Avian Use of Exotic Street Treescapes in Metropolitan Areas of Phoenix, Arizona
Brian E. Washburn1*, Kristen A. Hoss2, and David L. Bergman3
1US Department of Agriculture, Animal Plant Health Inspection Service, Wildlife Services, National Wildlife Research Center, 6100 Columbus Avenue, Sandusky, OH 44870 USA. 2Tanawha Presents, LLC, 6730 SW 10th court, North Lauderdale, FL 33068 USA. 3US Department of Agriculture, Animal Plant Health Inspection Service, Wildlife Services, 8836 North 23rd Avenue, Suite 2, Phoenix, AZ 85021 USA. *Corresponding author.
Urban Naturalist, No. 39 (2021)
Abstract
Urban areas are highly modified environments that are strongly influenced by a variety of anthropogenic factors. Consequently, these areas contain unique wildlife communities typically dominated by species that are generalist in nature or highly adaptable. We examined the use of five species of exotic treescapes by exotic and native birds in metropolitan areas of Phoenix, Arizona. House Sparrows [Passer domesticus (37%)], European Starlings [Sturnus vulagris (27%)], Mourning Doves [Zenaida macroura (11%)], and Great-tailed Grackles [Quiscalus mexicanus (7%)] were the most frequently observed species during the study. Approximately two-thirds (67%) of the birds observed during the study were exotic species. Avian community composition and diversity associated with these streetscapes varied among the tree species. Growth habits and other characteristics of the trees themselves, in addition to the landscaping components beneath and adjacent to the street trees, influenced bird use of these habitats in this highly urbanized desert environment. Our findings demonstrate that exotic street treescapes might provide some ecological value to urban birds.
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No. 39 Urban Naturalist 2021
Avian Use of Exotic
Street Treescapes in
Metropolitan Areas of
Phoenix, Arizona
Brian E. Washburn, Kristen A. Hoss,
and David L. Bergman
Urban Naturalist
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Cover Photograph: A view of an observation location in metropolitan Phoenix, Arizona where shoestring acacia trees
were found in a street-tree landscape setting. At this location, the landscaping beneath the trees was comprised of coolseason
grasses. Photograph © Kristen Hoss, USDA Wildlife Services.
Urban Naturalist
B.E. Washburn, K.A. Hoss, and D.L. Bergman
2020 No. 39
1
2020 Urban Naturalist 39:1–12
Avian Use of Exotic Street Treescapes
in Metropolitan Areas of Phoenix, Arizona
Brian E. Washburn1*, Kristen A. Hoss2, and David L. Bergman3
Abstract - Urban areas are highly modified environments that are strongly influenced by a variety
of anthropogenic factors. Consequently, these areas contain unique wildlife communities typically
dominated by species that are generalist in nature or highly adaptable. We examined the use of five
species of exotic treescapes by exotic and native birds in metropolitan areas of Phoenix, Arizona.
House Sparrows [Passer domesticus (37%)], European Starlings [Sturnus vulagris (27%)], Mourning
Doves [Zenaida macroura (11%)], and Great-tailed Grackles [Quiscalus mexicanus (7%)] were
the most frequently observed species during the study. Approximately two-thirds (67%) of the birds
observed during the study were exotic species. Avian community composition and diversity associated
with these streetscapes varied among the tree species. Growth habits and other characteristics
of the trees themselves, in addition to the landscaping components beneath and adjacent to the street
trees, influenced bird use of these habitats in this highly urbanized desert environment. Our findings
demonstrate that exotic street treescapes might provide some ec ological value to urban birds.
Introduction
As human populations increase, the growth of urban landscapes is occurring at an accelerating
rate throughout the world and has created major environmental concerns (Grimm
et al. 2008, McKinney 2002). Urbanization of landscapes results in an overall loss of biodiversity,
especially notable for avian and insect communities (Chace and Walsh 2006, Czech
et al. 2000, McKinney 2002). Within highly urban matrices, birds (and other wildlife) use
a variety of semi-natural and man-made habitats, including natural habitat fragments, roadsides,
railways, golf courses, parks, green roofs, gardens, and landscaped areas (Fernandez-
Juricic and Jokimaki 2001, Hudson and Bird 2009, Meffert and Dziock 2012, Vallego et
al. 2009, Washburn et al. 2016). Many of these habitats are small in size (e.g., <1 ha) and
contain exotic vegetation planted for aesthetic values (Dunnett and Kingsbury 2004, Green
and Baker 2003, Ikin et al. 2013).
