Avian Abundance and Guild Structure in Two Montana Coniferous Forests Author(s): David A. Manuwal Reviewed work(s): Source: The Murrelet, Vol. 64, No. 1 (Spring, 1983), pp. 1-11 Published by: Society for Northwestern Vertebrate Biology Stable URL: http://www.jstor.org/stable/3536012 . Accessed: 10/07/2012 22:12 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp
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THE MURRELET MURRELET THE
64:1-11 64:1-11
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AVIAN ABUNDANCE AND GUILD STRUCTURE IN TWO MONTANA CONIFEROUS FORESTS DAVID A. MANUWAL ABSTRACT-Aviandensities and guild structurewere studied in an open Douglasfir(Pseudotsugamenziesii)stand on a south-facingslope and in a dense lodgepole pine (Pinus contorta)stand on a north-facingslope. Both stands have been greatlymodified by past logging activitiesand forestfires.Twenty-sevenbird species were censused on the two forests.Fifteenspecies were common to both types,six species were unique to the Douglas-firplot, and six species were unique to the lodgepole pine plot. Both density and number of species increased in both stands from 1967 to 1968. Largest increasesoccurredin populations of the Dark-eyedJunco,Chipping Sparrow,Evening Grosbeak, Pine Siskin, Yellow-rumped Warbler,and Hermit Thrush. A spruce budworm outbreak apparently contributedto these increases. Differencesin guild composition are attributedto structuralfeaturesof the forests. There are few published accounts (e.g. Ramsden et al. 1979) describing avian abundance and community structure in the coniferous forests of western Montana. The purpose of this investigation was to determine the species composition, density, and trophic structure of birds breeding in two different widespread forest communities in western Montana. STUDY AREA The study was conducted in 1967 and 1968 on the North Fork of Elk Creek in the Lubrecht ExperimentalForest,approximately64 km NE of Missoula, Montana. The two foreststands studied are in the Pseudotsugamenziesiiseries described by Arno (1979). More specifically,they are the borealisand P. menziesii/Agropyron spicatumtypes (Pfisteret al. 1977). Pseudotsugamenziesii/Linnea However, forsimplicity,these two types will be referredto as the lodgepole plot and Douglas-fir plot respectivelysince these stands are now dominated by these tree species. A narrow riparian strip(Manuwal 1967a) separated the two forestcommunitieswhich were on sloping terrainat an elevation of 1400 m. The Elk Creek drainage was logged during 1927; subsequent forestfiresand cattle grazing have furthermodifiedthe successional vegetationfeatures.In the summerof 1929, a forestfireoccurred throughoutthe North Fork area. Following this, a heavy stand of lodgepole pine (Pinus contorta) became established, primarilyon the north-facingslope. However, many trees that survived the logging also survived the forestfire.This resulted in an uneven-aged stand of several coniferous species. By 1967, these trees were about 30 m tall (see photographs in Manuwal 1967b, 1967c). In 1967, the effectsof the 1927 logging were still evident as small clearings in the Douglas-firplot and as narrow logging skids in the lodgepole plot where western larch (Larix occidentalis)and were removed (Fig. 1). Engelmann spruce (Picea engelmannii) Plot Douglas-fir This plot is on an 18 ha (44 acre) gently-slopingsouthernexposure. Disturbanceby fire,logging, and grazing have probably altered the normal successional patternof this stand. Douglas-firremains the dominant overstorytree with ponderosa pine (Pinus ponderosa)being the second most numerous tree(Table 1). The vegetationpatternis characterizedby numerousshallow draws which contain small clumps of Douglas-firand scatteredshrubs, grasses and forbs (see Manuwal 1967b fora complete description). LodgepolePlot This plot is on a 10 ha (25 acre) steep northernexposure across the valley fromthe Douglas-fir plot. The vegetation is an uneven-aged, dense, mixed species stand dominated by lodgepole pine 1
2
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64(1)
FIGURE 1. Aerial photograph of the two forestplots. The Douglas-firplot is the upper portion; the lodgepole plot is the dark lower portion. The fine-grainappearance in the middle of the lodgepole plot is a dense even-aged stand of lodgepole pine. Numerous canopy openings are visible in the lodgepole plot. The edge of the aerial photograph corresponds with the edge of the bird census grids.
