California Partners in Flight Desert Bird Conservation Plan

 

Gila Woodpecker (Melanerpes uropygialis)

 


Photo by Ian Tait

Prepared by: Chris McCreedy (cmccreedy@prbo.org)

PRBO Conservation Science
3820 Cypress Drive
Petaluma, CA  94954

RECOMMENDED CITATION

McCreedy, C.  2008.  Gila Woodpecker (Melanerpes uropygialis). In The Desert Bird Conservation Plan.  California Partners In Flight. http://www.prbo.org/calpif/htmldocs/desert.html

SHORTCUTS

range map

references

management issues

SUBSPECIES STATUS:

Some dispute noted in Edwards and Schnell (2000), though Edwards and Schnell chose to recognize three subspecies.  Two are found in the Desert Plan’s extent: nominate M. u. uropygialis (s.e. California, Arizona, and Sonora) and M. u. cardonensis (eastern Baja California Norte to perhaps San Felipe).  Edwards and Schnell (2000) note that all previous attempts at subspecies classification have problems, and that more study is needed.

MANAGEMENT STATUS:

The Gila Woodpecker was added to the California State Endangered List in 1988 (CDFG 2008).  Previous surveys have placed the California population between 200 individuals (Hunter 1984) and less than 30 pair (Laymon and Halterman 1986).  The Gila Woodpecker is currently not listed as endangered or threatened in Arizona nor Mexico.

DISTRIBUTION:

Generally permanent resident where found.  In the United States, from extreme southwestern New Mexico (Hidalgo and Grant Counties), through southern Arizona north to the Mogollon Rim west to extreme southeast California.  Edwards and Schnell (2000) report Gila Woodpeckers in Clark County, Nevada, but Floyd et al. (2007) did not find evidence of Gila Woodpeckers breeding in the state.  Nevada and California populations generally constrained to the last riparian remnants of the Colorado River, though small numbers - perhaps up to 100 pair (G. McCaskie, pers. comm..) - may be still found in the Imperial Valley (Alcorn 1988, Hunter 1986).  Researchers recently discovered a small population in Cercidium – Olneya woodland near the Palo Verde Mountains, raising the possibility that more Gila Woodpeckers may be found in large, old-growth xeric riparian woodlands in Imperial County (McCreedy in prep.).

In Mexico, throughout Baja California, save northwest Baja California Norte, Sonora, southwest Chihuahua, Sinaloa, Durango, and Zacatecas.

Figure 1.  Gila Woodpecker summer distribution and density: http://www.mbr-pwrc.usgs.gov/bbs/htm03/ra2003_blue/ra04110.htm

Historically, Gila Woodpeckers were much more common in southeastern California, concentrated in riparian forests on the Colorado River (Grinnell and Miller 1944, Hunter 1984).  These forests have been nearly erased from the Lower Colorado River Valley, precipitating the Gila Woodpecker’s eventual addition to the California Endangered Species List.

ECOLOGY:

AVERAGE TERRITORY SIZE

Habitat dependent: three territories in a wash at Organ Pipe National Monument had an average size of 4.57 ha (Hensley 1954), while two territories in an “open desert area” averaged 9.88 ha in extent (Edwards and Schnell 2000). 

In southeastern California, the Milpitas Wash annually holds 4-5 territories over 315 hectares of old-growth Cercidium –Olneya woodland (McCreedy in prep).

TIME OF OCCURRENCE AND SEASONAL MOVEMENTS

Generally sedentary, though small-scale movements from higher elevations to low elevations during the non-breeding season have been noted (Edwards and Schnell 2000).

FOOD HABITS

FORAGING STRATEGY:

Omnivorous, focused on large trees and columnar cacti, also mistletoe (Phoradendron californicum), though will occasionally ground-feed when food is easily visible (Edwards and Schnell 2000).  Seasonal patterns include focusing on saguaro (Carnegia gigantea) and other cacti during the summer, when flowers and fruit are present, and focusing on mistletoe during the winter, when mistletoe berries are present.

DIET:

A widely varied, omnivorous diet.  In urban areas, will capitalize on fruit-bearing plantings, feeders and even food scraps (CM personal observation, Edwards and Schnell 2000)

Where saguaro less common or absent, focus on Coleopterans, Lepidopterans, ants, and cicadas (Anderson et al. 1982). 

