Breeding Bird Densities, Species Composition, and Bird Species Diversity of the Algodones Dunes

BREEDING BIRD DENSITIES, SPECIES COMPOSITION, AND BIRD SPECIES DIVERSITY OF THE ALGODONES DUNES

KATHLEEN E. FRANZREB, Bureau of Land Management, 1695 Spruce Street, Riverside, California 92507

The AlgodonesDunes, also known as the Imperial Sand Dunes, in Imperial County are consideredto be the largestsand dune systemin Californiaand one of the largestin the United States(California Division of Mines 1952, Miller 1957). Within the AlgodonesDunes are several different habitat typeswhich contribute to the biologicaluniqueness of the area. Few data are availableon the arian community of thesedunes. The purposeof this investigationwas to determinearian speciescomposition,breeding bird densities,bird speciesdiversity, and plant speciesdiversity in the varioushabitats associated with the AlgodonesDunes.

STUDY AREA The AlgodonesDunes are locatedin the extreme southeasternpor- tion of Californiawith the southernmost6.4 km extendinginto Mexico. The dunesare borderedon the west by East Mesa, on the north by MammothWash, on the eastby the ChocolateMountains and the Cargo MuchachoMountains, and on the south by the ColoradoRiver Delta. The CoachellaCanal lies alongthe westernboundary of the duneswhile the SouthernPacific Railroad borders the easternfringe, eventuallyin- tersectingthe CoachellaCanal to form the northernborder. This dune systemis approximately64-.4 km long,varying in width from 4.8 km to 9.7 km, and is oriented in a northwesterlydirection. The dunesare long and narrowat the northerntip, extendingto a wid- er, southernbase. Dune crestsmay exceed90 m althoughmost range between60-90 m (WesteeServices Inc. 1977). Elevationranges from 33-168 m.

Climate The AlgodonesDunes are locatedwithin the CahuillaBasin, an area of both low rainfalland humidity,and extremely high summer tem- peratures. Rainfall results from occasionaltorrential late summer show- ers or from infrequentlight winter rains. Averageannual rainfall for Amos, a site on the eastern side of the dunes, is 71.9 mm (Norris and Norris 1961).

Vegetation Severalbasic habitat types occur on the AlgodonesDunes. A considerable portion of the dunes consistsof barren, wind-blown sand form- Western Birds 9:9-20, 1978 9 BIRDS OF THE ALGODONES DUNES ing, in places,steep troughs and peaks. However,much of the area is not devoid of vegetationbut containspsammophytic ("sand-loving") specieswhich occur throughout this dune system,some of which are endemic. Fingersof partially stabilizedsand invade the bordersof the Algodo- nesDunes. Within thesefingers of land and alongmuch of the periphery of the dunes, Sonoran CreosoteBush scrub,characterized primarily by Creosote Bush, occurs. On the western side of the dunes, stands of almost pure CreosoteBush, some reachingenormous size, are found. Gigantism presumably results from continual water seepagefrom the unlined CoachellaCanal. This vegetationtype is referredto as "dense" Sonoran Creosote Bush scrub in this study. Although unusually large CreosoteBushes are alsoprevalent in the more mesicsites on the eastern dunesborder, there are substantialareas of relativelysparsely distributed CreosoteBushes referred to as "open" SonoranCreosote Bush scrub. Particularly interesting is the desert microphyll woodland habitat which has a patchy distributionalong the easterndune edge. In these areasthe vegetationis luxuriant by desertstandards, probably as a re- suit of temporaryimpounding of runoff water during infrequentrains (Norris and Norris 1961). As water drains from the Chocolate and CargoMuchacho mountains down the alluvial fans eastof the dunes,it is effectively impededby the large sand mass. This servesas a barrier, thereby creatingin some locationsa sufficiently mesicenvironment to permit developmentof a very dense,lush, desert microphyll woodland. Consequently,this habitat is found on only one sideof the dunesystem.