Street trees (i.e., trees growing along streets) in suburban environments can influence
local populations of wildlife, including both exotic and native species (Fernandez-Juricic
2000, Murgui 2007, White et al. 2005). Streetscapes that include trees and other woody vegetation
can serve as a functional, intermediary habitat between urban parks and streetscapes
with no vegetation (Fernandez-Juricic 2000, Murgui 2007) and can mitigate the negative effects
of anthropogenic noise (e.g., vehicle traffic) on avian communities (Pena et al. 2017).
In addition, wooded streetscapes might constitute a functional habitat corridor for some bird
species, facilitating their movement through the urban matrix and providing small parcels
of habitat (Fischer and Lindenmeyer 2002, Sodhi et al. 1999, White et al. 2005).
Vegetation (e.g., street trees) in highly urbanized areas is especially vulnerable to climate
change because many species rely on irrigation and other resource-heavy management
1US Department of Agriculture, Animal Plant Health Inspection Service, Wildlife Services, National
Wildlife Research Center, 6100 Columbus Avenue, Sandusky, OH 44870 USA. 2Tanawha Presents,
LLC, 6730 SW 10th court, North Lauderdale, FL 33068 USA. 3US Department of Agriculture, Animal
Plant Health Inspection Service, Wildlife Services, 8836 North 23rd Avenue, Suite 2, Phoenix, AZ
85021 USA. *Corresponding author – brian.e.washburn@usda.gov.
Manuscript Editor: David Krauss
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B.E. Washburn, K.A. Hoss, and D.L. Bergman
2020 No. 39
2
practices, especially in desert environments (Niinemets and Penuelas 2008, Van der Veken
et al. 2008). Consequently, the selection of exotic species is common when urban planners
are evaluating what species to use in urban projects (Arizona Municipal Water Users Association
2004, McPherson and Berry 2015, McPherson et al. 2017). However, the value of
exotic street trees and other vegetation to wildlife, especially birds, is generally unknown
and is in need of evaluation.
We investigated the composition and diversity of avian communities using small groups
of exotic street trees (i.e., treescapes) during an entire year at locations in a highly urbanized
area within a desert environment. The objectives of this study were to quantify (1) bird use
(e.g., abundance), (2) avian diversity, and (3) bird activities and foraging guilds associated
with five exotic treescapes commonly used for landscaping in urb an environments.
Methods
This study was conducted in Maricopa County and the City of Phoenix, which is located
in south central Arizona, USA (33° 27' N, 112° 03' W). Phoenix is located in the Sonoran
Desert and historically the natural vegetation of the area was a combination of Lower and
Upland Sonoran plant communities as well as vegetation associated with riparian corridors
(Brown, 1994). The study area has a hot and dry climate and an average annual precipitation
of 204 mm. Air temperatures in the study area average 30.8°C during summer and 11.3°C
during winter months (Arizona State Climate Office 2018). Approximately one-half of the
annual precipitation that falls in the Phoenix metropolitan area occurs during the summer
months as monsoons, often resulting in flash flooding along waterways and drainage areas.
Exotic Tree Species
We located sites within the Phoenix metropolitan area where the candidate tree species
were currently growing in a street-tree setting. From a larger pool of candidate study sites (i.e.,
streetscapes), we selected a total of 20 similar observation locations (4 replicates of each of 5
exotic street tree species) that were located in business areas, along major roadways (i.e., 4-lane
roads) and contained only one of the exotic street trees. The five exotic tree species included in
this study were: Shoestring Acacia (Acacia stenophylia A. Cunn ex Benth), Evergreen Elm (Ulmus
parvifolia Jacquin), Mulga (Acacia aneura F. Muell. ex Benth), the Sissoo tree (Dahlbergia
sissoo Roxb. ex DC), and Thornless Mesquite (Prosopis hybrid ‘Phoenix’ Linnaeus). These trees
are commonly used in urban landscaping in hot, dry climates due to their drought tolerance and
floristic characteristics (McPherson and Berry 2015, McPherson et al. 2017).