Cactus fruit (Carnegia, Stenocereus, etc.) in season.  Steenbergh and Lowe (1977) noted that Gila Woodpeckers pass saguaro seeds undamaged, hinting at effective seed dispersal.  Mistletoe berries December-March (CM personal observation).

Repeatedly observed to be a nest predator of Lucy’s Warbler (Vermivora luciae), Yellow Warbler (Dendroica petechia), and Bell’s Vireo (Vireo bellii) (Edwards and Schnell 2000), though no indication of how frequently Gila Woodpeckers depredate other species’ nests.

BREEDING HABITAT:

Require environments with large boles for nesting substrate, either columnar cacti or large trees, including riparian woodlands, old-growth xeric-riparian wash woodlands, uplands with concentrations of large columnar cacti, dry subtropical forests, and urban residential areas (Edwards and Schnell 2000).  It is questionable why Gila Woodpeckers have not succeeded in colonizing urban residential areas of southern California west of the species’ traditional breeding range.  It is also questionable why Gila Woodpeckers have not managed to occupy other large wash woodlands in California outside of Imperial County.

DOMINANT CANOPY SPECIES:

In riparian woodlands, Fremont Cottonwood (Populus fremontii), Goodding’s Willow (Salix gooddingii), Arizona Sycamore (Platanus wrightii).  In saguaro scrub, Giant Saguaro (Carnegia gigantea), Organ Pipe Cactus (Stenocereus thurberi), and Cardón (Pachycereus pringlei).  In xeric riparian woodland, Blue Palo Verde (Cercidium floridum), and Ironwood (Olneya tesota), in urban environments, various palms, Eucalyptus, Athel Tamarisk (Tamarix aphylla).  (CM personal observation, Edwards and Schnell 2000).

DOMINANT SHRUB SPECIES:

Not applicable, as Gila Woodpeckers spend majority of time foraging in large trees and/or saguaro, and require large-diameter boles for nesting substrate (Parker 1986).

DOMINANT FORB SPECIES:

Not applicable, as Gila Woodpeckers spend majority of time foraging in large trees and/or saguaro, and require large-diameter boles for nesting substrate (Parker 1986).

HEIGHT OF PLANT:

In one study, plant height ranged from 6 to 15 feet (n = 6 nests) (Parker 1988).

SLOPE/ASPECT:

Micro-climate attributes created by slope and aspect can promote columnar cacti growth, which in turn provide nesting habitat for Gila Woodpeckers.  However, saguaro-nesting Gila Woodpeckers generally nested in arroyos, with a negative correlation to slope (Kerpez and Smith 1990b).

TREE DBH/NEST HEIGHT::

Require mature saguaro cacti as nesting sites, of average height = 7.8 m, average nest height = 6.1 m, average DBH = 56 cm, average DGH (diameter at ground-height) = 54 cm (n=7, PRBO unpublished data).
Will not nest in saguaro smaller than 4-5 m in height (PRBO unpublished data, Korol and Hutto 1984, Kerpez and Smith 1990b).

In Olneya-Cercidium woodland in southeastern California, invariably nested in softer, old-growth Cercidium (n=10, PRBO unpublished data): average plant height = 7.3 m, average nest height = 4.2 m, average DBH = 39 cm, average DGH = 46 cm.  The smallest Cercidium on Milpitas Wash, Imperial County, CA (where saguaros do not exist) found to host a nest cavity was 25 cm in DBH and 7.3 m in height.

DISTANCE TO WATER::

Gila Woodpeckers require large trees or large columnar cacti for nesting substrate. Desert riparian woodland provides an abundance of cottonwood and willow boles for cavity sites, and Gila Woodpeckers are common in Sonoran Desert riparian areas. However, Gila Woodpeckers can also nest in rarely-watered xeric riparian washes and saguaro-dominated upland, and surface water is not a necessary circumstance for Gila Woodpecker nesting.

NEST SUBSTRATE::

In desert riparian woodland: Fremont Cottonwood (Populus fremontii), Goodding’s Willow (Salix gooddingii), and Arizona Sycamore (Platanus wrightii) (CM personal observation, Edwards and Schnell 2000). 

In mesquite bosque and xeric riparian washes, Blue Palo Verde (Cercidium floridum), Honey Mesquite (Prosopis glandulosa), and Screwbean Mesquite (Prosopis pubescens) (PRBO unpublished data, Edwards and Schnell 2000).