SPRING 1983
3
MANUWAL:AVIAN GUILD STRUCTUREIN MONTANA FORESTS
TABLE 1. Tree composition,densityand dbh of tree species in the Douglas-firand mixed lodgepole pine plots in western Montana.1
Tree species Douglas-fir Ponderosa pine Lodgepole pine Subalpine fir Westernlarch Engelmann spruce Tree density (stems/ha) Mean distance (m) between trees
Douglas-firplot (18 ha) % composition (N = 196 x dbh (cm) trees) 72.0 22.6 5.4
21.2 8.6
22.6 32.3 22.5
Lodgepole plot (10 ha) % composition (N = 120 x dbh (cm) trees) 33.1
22.0
48.8 7.4 7.4 3.3 91.5 4.1
17.4 15.1 23.1 14.2
' Data frompoint-centered-quarter method.
interspersedwith small open areas (Table 1). The ridge top formingthe southern boundary is composed of western larch and Douglas-fir;overstorytrees here vary in height between 17-27 m. The ground cover is characterizedby a thickmat of bearberry(Arctostaphylos uva-ursi)and several other species. One-third the way down the slope there is a dense stand of 7-10 m tall lodgepole pine and western larch. Most of the slope consists of lodgepole pine, Douglas-firand larch (Table 1). The overstoryalong the logging skids is mainly westernlarch or lodgepole pine. At the bottom of the slope at the edge of the riparian zone, the overstorybecomes a dense stand of Douglas-fir, Engelmann spruce, and subalpine fir(Abieslasiocarpa).Where the overstoryis dense, the ground cover is sparse or absent. METHODS The spot-map method (Williams 1936; Hall 1964) was used to census birds. The census grid consisted of 14 lines of 1 m wooden lath stakes spaced 20 m apart. The census grid was 18 ha on the Douglas-firplot and 10 ha on the lodgepole plot. Censuses were taken as soon as there was enough light to see birdsand consisted of walking slowly along alternategrid lines. The lodgepole plot was censused 13 times (2 May to 23 June) in 1967, and 7 times (2 May to 4 June) in 1968. The Douglas-firplot was censused 13 times (26 April to 21 June) in 1967 and 8 times (1 May to 5 June) in 1968. Singing males were recorded by locating their position on a mimeographed map of the grid. Territoryboundaries were determinedby using a map summarizingall observationsof territorial males fora particularspecies on one plot. The occurrenceof groupings of observationsindicated the extentof territories.Some species, especially juncos (see Table 2 forscientificnames of birds), Chipping Sparrows, and Western Tanagers required extra observations to determine territory boundaries since males did not travelextensivelyin theirterritories.In such cases, it was necessary to locate and follow the birds in question until enough observationswere obtained to completely delineate the territory.During the two years, approximately250 additional hours of observation were made on the two sites. Often, fightsor song-duelling between two territorialmales at the boundaryof theirterritoriesgave the best information.Location of nestshelped delineate territorial boundaries and confirmedthe mating of a territorialmale. The numberof territorieson the studyarea was determinedby summingthose territorieswhose perimeterslie entirelywithin the study area and