The Shoestring Acacia is a fast-growing, slender upright tree that has evergreen leaves
and is native to Australia (Arizona Municipal Water Users Association 2004). This tree
thrives under conditions of intense desert heat and drought, is characterized by a low amount
of litter fall, and provides filtered shade. Evergreen Elms (or Chinese Elms) are fast-growing,
umbrella-like trees that have semi-evergreen foliage and are native to eastern Asia, including
China, India, Taiwan, Japan, North Korea, and Vietnam (Perry 1992). Mature Evergreen
Elms have long, arching branches that provide dense summer shade. The Mulga is a thornless,
slow-growing large erect shrub (or small-size tree) that has evergreen leaves (Arizona
Municipal Water Users Association 2004). Native to the deserts of Australia, during extreme
dry periods, the Mulga drops much of its foliage to the ground, which provides a layer of
mulch and allows for nutrient recycling. Sissoo trees are deciduous and grow to be large trees
with dense, leathery leaves that produce heavy shade. Sissoo (also known as North Indian
Rosewood) is native to India and tolerates the Phoenix summer heat very well (Perry 1992).
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Thornless Mesquite trees are slower-growing, umbrella-like trees that have evergreen foliage.
Although some Mesquite species are native to North America, Thornless Mesquites are
native to Chile and Argentina (Arizona Municipal Water Users Association 2004).
Bird Observations
During the study, we conducted 98 bird survey days (i.e., series of avian fixed area
counts) centered on the 20 observation sites (Bibby et al. 2000, Ralph et al. 1995). Observations
were conducted on one randomly chosen day per week (Monday through Friday)
for a one-year period (8 April 2005–27 March 2006) at each location. We conducted these
observations so that, during each survey day, one series of observations (survey) was made
during the morning (06:00 to 12:00 h) and one series of observations was made during the
afternoon and evening (13:00 to 19:00 h). Each survey consisted of a series of 5-minute
fixed area counts at four pre-determined observation locations (replicates) that contained
five individual trees of the same species (Fig. 1). During each individual 5-minute fixed area
count, an observer walked very slowly through the survey area and recorded the presence,
number, and behavior of all birds observed on or in the street trees, on the ground within 5
m of, or flying within 25 m of the group of five trees. Birds observed during the fixed area
count were placed into one of several activity categories, including: (1) flying (locally), (2)
on the ground, (3) feeding on the ground, (4) perched in a study tree, (5) vocalizing in a
study tree, (6) feeding in a study tree, or (7) nesting in a st udy tree.
Nesting Observations
During each 5–minute fixed area count, we noted any nesting activity by birds. After the
observer completed the observations (at the end of each count) all bird nests were examined
for activity using binoculars and/or a mirror on a telescoping pole to determine the status of
the nest. The status (e.g., active or inactive), the species of bird using each nest, and the number
of eggs or chicks (if present) were recorded for each nest observed during each survey.
Figure 1. Hypothetical example of
a 5-minute fixed area count associated
with a study of five exotic tree
streetscapes in metropolitan areas
of Phoenix, AZ, April 2005 through
March 2006. Five individual exotic
street trees of the same species were
contained within the sampling area.
All birds within the sampling area,
here displayed as the dashed-line
box, were counted and their activities
recorded.
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Data Analyses
We compared the bird use (e.g., abundance) among the five exotic tree streetscapes (as
a fixed effect) for each individual bird species and for all species combined, using repeated
measures analysis of variance (survey day was the repeated factor) and Fisher’s Protected
LSD tests (Neter et al. 1990, Zar 1996). Prior to conducting these analyses, we used Shapiro-
Wilk tests (to ensure normality) and Levene’s tests (to ensure equality of variances) for
dependent variables (Neter et al. 1990). We considered differences significant at P ≤ 0.05
and conducted all analyses using SAS statistical software versi on 9.1 (SAS Institute, Cary,
NC). Data are presented as mean ±1 standard error (SE).