In urban environments, Athel Tamarisk (Tamarix aphylla), Eucalyptus (Eucalyptus sp.), Blue Fan Palm (Erythea armata) (CM personal observation, Edwards and Schnell 2000).

NEST TYPE::

Cavity nest.

 

BREEDING BIOLOGY:

TYPICAL BREEDING DENSITIES:

(See ECOLOGY, above).  Highest densities likely reached in cottonwood-willow desert riparian corridors.  In a study of ten xeric riparian washes throughout western Arizona (McCreedy 2007), highest density found on the Alamo Wash, La Paz County, within the Kofa NWR (0.942 individuals per ha, assumed constant detection rate).

DISPLAYS

Some information on antagonistic displays (Edwards and Schnell 2000) but not courtship displays.

MATING SYSTEM

No information, assumed monogamous (CM personal observation)

CLUTCH SIZE:

For 84 egg sets stored at the Western Foundation for Vertebrate Zoology, range 2-7 eggs, mean 3.74 ± 0.87 SD) (Edwards and Schnell 2000).

INCUBATION

Both sexes incubate, no information on which sex incubates more often (Edwards and Schnell 2000).

INCUBATION PERIOD

Poorly studied.  “About 2 weeks” (Bent 1939), 13 days (Baicich and Harrison 1997).

DEVELOPMENT AT HATCHING

Altricial.

NESTLING PERIOD

Not well known, about 4 weeks (Kaufmann 1996).

PARENTAL CARE

Both parents actively feed young – male 8 trips/hour, female 12 trips/hour (Martindale and Lamm 1984).  Males generally spend more time guarding the nest, while females spend more time gathering food (Martindale and Lamm 1984), though Martinsale and Lamm also found that if female decreased her rate of feeding, male increased his rate in response.  Food for young includes insect, fruit (saguaro fruit in particular), and scavenged items from human habitation.  Martindale (1983) found that 20% of deliveries were saguaro pollen or fruit. 

POST-FLEDGING BIOLOGY OF OFFSPRING

Little information, though Rosenberg et al. (1991) reported that family groups will hold even as adults tend young of their second brood.

DELAYED BREEDING

No information.

NUMBER OF BROODS

Often 2-3 (Edwards and Schnell 2000).  Multiple broods likely more common in eastern portion of range, where summer monsoonal moisture provides increased nesting opportunity.  More information is needed to confirm this.

BROOD PARASITISM

Not observed.

WINTERING GROUNDS NEEDS AND DISTRIBUTION

Generally sedentary.  Several observers have noted the importance of mistletoe berries to the winter diet, though Rosenberg et al. (1991) found that 100% of winter stomach contents were animal (n=3).  Will cache acorns, though oak trees are rarely found within this species’ distribution (Edwards and Schnell 2000).

LANDSCAPE FACTORS

Fire:

Extensive loss of habitat due to increased fire frequency in Sonoran Desert environments has become an increasingly critical threat to several Sonoran Desert endemic bird species.  Invasions of several fire-adapted exotic annuals (Schismus sp., Bromus sp., Brassica tournefortii, and Pennisetum ciliare) have fueled numerous, extensive catastrophic fires across the Mojave and Sonoran Deserts in recent years.  Vegetation that Gila Woodpeckers require for nesting habitat – columnar cacti, palo verde, and mesquites – are not evolved for high frequencies and require long periods to recover from burns.  Esque and Schwalbe (2002) suggested that of all wildlife, birds are most susceptible to declines in the face of loss of Sonoran Desert perennial vegetation.

Flooding:

Flooding of cottonwood-willow riparian due to poor hydrological management has resulted in additional habitat loss on the Colorado River (Monson 2007).

Grazing:

Grazing impacts on Gila Woodpeckers not well-studied.  Kreuper et al. (2003) found nearly zero change in Gila Woodpecker numbers ten years after grazing cessation on the San Pedro River, AZ.  Given that a cavity-nesting species would be likely to respond more slowly to restoration, it will be instructive to re-assess the San Pedro River’s bird populations now that it has been 21 years since cessation of grazing.  Overgrazing in riparian areas will continue to limit cottonwood and willow regeneration and thus limit future Gila Woodpecker nesting habitat.