by adding a percentage of each territorylying only partially on the study area. This percentage was derived by calculating the average size of whole territoriesand by observations of males holding these partial territories.The area of each whole territorywas measured with a planimeter.Only those territorieswith well-grouped observations were used. Birds breeding on the study area were classified by feeding guild, a systemdesigned by Salt (1953), and refinedby Root (1967). Salt divided the avifauna into eight distinctfeeding groups or guilds, based on the feeding location and type of food sought or method of feeding. Some species,
4 THE MURRELET
64(1)
Td
I
Ts
Fi
rn n I
Gi
IsI
Fs Gs
F-
I
I
1
I
I
I
E-
m---
1
I *1
ON
I
u-i
I
LODGEPOLE
PINE
DOUGLAS-FIR
* 200 FIGURE2. Avian standingcrop biomass(g) partitionedinto seven foragingguilds (afterSalt Td timber-drilling, Ts timber-searching, Fi foliage-insect, Fs Gi ground-insect, 1953);Ai air-insect, Gs ground-seed. Darkbarsare 1967data,lightbarsare 1968. foliage-seed, forinstancetheChippingSparrow,drastically changetheirdietseasonally.In suchcases,thefood was basedon whethera certainitemformedat least50%ofthediet.In mostcases,Martin category et al. (1951)was used as thedietreference. ofbirdswerederivedfromspecimensin Wet-weights the University of Montanavertebrate collectionand fromthe Museumof Vertebrate Zoology, ofCalifornia, University Berkeley. The point-centered-quarter method(Curtisand Cottam1962) was used to obtainquantitative Leaf surfacearea was calculatedusingbasal area and densityvalues derived plantinformation. fromthe point-centered-quarter methodand by the regressionequationsgiven by Gholz et al. (1979).SpecificleafsurfacevalueswereobtainedfromGholz et al. (1976)and Grierand Running in aviandensities. U-test(Siegel 1956)was used to comparedifferences (1977).The Mann-Whitney RESULTS SpeciesComposition Twenty-sevenbird species were censused and territoriesdelineated during 1967 and 1968 forthe two forestplots (Table 2). Fifteenbird species were common to both plots. Six species were unique to the Douglas-firplot: Cassin's Finch, NorthernFlicker,American Robin, Warbling Vireo, Common Nighthawk, and Dusky Flycatcher.Six species were found breeding only on the lodgepole plot: Spruce Grouse, RuffedGrouse, Gray Jay,Pileated Woodpecker, Golden-crowned Kinglet, and Olive-sided Flycatcher.In addition, four wide-ranging species were present in both plots, but territoriescould not be delineated and thereforethey are not included in Table 2. These were the Goshawk (Accipitergentilis),Cooper's Hawk (A. cooperii),Red-tailed Hawk (Buteojamaicensis),and Great Horned Owl (Bubo virginianus). Density The bird densities in the two coniferous forestplots (Table 2) were not significantly different(U = 155, p = 0.05). Total bird densities of the two foresttypes were higher in
SPRING 1983
MANUWAL: AVIAN GUILD STRUCTUREIN MONTANA FORESTS
5