METHODS

Arian Densities

Arian speciescomposition and breedingbird densitieswere determined for four habitats: desertmicrophyll woodland, desert psammophytic scrub, open Sonoran CreosoteBush scrub and dense Sonoran Creosote Bush scrub. Birds were sampled using the spot-map method (Williams 1936), whereby one 20 ha grid pattern (11.4 ha in the microphyll forest due to habitatpatchiness) was established in eachof the four habitats,using plastic flaggingand wooden stakesplaced at 50 m intervalsalong 5 par- allel lines, each 450 m in length and 112.5 m apart. Each flag or stake was labeled with a number correspondingto the transectline and a let- ter correspondingto the distance traveled from the beginningof the line. Each habitat was censusedthree to six times from 20 April to 6 May 1977. Censusescommenced one-half hour after sunriseand were completedby 0900. Density valuesare expressedin terms of number of breeding individualsper 40 ha (100 acres),a commonlyused base for arian studies.

10 BIRDS OF THE ALGODONES DUNES

Vegetation Analysis Vegetation in each plot was sampledusing the line-interceptmethod describedby Mueller-Domboisand Elenberg(1974). A 100 m transect was sampledin each habitat type, thus providinginformation on plant speciesdominance (an indicationof cover),relative dominance, frequency and relativefrequency. The desertmicrophyll woodland habitat was located on the eastern edge of the dunes,approximately 4.8 km north of the intersectionof Glamis and Highway 78 (T 13 S, R 17• E, sectionunsurveyed). The psammophyticscrub plot was placedroughly 500 m southof Highway 78 and 200 m eastof the Gecko CampgroundRoad (T 13 S, R 17• E, Sec. 36). The open Sonoran CreosoteBush scrub plot was established 4.5 km north of Glamis(T 13 S, R 17• E, sectionunsurveyed), whereas the denseCreosote Bush plot was located approximately50 m east of the CoachellaCanal and 2.8 km southof Highway78 (T 14 S, R 17 E, sections I and 12).

Diversity Diversity values (to base e) were determinedfor the avian community in eachhabitat type usingShannon's (1948) formula. Plantspec- ies diversityindices were calculatedfrom relativedominance values for each habitat.

RESULTS

Vegetation Resultsof the vegetationalanalysis indicate that the desertmicrophyll woodlandhad the highestnumerical dominance and frequency (Table 1). The latter valueis an indicationof plant density. The nu- merousPalo Verde trees were denseand tall, many reaching8 m and above(Figure 1). Much of the understorywithin the plot wasrelative- ly thick and provideddense cover for speciessuch as Gambel's Quail. In contrastto the microphyllwoodland habitat, the other plots con- tainedmuch lesscover. Vegetationin the psammophyticplot wasun- evenlydistributed, low in stature,and relatively sparse (Figure 2). Within the westerncorner of the open CreosoteBush plot were shal- low, narrow washescontaining Palo Verde which were utilized for nesting by severalspecies such as Verdins and Black-tailedGnatcatchers. The individualCreosote Bushes were fairly evenlyspaced in the open CreosoteBush habitat (Figure 3), and were not inordinatelylarge as they were in areasbordering the canal. Analysisof the denseSonoran CreosoteBush scrub habitat yielded much larger dominanceand frequency valuesthan did the sparsecreosote area. The denseCreosote Bushplot is shownin Figure4. 11 BIRDS OF THE ALGODONES DUNES

Fibre 1. Desertmicrophyll woodland, Algodones Dunes, California (for location see Methods), October 1977.

Figure 2. Desert psammophyticscrub, October 1977. 12 BIRDS OF THE ALGODONES DUNES

Figure 3. Open SonoranCreosote Bush scrub, October 1977o

Figure 4. Dense Sonoran Creosote Bush scrub, October 1977.

13 BIRDS OF THE ALGODONES DUNES

A comparisonof the plant speciesdiversity (PSD) valuesfor the var- ious communitiesindicates that the microphyll woodland plot had the highestPSD (1.26), whereasthe pure standof densecreosote had a diversityvalue of zero. The psammophyticscrub and open Creosote Bushscrub had intermediatevalues (0.77 and 1.05 respectively)between these two extremes.