Adjacent Land Use:

No information.  As noted above, Gila Woodpeckers can exist in urban environments given enough nesting habitat, and will actually thrive in urban environments given sufficient saguaro habitat. 

Pesticide:

No information.  Perhaps a nonissue across the majority of this species’ distribution, due to the relatively low amount of agriculture across the Sonoran Desert.  However, many Gila Woodpeckers still survive in urban and particularly riparian areas near agricultural development, and pesticide impacts on these populations is unknown.

Predators:

No information.  European Starlings (Sturnus vulgaris) thought to frequently evict Gila Woodpeckers from cavity sites (Kerpez and Smith 1990a).

DEMOGRAPHY AND POPULATION TRENDS:

DEMOGRAPHICS:

Age and Sex Ratios: No information.

Productivity Measurements: No information.

Survivorship: No information.

Dispersal: No information.

POPULATION TREND

Though Gila Woodpeckers are not difficult to find in sprawling cities like Phoenix and Tucson, this species faces significant declines across its range and an increased and persistent threat of habitat loss to fire and urbanization into the foreseeable future.

Using Breeding Bird Survey data, the Patuxent Wildlife Research Center reports a significant negative population trend of -2.2% (P= 0.04) for Gila Woodpeckers in Arizona from 1980-2007, which is the time period for which most surveys have occurred (Sauer at al. 2008).  In addition, Rosenberg et al. (1991) and Hunter (1984) have recorded near extirpation of this species from southeastern California, and Laymon and Halterman (1986) estimated that less than 30 pairs survive in California altogether.  Population trend in Mexico is totally unknown, though conversion of Sonoran Desert scrub to exotic bufflegrass has been much more dramatic than in the United States (Búrquez-Montijo et al. 2002), and it is reasonable to suggest that Gila Woodpecker population declines in Mexico may be even more significant than in the United States.

Figure 2 shows Gila Woodpecker population trends from 1996 – 2003.  Gila Woodpecker populations are declining significantly, particularly over the Lower Colorado River Valley section of the Sonoran Desert portion of their range.

Figure 2.  Gila Woodpecker population trend, 1966-2003.  http://137.227.242.23/bbs/htm03/trn2003/tr04110.htm

 

MANAGEMENT ISSUES AND OPTIONS

EXOTIC SPECIES INVASION/ENCROACHMENT

Large-scale conversion of cottonwood-willow riparian forest to monotypic salt-cedar (Tamrix sp.) stands due to altered hydrology and fire regimes has robbed Gila Woodpeckers of much of the remaining desert riparian left in the Sonoran Desert.  Though Gila Woodpeckers will occasionally nest in large Athel Tamarisk, the more common salt-cedar stands that dominate the Lower Colorado River and Gila River are not viable Gila Woodpecker nesting habitat (Rosenberg et al. 1991).  Large-scale cottonwood-plantation and Tamarix-removal projects are underway in the Lower Colorado River Valley and may add Gila Woodpecker habitat in the future.

Widespread invasion of Sonoran Desertscrub by exotic grasses has resulted in increased fire frequency and large-scale eradication of saguaro cactus stands across southern Arizona and Sonora (see FIRE, above).

MANAGEMENT

Kerpez and Smith (1990a) noted that European Starlings compete with Gila Woodpeckers for nest cavities, which could 1) limit Gila Woodpecker populations and 2) subsequently limit populations of Sonoran Desert secondary cavity nesters which rely on Gila Woodpeckers to construct their nest sites.  However, Koenig (2003) could not find significant evidence that the European Starling invasion of the Southwest (they first nested in Arizona in 1954, for example) is directly tied to Gila Woodpecker population declines.  Yet Gila Woodpecker populations are declining, and significantly, in both Arizona and California (POPULATION TREND, above).  Both Koenig and Kerpez and Smith had relatively small sample sizes for their analyses (Koenig 2003, Kerpez and Smith 1990a), and Koenig’s data set ended in 1996.  More study is needed on the effect of European Starlings, for Gila Woodpeckers still persist in urban areas in the Sonoran Desert, and European Starling impacts on Gila Woodpecker populations may be overstated.    
 