TABLE 2. Breeding bird density (pairs/40 ha) in Douglas-firand lodgepole pine forests. Bird species Ground-seed Spruce Grouse (Canachitescanadensis) RuffedGrouse (Bonasa umbellus) Cassin's Finch (Carpodacuscassinii) Dark-eyed Junco (Juncohyemalis) Foliage-seed Evening Grosbeak (Coccothraustes vespertina) Pine Siskin (Carduelispinus) Gray Jay (Perisoreuscanadensis) Ground-insect Northern Flicker (Colaptesauratus) Pileated Woodpecker (Dryocopuspileatus) American Robin (Turdusmigratorius) Hermit Thrush (Catharusguttata) Swainson's Thrush (C. ustulata) Townsend's Solitaire (Myadestestownsendi) Chipping Sparrow (Spizella passerina) Foliage-insect Mountain Chickadee (Parus gambeli) Golden-crowned Kinglet (Regulussatrapa) Ruby-crownedKinglet (R. calendula) Solitary Vireo (Vireosolitarius) Warbling Vireo (V. gilvus) Yellow-rumped Warbler (Dendroicacoronata) WesternTanager (Pirangaludoviciana) Air-insect Common Nighthawk (Chordeilesminor) Hammond's Flycatcher (Empidonaxhammondii)
Douglas-fir 1968 1967
Lodgepole pine 1968
1967
5.0
5.0 +
5.8 24.0
28.8
33.0
32.0
4.5
9.2
8.0
20.0
4.5
9.2
8.0
16.0
+
+
+
+
1.2
4.5
6.9
0.8
2.3
+
4.0
9.0
6.9
23.0
14.0
2.3
4.0
2.0
36.0
51.8
28.0
22.0
3.0
5.8
4.0
8.0
8.0
9.0
5.0
5.8
7.0
5.2
8.0
7.5
10.9
3.0 4.0
8.0
19.0
12.0
26.0
10.5
11.5
4.0
4.0
+
+
7.0
6.9
4.0
6
THE MURRELET
64(1)
TABLE 2. Continued. Lodgepole pine
Douglas-fir Bird species
1967
1968
6.0
7.5
Dusky Flycatcher (E. oberholseri) Olive-sided Flycatcher (Contopusborealis)
1968
1967
+
Timber-drilling Williamson's Sapsucker throideus) (Sphyrapicus Timber-searching Red-breastedNuthatch (Sittacanadensis) Total density
8.0 149.0
5.8 200.6
2.5 143.2
2.0
+
1.2
9.0 188.0
+ Species present,but densityless than 0.5 territories/40 ha.
1968 than in 1967, but not significantly (U= 154, Douglas-fir plot; U= 105, lodgepole plot). On the Douglas-fir plot, 16 species increased, three decreased, and two showed no change. Largest increases occurred with the Dark-eyed Junco, Evening Grosbeak, Pine Siskin, Chipping Sparrow and Yellow-rumped Warbler (Table 2). On the lodgepole plot, 13 species increased, three decreased and five were unchanged. Largest increases were the Evening Grosbeak, Pine Siskin, Hermit Thrush, and Yellow-rumped Warbler. Swainson's Thrush showed a notable decrease. Trophic Structure Avian wet-weight biomasses were higher in the lodgepole plot (5779 g/40 ha) (Fig. 2) than the Douglas-fir plot (4165 g/40 ha) due to the presence of the Spruce Grouse, which accounted for 2100 g/40 ha. Even without this species, the lodgepole plot supported more primary consumers than the Douglas-fir plot. In this study the primary consumers consisted of two foraging guilds (ground-seed and foliage-seed) totalling 7 species. The Douglas-fir plot (Table 3) supported more secondary consumer biomass which consisted of five foraging guilds (air-insect, timber-drilling, timber-searching, foliage-insect and ground-insect) totalling 17 species.
TABLE 3. Foraging guilds and standing crop biomass of birds in the Douglas-firand lodgepole pine habitatsin 1968. Douglas-firtype
Foraging guild Air-insect Timber-drilling Timber-searching Foliage-insect Ground-insect Foliage-seed Ground-seed Total
No. of species 3 1 1 6 6 2 2 21
Percentl 7.1 0.6 2.9 27.3 35.6 9.2 17.3
'Percent of total population density(see Table 2). 2Expressed as grams/40ha.