Table1. Vegetationanalysis of fourAlgodones Dunes habitats in ImperialCounty, California. DESERT MICROPHYLL WOODLAND

Relative Relative Domi- Domi- Fre- Fre- SPECIES nance1 nance2 quency3 quency4 Brandegea Brandegeabigelovii 22.3 16.4 28 43.1 Palo Verde Cercidiurnfloridurn 74.8 54.9 12 18.4 Galleta Grass Hilaria rigida 1.5 1.1 2 3.1 Creosote Bush Larrea tridentata 22.5 16.5 6 9.2 Hairy-podPeppergrass Lepidiurnlasio carpurn O.7 O.5 i 1.5 Tumble-mustard Sisyrnbriurnaltissirnurn O.7 0.5 i 1.5 Globemallow Spaeralceaemoryi 13.7 10.1 15 23.1 TOTAL 136.2 100.0 65 99.9

DESERT PSAMMOPHYTIC SCRUB

Croton Croton wigginsii 9.9 77.3 11 68.8 Desert Dicoria Dicoria canescerts 0.8 6.3 1 6.3

Mormon-tea Epbedratrifurca 1.5 11.7 3 18.8 Blazing star Mentzelia Iongiloba 0.6 4.7 1 6.3 TOTAL 12.8 100.0 16 100.2

OPEN SONORAN CREOSOTE BUSH SCRUB

Palo Verde Cercidiurn floi'idurn 1.1 9.6 1 lO Indigo Bush Dalea emoryi 0.5 4.4 1 lO Jimson Weed Datura discolor 0.2 1.8 1 lO Brittle-Bush Enceliafarinosa 0.3 2.6 1 lO 14 BIRDS OF THE ALGODONES DUNES

Table 1 (cont.) Relative Relative Domi- Domi- Fre- Fre- SPECIES nance• nance2 quency3 quency4 Galleta Grass Hilaria rigida 0.8 7.0 1 10 Creosote Bush Larrea tridentata 8.2 71.9 4 4O SpanishNeedles Palafoxia arida var. arida 0.3 2.6 i 10 TOTAL 11.4 99.9 10 100

DENSE SONORAN CREOSOTE BUSH SCRUB

Creosote Bush Larrea tridentata 68.9 100 23 100

i Dominance= Number of metersa givenspecies was encountered along a 100 m line-intercepttransect. 2 Relative dominance(percent) = (Dominanceof a species/Totaldominance of all species)x 100. 3 Frequency=The number of plants of a given speciesencountered on the line- intercept.. 4 Relativefrequency (percent) -- (Frequencyof a species/Totalfrequency) x 100.

Table 2. Avian speciescomposition, densities, and distributionin the Algodones Dunes,Imperial County, California.

BREEDING DENSITY (BIRDS/40 ha)

Desert Open Dense Micro- Desert Sonoran Sonoran phyll Psammo- Creosote Creosote Wood- phytic Bush Bush SPECIES land Scrub Scrub Scrub Turkey Vulture Catbartes aura V V Red-tailed Hawk Buteo jarnaicensis V V Marsh Hawk Circuscyaneus 4.0 Prairie Falcon Falco mexicanus American Kestrel Falco sparverius Gambel's Quail Lopbortyx gambelii 20.6 White-wingedDove Zenaida asiatica 8.3 v Mourning Dove Zenaida macroura 57.8 v 4. o P Ground Dove Columbinapasserina v 15 BIRDS OF THE ALGODONES DUNES

Table 2 (cont.) Desert Open Dense Micro- Desert Sonoran Sonoran phyll Psammo- Creosote Creosote Wood- phytic Bush Bush SPECIES land Scrub Scrub Scrub Roadrunner Geococcyxcalifornianus 8.3 P Long-earedOwl Asio otus 8.3 LesserNighthawk Cbordeilesacutipennis * 4.0 V 4.0 Costa'sHummingbird Calypte costae 8.3 Ladder-backedWoodpecker Picoides scalaris 8.3 Ash-throatedFlycatcher Myiarcbuscinerascens 24.8 Black Phoebe Sayornisnigricans V Say'sPhoebe Sayornissaya V Cliff Swallow Petrocbelidonpyrrbonota V Verdin Auriparusflaviceps 54.5 12.0 4.0 Cactus Wren Carnpylorbyncbusbrunneicapillus 24.8 LeConte's Thrasher Toxostorna lecontei 16.6 V 4.0 Crissal Thrasher Toxostorna dorsale 8.3 V 4.0 Black-tailed Gnatcatcher Polioptila melanura 30.3 9.3 20.0 LoggerheadShrike Lanius ludovicianus 4.2 V Starling Sturnusvulgaris V House Finch Carpodacusmexicanus 24.8 V