Bird species were divided into two groups, exotic and native. The exotic birds group
consisted of House Sparrow (Passer domesticus Linnaeus), European Starling (Sturnus
vulgaris Linnaeus), and Rock Pigeon (Columba livia Gmelin). The native birds group consisted
of all birds that are native to the Phoenix area. Although Inca Doves (Columbina inca
Lesson) and Great-tailed Grackles (Quiscalus mexicanus Gmelin) arrived in the Phoenix
area around 1885 and in the 1940s, respectively, we considered them to be native species
as they came to the area through natural range expansions (Emlen 1974, Phillips 1950).
We evaluated bird use patterns among exotic tree streetscapes for exotic and native species
(individually) to determine if differences existed.
We calculated bird species richness (S) associated with each of the five street trees. Comparison
of avian species diversity was conducted by calculating the Shannon’s diversity index
(H’), the Simpson’s diversity index (1/D), and the Berger–Parker dominance index (BP) for
each of the five tree species (Gotelli and Entsminger 2001, Magurran 2004, Morris et al. 2014).
We examined bird activities and compared the proportion of birds (all species combined)
flying, using the trees, and on the ground among the five tree species using G-tests for independence
(Sokal and Rohlf 2011, Zar 1996). In addition, we assigned all birds observed
into foraging guilds using a standard classification (DeGraff et al. 1985). We compared the
proportion of birds within avian foraging guilds among the five tree species using G-tests
for independence (Sokal and Rohlf 2011, Zar 1996).
As the study progressed, we observed that the landscaping below the exotic street trees
might be an important influence on bird use. Although 16 of the 20 observation locations
had gravel landscaping, four of the locations (3 Shoestring Acacia and 1 Evergreen Elm) had
manicured cool-season grass landscaping. We compared the bird use among the Evergreen
Elm and Shoestring Acacia streetscapes with grass landscaping and gravel landscaping (tree
species and landscaping type were fixed effects) and tested for interactions between these
factors for all species combined and for individual bird species, using repeated measures
analysis of variance (survey day was the repeated factor) and Fisher’s Protected LSD tests
(Neter et al. 1990, Zar 1996).
Results
During 196 avian fixed area count surveys, we observed at total of 6,305 birds, representing
23 different species. House Sparrows (37%), European Starlings (27%), Mourning
Doves (Zenaida macroura Linnaeus [11%]), and Great-tailed Grackles (7%) were
the most frequently observed species during the study. Approximately two-thirds (67%)
of the birds observed during the study were exotic species, whereas the other one-third
were native.
Overall, bird use (abundance) varied (F 4,1900 = 6.38, P = 0.0004) among the five street
treescapes. The lowest number of birds were observed in Mulga streetscapes, whereas the
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most birds were found in Shoestring Acacia streetscapes; bird use of Evergreen Elm trees,
Sissoo trees, and Thornless Mesquite streetscapes was intermediate relative to Mulga and
Shoestring Acacia (Table 1).
Exotic Bird Species
House Sparrows were very common and were observed flying in the vicinity of all tree
species studied (Table 1). House Sparrows were present on or in Evergreen Elm, Sissoo, and
Thornless Mesquite trees more (F 4,1900 = 13.31, P < 0.0001) than in Mulga and Thornless
Mesquite streetscapes.
The number of European Starlings varied (F 4,1900 = 16.74, P < 0.0001) among the tree
streetscapes (Table 1). European Starlings used Shoestring Acacia streetscapes exclusively;
these birds typically fed in the grassy areas beneath these trees and used the acacias as a
refuge when startled. Although European Starlings were commonly observed flying in the
vicinity of Mulga trees, they did not use these trees or the gr ound beneath them.
Rock Pigeons were typically observed flying and only rarely observed feeding or resting
under trees. Pigeons used Mulga, Sissoo, and Thornless Mesquite streetscapes more (F 4,1900
= 4.45, P = 0.004) than Shoestring Acacia streetscapes (Table 1).
Native Birds
Doves were commonly observed during the study (Table 1). Mourning Doves used all
five exotic tree streetscapes relatively equally (F 4,1900 = 1.29, P = 0.29). Inca Doves were observed
almost exclusively (F 4,1900 = 20.18, P < 0.0001) in association with Thornless Mesquite
streetscapes, using these trees and the ground beneath (Table 1). White-winged Doves
(Zenaida asiatica Linnaeus) were commonly observed flying within all of the streetscapes
we studied, but they used only Shoestring Acacias.