Lodgepole type
Standing crop biomass2 159 54 59 837 1760 626 670 4165
No. of species 2 1 1 6 5 3 3 21
Percent 2.1 1.1 4.8 30.9 22.3 19.1 19.7
Standing crop biomass 41 90 91 716 861 1314 2666 5779
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MANUWAL: AVIANGUILDSTRUCTURE IN MONTANAFORESTS
7
DISCUSSION Many authorshave shown thathabitatpatchinessand vegetativeheterogeneitystrongly influencebird species diversityand richness(e.g. MacArthurand MacArthur1961,Cody 1968, James1971, Anderson and Shugart 1974, Balda 1975, Wiens 1976, Rotenberryand Wiens 1980, etc.). The two forestplots I studied differedgreatlyin general appearance. The Douglas-firplot is essentiallyan open standwith much edge and few coniferspecies whereas the lodgepole plot is a closed canopy stand with a few openings but it has several species of coniferscomprisingthe canopy. Despite the differencesin foreststructure,the bird species composition of the two plots was very similar (Sorensen's Index of Similarity(IS) = 0.85 (1967), 0.71 (1968); Mueller-Dombois and Ellenberg 1974). Lodgepole stands are typicallyhomogeneous. Bird communitiesoccupying pure or nearly pure lodgepole pine foreststend to be depauperate in species richnessand abundance. For example,Salt (1957) foundonly 8 breedingspecies (37 pairs/40ha) in a nearly pure lodgepole stand in Wyoming.Austin and Perry(1979) found 10 species (40 pairs/ 40 ha) in a pure stagnated stand in Utah. Kingery (1970) found 10 species (58 pairs/40 ha) in a mixed stand (70% lodgepole) in Colorado. On the other hand, lodgepole stands with apparentlygreatercrown structuralcomplexitydue to maturityor the presence of other tree species (usually presentdue to disturbanceor changes in soil characteristics) appear to provide suitable habitat for additional bird species. For example, Austin and Perry(1979) found 20 species (134 pairs/40 ha) in a maturelodgepole stand with lesser amounts of subalpine firand Engelmann spruce. In my mixed lodgepole stand,I found 17-21 bird species (149-188 pairs/40 ha). The higher bird densities in 1968 in both plots may reflectnormal fluctuationsin numbers but some population increases may have represented the beginning of numerical responses to an outbreakof western spruce budworm (Choristoneura occidentalis) (Manuwal 1983). The estimated leaf biomass on the Douglas-firplot was 3 times greaterthan on the lodgepole plot (Table 4). The leaf surface area however was 2.5 times greateron the lodgepole plot. This differencecan be attributedto the differencesin tree species composition and crown geometryof individual tree species (Table 1). The higher avian standing crop biomass of the primaryconsumers on the lodgepole plot may be due to the higher leaf surfacearea. Gholz (1982) has shown that net primaryproduction (including production of seeds) tends to increase with leaf surfacearea. The Douglas-fir plot, however, supported more avian biomass in secondary consumers, especially foliage-, ground-, and air-insectivoresthan did the lodgepole plot. This is apparently because of the greaterleaf biomass since Grierand Logan's (1977) data stronglysuggest that insect abundance increases with leaf biomass in Douglas-firforests. Birds have been shown to respond to differencesin foreststructureby modifying theirforagingbehavior (Mauer and Whitmore1981). Furthermore,Holmes et al. (1979) determined that there are three distinctforagingenvironmentsin the mixed coniferdeciduous foresttheystudied: the forestfloor,the trunksof trees,and the outerbranches and foliage. A fourthenvironmentin my plots,the air space above and beside treesand shrubs,was utilized extensivelyby air-insectivores.One mightexpect then thatforests that differin general structurewill also exhibit differencesin foragingguild composition. A briefdiscussion of foragingguilds will depict more clearlythe responses of birds to the structuralfeaturesof the two forestplots in this study. The air-insectguild is well-representedon the Douglas-firplot (3 species, 13-14 pairs) but only two species (4 pairs) on the lodgepole plot. This can easily be attributedto the open canopy of the Douglas-firplot which offersforagingopportunitiesforthe Common Nighthawk, Dusky Flycatcherand Hammond's Flycatcher.These species specialize on flyinginsects. There appears to be little differencebetween the two stands in the timber-drilling
TABLE 4.