TOTAL DENSITY 308.2 4.0 25.3 40.0 Number of breedingspecies 15 1 3 6 Number of migrant species 28 3 8 11 Censusdates 20, 21, 21, 22, 27, 28, 20, 21, 22, 23, 27, 28 29 Apr. 22, 29 27 Apr. Apr. Apr. 4, 6 May V = visitor (seen throughout the springand summerin the area but not nesting on the plot). P = Present,breeding density undetermined. * = Breedingwithin 0.5 km in nearby creosotescrub. 16 BIRDS OF THE ALGODONES DUNES

Avian SpeciesComposition and Densities The desertmicrophyll woodland contained the highestdensity of breedingbirds as well asthe greatestnumber of breedingspecies (Table 2). More speciesincluding migrants and visitors,were observedthere than in any other habitat. Verdins,Black-tailed Gnatcatchers and House Fincheswere particularly abundantin the woodland. The denseCreo- sote Bushscrub supported the next highestdensity and containedfive breedingspecies. Of 17 speciesnoted in the open creosotehabitat, only 3 were nesting,with the Verdin achievingthe highestdensity. Desert psammophyticscrub was the least utilized habitat. Only the LesserNighthawk was observednesting there. The avifauna of the microphyll woodland habitat representedthe highestbird speciesdiversity (BSD) value (2.44) in contrastto a diversity of zero in the psammophyticscrub plot. DenseCreosote Bush scrub possesseda higher BSD than did open CreosoteBush scrub (1.64 versus1.01 respectively).