Other birds, such as native songbirds and raptors, were occasionally observed flying,
beneath, or in the street trees being studied. Verdins (Auriparus flaviceps Sunduvail)
(F 4,1900 = 1.69, P = 0.17) and raptors (F 4,1900 = 1.87, P = 0.13) used all five streetscapes
relatively equally. In contrast, Northern Mockingbirds (Mimus polyglottos Linnaeus)
were more abundant (F 4,1900 = 6.38, P = 0.0004) on or in Evergreen Elms compared to
the other street trees. Great-tailed Grackle use varied (F 4,1900 = 10.63, P < 0.0001) among
the streetscapes (Table 1). Great-tailed Grackles used the ground beneath the Shoestring
Acacia trees for resting and foraging. In addition, our observations suggest Great-tailed
Grackles also use Evergreen Elm and Thornless Mesquite trees themselves.
Bird Community Diversity
Avian diversity varied among the five exotic tree streetscapes (Table 2). Overall, the
avian community associated with Mulga streetscapes had the highest levels of diversity
(a consistent finding across all four diversity measures). In contrast, the diversity of avian
communities was lowest within streetscapes comprised of Evergreen Elms (Table 2). Shoestring
Acacia, Sissoo, and Thornless Mesquite streetscapes had avian communities with
intermediate diversity relative to Mulga and Evergreen Elm habitats.
Bird Activities
The proportion of birds (all species combined) flying near (G 4 = 699.7, P < 0.0001), using
the street trees (G 4 = 1499.9, P < 0.0001), and on the ground under or near study trees
(G 4 = 1083.9, P < 0.0001) varied among the five tree streetscapes. The highest amount of
local flying occurred near Mulga trees, whereas the ground under Shoestring Acacias was
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Table 1. Mean (± SE) no. of birds observed per 5-minute fixed area count of individual species and guilds of birds associated with five exotic tree streetscapes in
metropolitan areas of Phoenix, AZ, April 2005 through March 2006.
Species or Guild Evergreen Elm Mulga Shoestring Acacia Sissoo Thornless Mesquite
House Sparrow
(Passer domesticus)
2.21 ± 0.23 a* 0.24 ± 0.08 b 0.12 ± 0.04 b 1.58 ± 0.14 a 1.87 ± 0.15 a
Mourning Dove
(Zenaida macroura)
0.13 ± 0.02 a 0.47 ± 0.08 a 1.05 ± 0.38 a 0.32 ± 0.04 a 0.31 ± 0.04 a
European Starling
(Sturnus vulgaris)
0.04 ± 0.02 a 0.20 ± 0.06 a 3.53 ± 0.55 b 0 a 0.02 ± 0.01 a
Great-tailed Grackle
(Quiscalus mexicanus)
0.14 ± 0.04 a 0.04 ± 0.01 a 0.76 ± 0.10 b 0.03 ± 0.01 a 0.21 ± 0.05 a
House Finch
(Haemorhous mexicanus)
0.14 ± 0.03 ab 0.07 ± 0.02 a 0.16 ± 0.04 ab 0.30 ± 0.04 c 0.26 ± 0.04 bc
Inca Dove
(Columbina inca)
< 0.01 ± < 0.01 a 0.08 ± 0.02 b 0.01 ± 0.01 a 0.02 ± 0.01 a 0.42 ± 0.05 c
Verdin
(Auriparus flaviceps)
0.07 ± 0.01 a 0.08 ± 0.02 a 0.03 ± 0.01 a 0.08 ± 0.02 a 0.07 ± 0.01 a
Rock Pigeon
(Columba livia)
0.03 ± 0.01 ab 0.10 ± 0.04 bc 0 a 0.12 ± 0.03 c 0.08 ± 0.02 bc
Northern Mockingbird
(Mimus polyglottos)
0.09 ± 0.04 a 0.04 ± 0.02 b 0.03 ± 0.01 b 0.01 ± 0.01 b 0.02 ± 0.01 b
Anna’s Hummingbird
(Calypte anna)
< 0.01 ± < 0.01 a < 0.01 ± < 0.01 a 0.02 ± 0.01 a 0 a 0 a
Songbirds** 0.01 ± 0.01 a 0.09 ± 0.02 bc 0.10 ± 0.03 bc 0.05 ± 0.01 ab 0.12 ± 0.02 c
Raptors*** 0 a 0.01 ± 0.01 a 0.01 ± 0.01 a 0 a < 0.01 ± < 0.01 a
All Species Combined 2.89 ± 0.27 a 1.46 ± 0.14 b 5.84 ± 0.68 c 2.54 ± 0.16 a 3.35 ± 0.19 a
*Means within the same row with the same letter are not significantly different (p > 0.05).
