly every how competitive exclusion would operate in montane island of the northern Great Plains a community of birds which are largely that has been studied, including the Sweet- migratory, such as the Sweetgrass Hills ati- fauna. Much of the theoretical work of grass Hills (Table 2). This group includes MacArthur and others was based on resident the Dusky Flycatcher, Clarks’ Nutcracker, avifaunas, which have had more time to Mountain Chickadee, Ruby-crowned Kinglet, deplete resources or to become “mutually Red-breasted Nuthatch, Swainsons’ Thrush, adiustecl” than migrants which arrive within Townsends’ Solitaire, Yellow-rumped Warbler, a few days or weeks of one another. Indeed, Western Tanager, Pine Siskin, Red Crossbill, I found no clear evidence of such exclusion and Dark-eyed Junco. Thus, the bsasicforag- in this study, which deals with highly migra- ing niches are filled by a relatively small tory species. 10 LARRY S. THOMPSON

While avifaunal diversity and community ACKNOWLEDGMENTS structure are intimately related in montane I gratefully acknowledge the help of J. W. Crane, island systems, the absence of several bird R. Daubenmire, N. K. Johnson, R. E. Johnson, and species from the Sweetgrass Hills is still un- R. E. Jonas who provided comments and criticisms explained. These absences may reflect the of an earlier version of this paper. N. K. Johnson random element in insular colonizations and and D. West kindly verified the identity of certain bird specimens, and C. V. Davis and P. D. Skaar extinctions. Many species which migrate to freely shared their extensive knowledge of Montana the montane islands in spring may find suit- bird distribution. Special thanks go to my wife able habitat there, but mav not remain due Susan. who assisted with all stages of this study. to sltrong philopatry to a distant nesting site. Field work‘ was undertaken while1 was at the Dk- Likewise, the very small breeding populations partment of Zoology, Washington State University, Pullman, and was funded in part by a NASA trainee- inhabiting such islands are highly susceptible ship under budget No. 12U-2940-8251. to “random” extinction. which would result in “unexplained”‘ absences from apparently suit- LITERATURE CITED able habitat. The dramatic year-to-yea] changes in avifaunal composition of monltane BEHLE, W. H. 1977. Avian biogeography of the intermountain west. In K. Harper Led.], Pro- islands (Johnson 1974) reflect such random- ceedings of the Intermountain Biogeography ness or “ecologic noise” which tends to ob- Symposium. Brigham Young University, Provo, scure more easily defined relationships. Utah. BENDIRE, C. E. 1895. Life histories of North American birds from parrots to the grackles, SUMMARY with special reference to their breeding habits The summer birds of the Sweetgrass Hills, and eggs. U.S. Natl. Mus., Special Bull. No. 3. BINFORD, L. C. 1973. A short method for treating an isolated mountain range in the northern avian breeding data in regional accounts. Wil- Great Plains region of Montana, were in- son Bull. 85 ~244-246. vestigated. Of the 100 breeding species re- COVES, E. 1878. Field notes on birds observed in ported, 18 are restric.ted to montane habitats Dakota and Montana along the 49th Parallel during the seasons of 1873 and 1874. Bull. U.S. during the breeding season. Data are pre- Geol. and Geogr. Surv. Terr. 4:545-661. sented on local distribution, sample abun- CHOWELL, K. L. 1962. Reduced interspecific com- dances, distribution among major habitats, petition among the birds of Bermuda. Ecology summer altitudinal range, and evidence of 43: 75-88. CHOWELL, K. L. 1973. Experimental zoogeog- breeding for each species observed. Sample raphy : introduction of mice to small islands. abundance data were used to construct a Am. Nat. 107:535-558. species curve with which to test thoroughness DAVIS, C. V. 1961. A distributional study of the birds of Montana. Ph.D. diss., Oregon State of avifaunal sampling. While montane habi- University, Corvallis. tats supported fewer breeding bird species DELAP, D. 1961. Breeding land birds of the Little than the more extensive steppe habitats, they Rocky Mountains in northcentral Montana. Proc. Montana Acad. Sci. 21:38-42. provided a sli&tly greater representation of DELAP, D., AND F. FEIST. 1963. Summer birds breeding-season guilds. The number of breed- seen and heard in the Highwood Mountains. ing montane species inhabiting montane is- Proc. Montana Acad. Sci. 23:138-143. DELAP, D., AND G. THOMPSON. 