DISCUSSION

Speciesoccurrence is determined by how closelya givenhabitat meets the species'niche requirements. Such requisitesinclude, but are not limited to, food (quality, accessibility,quantity), nestsites, song posts, cover, water availability (critical for some species),presence of predators, foliage volume, and climate. The quality of the habitat influences the total densityof the avifauna. Coverand the amountof foliageavailable are particularly important sincethey provide, at least for many species,nest sites,song posts, food, habitat for insectsupon which in- sectivorousbirds prey, foragingsurface, and protection againstboth inclement weather conditionsand predators. The resultsof this study providean estimateof breedingbird density and are not absolutevalues. Becauseof the rather broad nestingperiod for these desertbirds the plots could not be censusedat the peak of the breedingseason for every species. For example,when the study was initiated one pair of Long-earedOwls was feeding nestlings. In con- trast, LesserNighthawks were in the processof establishingterritories and young did not hatch until June. Eventhough the censuseswere somewhatlate for some speciesand early for at least.one other, the resultsrepresent an estimationof breedingbird density at a particular stagein the breedingseason. For most species,this representsa reason- ably accurateestimate. Severalspecies undoubtedly foragedin a habitat type different from that in which they nested. Such was the casefor the Long-earedOwl which nestedin microphyllwoodland but foragedover the nearbyopen creosotescrub and psammophyticscrub habitats. Becauseof this, their density was probably overestimatedsince their territory encompassed 17 BI,RDS OF THE ALGODONES DUNES a largerarea than just the woodland.Most of the species,however, both foragedand nestedwithin the sameterritory. In this study the microphyll woodland provided far more cover, as reflected in the dominancefigures, than did any of the other habitats. The robust Palo Verde trees and denseunderstory were particularly at- tractive to certain species. For example, thicket-dwellingspecies such as the CrissalThrasher, Gambel's Quail and Ladder-backedWoodpecker appearedto be restrictedto this habitat. Somespecies such as the Long- eared Owl, Ladder-backedWoodpecker, Ash-throated Flycatcher and HouseFinch requiretall plantsfor nesting(Raitt and Maze 1968). The vegetationin the microphyllwoodland provided considerable foraging substratein addition to nestsites and songposts. It alsoserved as a ref- uge againstthe hot summertemperatures. During the migratoryseason many migrantstook advantageof the attributesof this habitat. The total avifaunadensity in the microphyllwoodland approximates the estimationof the breedingbird densityfor MilpitasWash (328-342 birds/40. ha, Tomoff, unpubl.), a microphyll woodland about 32 km east of the microphyll woodland plot in this study. In contrastto the microphyll woodland, the psammophyticscrub habitat provided very little coverand a relativelylow plant speciesdiversity. Consequently this habitat type supporteda very meageravifauna. It is not just the foliage volume or density but, perhapsmore im- portantly in many cases,the kind or quality of vegetationavailable in a habitat that influencesspecies density. Although extensivestands of pure creosoteare prevalentthroughout the desertsof the Southwest, few birds selectthem as breedinghabitats. Hensley(1954) found no nestingbirds in a pure SonoranCreosote Bush fiat in OrganPipe Cactus National Monument, Arizona. However, if creosoteis part of a fairly diverseplant community and is intermingledwith cacti, mesquite(Pro- sopis sp.) or Catclaw (Acaciagreggii), a fairly substantialnumber of birds Will use it (Anderson and Anderson 1946). A habitat of Creosote Bush, Burro-Weed (Ambrosia dumosa), Brittle-Bush, Buckhorn Cholla (0t9untia acantbocart9a),and Catclaw, supported127 birds/40 ha and 14 species(Hutchinson 1942). In another study in Arizona, a Sonoran Creosote Bush scrub plot supporteda densityof 70 birds/40 ha and 16 species(Hensley 1954). Thoughfew studieshave examinedits use,some foraging may occur within CreosoteB.•ushes. Anderson and Anderson(1946) found Verdins andgnatcatchers (Poliot9tila sp.)apparently searching for insectson creosote. They observedCactus Wrens walking alongthe ground, peeringat the twigs,and jumping upward to catchfoliage-inhabiting insects. They also noted House Finchesand HouseSparrows (Passer domesticus) nib- bling on the creosotebuds, pulling off the fruits, crushingthem, and eatingthe seeds.

18 BIRDS OF THE ALGODONES DUNES

PresumablyCreosote Bushes are of limited value in providingsuitable nest sitesbecause of their relativelythin twigsand non-supportive branches. However, the large Creosote Bushespresent in the dense CreosoteBush habitat providedmore elaboratebranches and stronger support. Black-tailedGnatcatchers and Verdinsevidently had little dif- ficulty in usingthese Creosote Bushes for nest sites. It has generallybeen thought that the more floristicallydiverse habitats havea more diverseand complexfauna. Both the numberof plant speciesand amount of vegetativecover may influence arian densities. In the dense CreosoteBush plot, the plant speciesdiversity was zero, yet this habitat containedmore birds than did the more botanicallydi- verse sparseCreosote Bush plot. Presumablythis was becausethe dense CreosoteBush habitat provided a greaterstructural diversity than the open Creosote Bush. In a study of varioushabitats in the Sonoran Des- ert of southernArizona, Tomoff (1974) found a positiverelationship betweenBSD and physiognomiccoverage diversity along a habitatgra- dient. In desertscrub habitats, plant speciescomposition was extremely significantin determiningmakeup of breedingbird communitiesand in providingsuitable nest sites(Tomoff 1974). Apparentlyit is not just the amount of and distributionof coveramong the variouslife-forms but alsothe floral compositionwhich is important. Other workers have noted a positivecorrelation between BSD and the foliage height diversity (FHD) (MacArthur and MacArthur 1961, MacArthur et al. 1962, 1966, Karr and Roth 1971, and others). Yet this relationship does not hold for all habitats (Balda 1969, Carothers et al. 1974, and others). Tomoff (1974) found the MacArthur model to be inconsistentin predictingBSD from FHD valuesin desertscrub communities.Since FHD valuesfollowing the abovecited methodwere not derivedfor the plots in this study, a comparisonwith the aforemen- tioned results cannot be made. Presumably,along with FHD and PSD, the actualplant speciespres- ent should be considered. Certain plant speciesevidently are of more value to birds than are others. Hence, it is not just the diversityof plants (either with respectto FHD or PSD) which may influenceBSD but also factors such as the compositionof the flora, the amount of foliageavailable, and the structuralcomplexity of the habitat.