Basal area, leaf biomass, and leaf surface area for two western Montana coniferous forests. Tree species Pseudotsuga men7ZwiCSii
Piuuls ponderosa
Pinus conItorta
Larix occident
Douglas-fir type Basal area (m2/ha) Leaf biomass (kg) Leaf biomass/basal
area (kg/ m2)
Leaf biomass/basal area X basal area Leaf area (M2/m2)
Lodgepole type Basal area Leaf biomass Leaf bioinass/basal Leaf biomass/basal Leaf area
3.75 1862 278
area area X basal area
(m2/m2)
1042
1.74
7.25 477 279 2023 3.38
3.0 1126 262
786
1.02
0.35 118 200
70 0.10
8.40 382 206 1730 2.25
0.82 136 316 259 0.52
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MANUWAL:AVIAN GUILD STRUCTUREIN MONTANA FORESTS
9
and timber-searchingguilds. Nest-siteavailability and the abundance of wood-boring insects largely determines primarycavity-nesterabundance (Conner et al. 1975) and neitherof these variables were measured in this study. The foliage-insectguild had the highest number of species and individuals in both plots. There was a large amount of overlap in species composition(IS = 0.83). The Warbling Vireo was unique to the Douglas-firplot probably because of the relativelylarge but patchy deciduous shrub component. The Golden-crowned Kinglet was unique to the lodgepole plot, presumablybecause of its affinityfordense canopy coverage. The ground-insectguild had the second highest number of species and individuals but the two stands showed less overlap in species composition (IS = 0.72) than did the foliage-insectguild. There was a large differencehowever in the avian biomass, since the Douglas-fir plot had double the avian biomass in this guild compared with the lodgepole plot. This is probablydue to the greateramount of ground vegetationon the open Douglas-firplot. The dense canopy of the lodgepole plot prevented the establishment of a lush understoryin most areas. The ground vegetationon the Douglas-firplot would thereforeattractlarger numbersof insects.The avian biomass differentialis also the result of the presence of two relatively large species on the Douglas-firplot, the Northern Flicker and American Robin, and the very large number of Chipping Sparrows.
The foliage-seed guild contained two major species, the Evening Grosbeak and the Pine Siskin. Densities of these two were twice as high on the lodgepole plot as on the Douglas-firplot. During the time of this study, these two species fed extensivelyon spruce budworm larvae (Manuwal 1983). Garton(1980) has found thatthese two species respond both functionallyand numericallyto outbreaksof westernspruce budworm. It seems likely thatthe densitydifferencesbetween the two forestplots may reflecteither nest-sitepreferencesor the response of the birds to budworm distributionand abundance and thereforehave littlerelevance to seeds. The differencesin species composition of the ground-seed guild can be largely explained by habitatpreference.The Spruce Grouse is rarelyfound in open coniferstands and in Montana is stronglyassociated with lodgepole pine (Stoneberg 1967). The Ruffed Grouse was primarilyassociated with a nearbyriparianzone but did occasionally utilize a portion of the lodgepole plot. My observations indicate that Cassin's Finch prefers open xeric sites like that of the Douglas-firplot. Only the Dark-eyed Juncowas found abundantly on both plots. This species exhibits a wide habitat preference,occupying virtuallyall coniferous foresthabitats in western Montana (pers. obs.). It is therefore not surprisingthat it is an abundant species in both study plots. CONCLUSIONS
Although quantitativemeasures of only a few structuralfeatureswere made in this study,my results agree with more recent investigationson the relationshipsbetween breeding bird populations and foreststructure.Vertical and horizontal structuralcomplexityappear to influencethe location and abundance of food resourceswhich in turn provide opportunities for guild development. The quantitative relationship between foliagebiomass and structureand insectabundance needs to be more intensivelystudied so that the bird-vegetation-foodrelationshipscan be more fullyunderstood. ACKNOWLEDGEMENTS
I would liketo thankRussellBalda,RichardHutto,RichardConner,and especiallyNaomiManuwal forreviewingvariousdraftsof thismanuscript. CharlesGrierassistedin calculatingleaf surfaceareavalues.RichardTaberprovidedadvicein theinitialstageofthisstudy.The Montana Forestand Conservation ofMontana,providedfinancial Station,University Experiment support.
10
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64(1) LITERATURECITED
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Wildlife Science Group, College of Forest Resources, University of Washington, Seattle, WA 98195. Received 21 May 1982, accepted 11 January 1983.