1962. Summer lands was found to be significantly correlated birds of the Pryor Mountains. Proc. Montana with island area, forested area, distance from Acad. Sci. 22:14-20. mainland, and several measures of habitat DIAMOND, J. M. 1969. Avifaunal equilibria and species turnover rates on the Channel Islands of diversity. Only species numbers of sedentary California. Proc. Natl. Acad. Sci. USA 64:57- or weakly migratory montane species were 63. correlated with barrier width. The relations DIAMOND, J. M. 1970a. Ecological consequences of island colonization by southwest Pacific birds. between species numb,er and community I. Types of niche shifts. Proc. Natl. Acad. Sci. structure is discussed; while no evidence of USA 67:529-536. competitive exclusion was noted in this DIAMOND, J. M. 1970b. Ecological consequences of island colonization by southwest Pacific birds. largely migratory avifauna, evidence of niche II. The effect of speciesdiversity on total popu- expansion and abundance compensation was lation density. Proc. Natl. Acad. Sci. USA 67: fo;nd, and may be reflected in-differences in 1715-1721. DIAMOND, J. M. 1971. Comparison of fauna1 equi- the species-abundance relation between main- librium turnover rates on a tropical and a tem- land and insular avifaunas. Philopatry and perate island. Proc. Natl. Acad. Sci. USA 68: chance may be the best explanation for the’ 2742-2745. GODFREY, W. E. 1950. Birds of the Cypress Hills absence of species not clearly related to lack and Flatten Lake regions, Saskatchewan. Bull. of suitable habitat or inability to colonize. Natl. Mus. Canada, No. 20. INSULAR MONTANE BIRDS 11

GRANT, P. R. 1966a. The density of land birds on sehl and F. Howell [eds.], Game management the Tres Marias Islands in Mexico. I. Numbers in Montana. Mont. Dept. Fish Game, Helena. and biomass. Can. 1. Zool. 44:805-815. PARRATT, L. P. 1964. Birds of Glacier National GRANT, P. R. 1966b. The density of land birds on Park. Glacier Nat. Hist. Assoc., Special Bull. the Tres Marias Islands in Mexico. II. Distribu- No. 9. tion of abundances in the community. Can. J. PRESTON, F. W. 1948. The commonness and rarity Zool. 44: 1023-1030. of species. Ecology 29:254-283. GRANT, P. R. 1969. Colonization of islands by PRESTON, F. W. 1962. The canonical distribution ecologically dissimilar species of birds. Can. J. of commonness and rarity. Ecology 43:185-215. Zool. 47:4143. ROOT, R. B. 1967. The niche exploitation pattern GRANT, P. R. 1970. Colonization of islands by of the Blue-gray Gnatcatcher. Ecol. Monogr. ecologically dissimilar species of mammals. Can. 37:317-350. J. Zool. 48:545553. SAUNDERS, A. A. 1921. A distributional list of the HALLADAY, I. R. 1965. A general survey of the birds of Montana. Pacific Coast Avifauna No. natural history, p. 3754. In R. C. Zell led.], 14. Cypress Hills Plateau guidebook no. 1. Alberta SHEPPARD, D. H., P. H. KLOPFER, AND H. OELKE. Sot. Petroleum Geologists. 1968. Habitat selection: differences in stereo- HOFFMANN, R. S. 1960. Summer birds of the Lit- typy between insular and continental birds. Wil- tle Belt Mountains, Montana. Oct. Pap. Mon- son Bull. 80~452-457. tana State Univ. No. 1. SILLOWAY, P. M. 1903. Birds of Fergus County, HUBBARD, J. P. 1965. The summer birds of the Montana. Fergus County Free High School, forests of the Mogollon Mountains, New Mexico. Lewistown. Bull. No. 1. Condor 67:404415. SKAAR. P. D. 1975. Montana bird distribution. JOHNSON, N. K. 1965. The breeding avifaunas of P: D. Skaar, Bozeman, Montana. the Sheep and Spring ranges in southern Ne- TATSCHL, J. L. 1967. Breeding birds of the Sandia vada. Condor 67:93-124. Mountains and their ecological distributions. JOHNSON, N. K. 1974. Montane avifaunas of Condor 69:479-490. southern Nevada: historical changes in species TEHBOHGH, J., AND 1. S. WESKE. 1975. The role composition. Condor 76:334-337. _ of competition in the distribution of Andean TOHNSON. N. K. 1975. Controls of numbers of bird birds. Ecolorrv 56:562%576. species on montane islands in the Great Basin. THOMPSON, L. S.; ANI) J. KUIJT. 1976. Montane Evolution 29:545-567. and subalpine plants of the Sweetgrass Hills, JOHNSON, R. E. 1966. Alpine birds of the Little Montana, and their relation to early postglacial Belt Mountains. Montana. Wilson Bull. 78:225- environments of the northern Great Plains. Can. 227. Field-Nat. 90:432448. JONES, E. T. 1972. Townsends’ Solitaire nesting in VUILLEUMIER, F. 1970. Insular biogeography in the Cypress Hills, Alberta. Blue Jay 30:33. continental regions: I. The northern Andes of LACK, D. 1969. Population changes in the land South America. Am. Natur. 104:373-388. birds of a small island. J. Anim. Ecol. 38:211- WALCHECK, K. C. 1969. Avian populations of four 218. plant communities in the Bear Paw Mountains, MACARTHUR, R. H., J. M. DIA~~ONU, AND J. P. KARR. Montana. Proc. Montana Acad. Sci. 29:73-83. 1972. Density compensation in island faunas. WAUER, R. H. 1971. Ecological distribution of the Ecology 53 :330342. birds of the Chisos Mountains, Texas. South- MACARTHUR, R. H., AND E. 0. WILSON. 1963. An west. Nat. 16:1-29. equilibrium theory of insular zoogeography. WILLIAMS, R. S. 1882. Notes on some birds of the Evolution 17:373-387. Belt Mountains, Montana Territory. Bull. Nut- MACARTHUR, R. H., AND E. 0. WILSON. 1967. The tall Ornithol. Club 7:61-63. theory of island biogeography. Princeton Univ. WILLSON, M. F. 1974. Avian community organiza- Press, Princeton, N. J. tion and habitat structure. Ecology 55:1017- MILLER, A. H. 1955. The avifauna of the Sierra 1029. de1 Carmen of Coahuila, Mexico. Condor 57: 154-178. Montana Department of Natural Resources and Con- MUSSEHL, T., P. SCHLADWEILER, AND R. WECKWERTH. seraation, 32 South Ezuing, Helena, Montana 59601. 1971. Forest grouse, p. 142-151. In T. Mus- Accepted for publication 15 December 1976.