SUMMARY

Avian speciescomposition, breeding bird densitiesand habitat pref- erenceswere determinedfor four habitatsin the AlgodonesSand Dunes in southeasternCalifornia. Desertmicrophyll woodland supported the highestbreeding bird density (308.2 birds/40 ha), the most breeding species(15), the greatestoverall number of species(49, includingmi- grants and visitors)and the highestbird speciesdiversity (2.44). This 19 BIRDS OF THE ALGODONES DUNES

is in contrastto sparselyvegetated psammophytic scrub habitat which contained a density of 4.0 birds/40 ha and only one breedingspecies. Dense, luxuriant, pure CreosoteBush scrub habitat and an open Creo- sote Bushscrub habitat supporteddensities of 40.0 and 25.3 birds/40 ha, respectively. A vegetationalanalysis indicated that the microphyll woodlandhad the highest dominance value (136.2), which is an indication of cover, and both the highestfrequency and plant speciesdiversity values (65 and 1.26 respectively). The open Creosote Bush scrub contained the least amount of cover, whereas the dense Creosote Bush scrub achieved a value of zero for plant speciesdiversity.

LITERATURE CITED

Anderson,A. and A. Anderson. 1946. Notes on the useof the CreosoteBush by birds. Condor 48:179. B-alda,R. P. 1969. Foliageuse by birds of the oak-juniperwoodland and Ponder- osa Pine forest in southeastern Arizona. Condor 71:399-412. California Division of Mines. 1952. Evolution of the California landscape.Bul- letin 158. Carothers,S. W., R. R. Johnsonand S. W. Aitchison. 1974. Populationstructure and socialorganization of southwesternriparian birds. Am. Zool. 14:97-108. Hensley,M. M. 1954. Ecologicalrelations of the breedingbird populationof the desertbiome in Arizona. Ecol. Monogr. 24:185-207. Hutchinson, A. E. 1942. Colorado Desert. Sixth Breeding-birdcensus. Aud. Mag. pp. 19-20. Karr, J. R. and R. R. Roth. 1971. Vegetation structureand arian diversitiesin several New World areas. Am. Nat. 105:423-435. MacArthur, R. H. and J. w. MacArthur. 1961. On bird speciesdiversity. Ecology 42:549-598. MacArthur, R. H., J. W. MacArthur and J. Preer. 1962. On bird speciesdiversity. II. Prediction of bird census from habitat measurements. Am. Nat. 96:167-174. MacArthur, R. H., H. Recher and M. Cody. 1966. On the relation between habitat selectionand speciesdiversity. Am. Nat. 100: 309-332. Miller, W. J. 1957. California through the ages;the geologichistory of a great state. WesternlorePress, Los Angeles. Mueller-Dombois, D. and H. Elenberg. 1974. Aims and methods of vegetation ecology. John Wiley and Sons,New York. Norris, R. M. and K. S. Norris. 1961. AlgodonesDunes of southeasternCalifornia. Geol. Soc. of Am. Bull. 72:605-620. Raitt, R. J. and R. L. Maze. 1968. Densitiesand speciescomposition of breeding birds of a Creosote Bush community in southernNew Mexico. Condor 70:193-205. Shannon, C. E. 1948. A mathematical theory of communication. Bell Syst. Tech. J. 27:379-343,623-656. Tomoff, C. S. 1974. Arian speciesdiversity in desertscrub. Ecology55: 396-403. WestecServices, Inc. 1977. Surveyof sensitiveplants of the AlgodonesDunes. Unpubl. report, Dept. Interior, Bur. Land Management,Riverside, California. Williams,A. B. 1936. The compositionand dynamicsof a beech-mapleclimax community. Ecol. Monogr. 6:317-408.

Accepted 28 March I978 20