Outside of starling removal/management, the best way to circumvent this threat may be to maintain and if possible, increase saguaro populations (Tweit and Tweit 1986).  Given that some of the most rapidly-growing urban areas in the United States are found in Arizona, and that large chunks of Sonoran Desertscrub are now lost annually to an increasing number of wildfires, maintaining saguaro populations is becoming much more difficult.  Any development that will impact saguaros must be reported in Arizona, but as saguaros are not listed as threatened nor endangered in Arizona, there is currently no effort to inventory the state’s saguaro nor to increase saguaro populations by the State Government.

Loss of native riparian habitat to development and altered hydrological and fire regimes has nearly eradicated Gila Woodpeckers from the Lower Colorado River Valley, and continues across Arizona and Mexico.  Reduction of riparian grazing, restoration of hydrological function, and restoration of cottonwood-willow overstory is necessary for Gila Woodpeckers to rebound in riparian systems.  Unfortunately, as populations and urbanization increase in the Southwest, demand for limited water resources has increased dramatically, generally out-competing conservation considerations in riparian corridors in the southwestern United States.

While Gila Woodpeckers are a California State Endangered Species, there are currently no management programs to conserve this species in California.  Though it is recognized that Gila Woodpeckers have lost nearly all of their former habitat in California’s portion of the Colorado River bottomland, new development still continues there.

ASSOCIATED SPECIES

Management to conserve Gila Woodpeckers will conserve a great number of other bird species, several of which are listed as special status species.  This is because 1) Gila Woodpeckers are tied to threatened Sonoran Desert plant communities, including desert riparian and saguaro Desertscrub, and 2) Gila Woodpeckers create cavities that are re-used by a number of secondary-cavity nesting bird species.

Conserving native, gallery forest desert riparian would help: Lucy’s Warblers, Crissal Thrashers, Bell’s Vireos, Gilded Flickers, Vermillion Flycatchers, Yellow Warblers, and federally endangered Willow Flycatchers and Yellow-billed Cuckoos.  A number of secondary-cavity nesters benefit directly from Gila Woodpecker cavities in both desert riparian and saguaro habitats, including: Ash-throated Flycatchers, Brown-crested Flycatchers, Dusky-capped Flycatchers, American Kestrels, Ferruginous Pygmy-Owls, Elf Owls, Western Screech-Owls, and Lucy’s Warblers.

MONITORING METHODS AND RESEARCH NEEDS

Demographic data virtually nonexistent for this species, including productivity, productivity in rural versus urban environments (which would consider Starling presence/absence), survivorship, reliance on urban populations for immigration from rural populations, individual response to catastrophic fire events (nest success, emigration, carrying capacity of habitats adjacent burns, etc.). 

An unknown number of Gila Woodpeckers may breed in xeric riparian habitats in Imperial County, CA:  only Milpitas Wash has been surveyed.  The remainder of the largest wash systems in Imperial County should be censused, and it would be instructive to learn what happens to young produced at these sites/why more xeric riparian habitats in southeastern California are not colonized.

Virtually no demographic information exists for Mexico.  Distribution in should be addressed throughout Sonora in the face of increasing development, competition for water resources, and conversion of Desertscrub habitats to exotic grass-dominated habitats.

REFERENCES:

Alcorn, J. R.  1988.  The Birds of Nevada. Fairview West Publ., Fallon, NV.

Anderson, B. W., Ohmart, R.D., and Fretwell, S.D.  1982.  Evidence for social regulation in some riparian bird populations. American Naturalist 120: 340–352.

Baicich, P. J. and Harrison, C.J.O.  1997.  A Guide to the Nests, Eggs, and Nestlings of North American Birds. 2nd ed. Academic Press, San Diego, CA.

Bent, A. C.  1939.  Life histories of North American woodpeckers. Bulletin of the United States National Museum. 174: 250–257.

Búrquez-Montijo, A., Miller, M.E., and Martínez-Yrízar, A.  2002.  Mexican grasslands, thornscrub, and the transformation of the Sonoran Desert by exotic bufflegrass.  In Invasive Exotic Species in the Sonoran Region.  B. Tellman ed.  The University of Arizona Press and The Arizona-Sonora Desert Museum.  Tucson, AZ. 

California Department of Fish and Game.  2008.  State and federally listed endangered and threatened animals of California: October 2008.  http://www.dfg.ca.gov/biogeodata/cnddb/pdfs/TEAnimals.pdf

Edwards, H. H. and Schnell, G.D.  2000.  Gila Woodpecker (Melanerpes uropygialis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/532

Emlen, J. T. 1974.  An urban bird community in Tucson, Arizona: derivation, structure, regulation. Condor 76: 184–197.

Esque, T.C. and Schwalbe, C.R.  2002.  Alien annual grasses and their relationships to fire and biotic change in Sonoran Desertscrub.  In Invasive Exotic Species in the Sonoran Region.  B. Tellman ed.  The University of Arizona Press and The Arizona-Sonora Desert Museum.  Tucson, AZ. 

Floyd, T., Elphick, C.S., Chisolm, G., Mack, K., Elston, R.G., Ammon, E.M., and Boone, J.D.  2007.  Atlas of the Breeding Birds of Nevada.  University of Nevada Press.  Reno, NV.

Grinnell, J. and Miller, A.H.  1944.  The distribution of the birds of California. Pacific Coast Avifauna 27: 1–608.

Hensley, M. M.  1954.  Ecological relations of the breeding bird population of the Desert Biome in Arizona. Ecological Monographs. 24: 185–207.

Hunter, W.C.  1984.  Petition to the State of California Fish and Game Commission.  http://nrm.dfg.ca.gov/FileHandler.ashx?DocumentVersionID=3356

Kerpez, T. A. and Smith, N.S.  1990a.  Competition between European Starlings and native woodpeckers for nest cavities in saguaros. Auk 107: 367–375.

Kerpez, T. A. and Smith, N.S. 1990b.  Nest-site selection and nest-cavity characteristics of Gila Woodpeckers and Northern Flickers. Condor 92: 193–198.

Korol, J. J. and Hutto, R.L.  1984.  Factors affecting nest site location in Gila Woodpeckers. Condor 86: 73–78.

Kreuper, D., Bart, J., and Rich, T.D.  2003.  Response of vegetation and breeding birds to the removal of cattle on the San Pedro River, Arizona (U.S.A.).  Conservation Biology 17:2 607-615.

Laymon, S.A. and Halterman, M.  1986.  Distribution and status of Yellow-billed Cuckoo in California: 1986-1987.  Contract No. C-1845.  Draft administrative report.  Sacramento, CA: Wildlife Management Division, Nongame Bird and Mammal Section, California Department of Fish and Game.

Martindale, S. 1983. Foraging patterns of nesting Gila Woodpeckers. Ecology 64: 888–898.

Martindale, S. and Lamm, D.  1984.  Sexual dimorphism and parental role switching in Gila Woodpeckers. Wilson Bulletin 96: 116–121.

McCreedy, C.  2007.  Potential off-road vehicle impacts on bird populations of Sonoran Desert Xeroriparian habitats of western Arizona.  PRBO Conservation Science, 3820 Cypress Drive, Petaluma, CA, 94954.  PRBO Contribution No. 1611.

Mills, G. S., Dunning, J.B., and Bates, J.M.  1989.  Effects of urbanization on breeding bird community structure in southwestern desert habitats. Condor 91: 416–428.

Monson, G.  2007.  In Arizona Breeding Bird Atlas. Corman, T.E. and Wise-Gervais, C. eds.  Then University of New Mexico Press.  Albuquerque, NM.

Parker, K. C.  1986.  Partitioning of foraging space and nest sites in a desert shrubland bird community. American Midland Naturalist 115: 225–267.

Rosenberg, K. V., Ohmart, R.D., Hunter, W.C. and Anderson, B.W. 1991. Birds of the Lower Colorado River Valley. Univ. of Arizona Press, Tucson.

Sauer, J.R., Hines, J.E., and Fallon, J.  2008.  The North American Breeding Bird Survey, Results and Analysis 1966-2007.  Version 5.15.2008.  USGS Patuxent Wildlife Research Center, Laurel, MD.  http://www.mbr-pwrc.usgs.gov/bbs/

Steenbergh, W. F. and Lowe, C.H.  1977.  Ecology of the saguaro. II. Reproduction, germination, establishment, growth, and survival of the young plant. National Park Service Science Monograph Series, no. 8.

Tweit, R. C. and Tweit, J.C.  1986.  Urban development effects on the abundance of some common resident birds of the Tucson area of Arizona. American Birds 40: 431–436.

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