**The songbirds category in this study includes Abert’s Towhee (Pipilo alberti), Brown-crested Flycatcher (Myiaarchus tyrannulus), Brown-headed Cowbird (Molothrus
atar), Black Phoebe (Sayornis nigricans), Curve-billed Thrasher (Toxostoma curvirostre), Cliff Swallow (Hirundo pyrrhonota), Common Raven (Corvus corax), Gila
Woodpecker (Melanerpes uropygialis), Say’s Phoebe (Sayornis saya), White-winged Dove (Zenaida asiatica), and Yellow-rumped Warbler (Dendroica coronate).
***Raptors include American Kestrel (Falco sparverius) and Harris’ Hawk (Parabuteo unicinctus).
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the most used by birds (Fig. 2). Bird use of the trees themselves was highest for Evergreen
Elm, Sissoo, and Thornless Mesquite trees (Fig. 2).
A total of 31 bird nests was observed and monitored during our study (Table 3). Seventyseven
percent (24 of 31 nests) of all bird nests were located in Sissoo trees; 16% (5 of 31)
were located in Thornless Mesquite trees; and 7% (2 of 31) were located in Evergreen Elms.
We found no bird nests in either the Mulga trees or the Shoestring Acacia trees. Doves (i.e.,
Mourning Doves and Inca Doves) accounted for two-thirds of the nests, whereas native species
(overall) accounted for 92% of the bird nests identified to species during the study (Table 3).
Foraging Guilds
We found significant variation in the proportion of avian foraging guilds for omnivorous
ground foragers (G 4 = 4210.2, P < 0.0001), granivorous ground gleaners (G 4 = 831.3, P <
0.0001), insectivorous air salliers (G 4 = 51.6, P < 0.0001), tree focused foragers (G 4 = 12.1,
P = 0.02), and other guilds (G 4 = 27.4, P < 0.0001) when comparing across tree streetscapes.
Although omnivorous ground foragers was the dominant avian foraging guild observed within
Shoestring Acacia streetscapes, granivorous ground gleaners was the most frequently observed
avian foraging guild at streetscapes comprised of the other four exotic tree species (Fig. 3). The
highest proportion of insectivorous air salliers, tree focused foragers, and other guilds were
found in association with Mulga streetscapes, whereas the composition of avian foraging guilds
were very similar among Evergreen Elm, Sisso, and Thornless Mesquite streetscapes (Fig. 3).
Landscaping
We found that, for all birds combined, exotic tree streetscapes with grass landscaping
(7.61 ± 0.51 birds per 5-minute count) had higher (F 1,194 = 79,29, P < 0.0001) bird use than
Table 2. Avian diversity indices associated with five exotic tree streetscapes in metropolitan areas of Phoenix, AZ,
April 2005 through March 2006.
Diversity Index Evergreen
Elm
Mulga Shoestring
Acacia
Sissoo Thornless
Mesquite
Species richness 13 20 16 10 14
Shannon’s diversity (H’) 0.977 2.123 1.076 1.257 1.533
Simpson’s diversity (1/D) 1.656 5.682 1.927 2.315 2.874
Berger–Parker dominance (BP) 0.772 0.339 0.702 0.631 0.560
Figure 2. Proportion of bird activities
associated with five exotic tree
streetscapes in metropolitan areas
of Phoenix, AZ, April 2005 through
March 2006.
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2020 No. 39
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those with gravel streetscapes (1.42 ± 0.51). European Starlings (F 1,194 = 21.62, P < 0.0001)
and Great-tailed Grackles (F 1,194 = 10.60, P = 0.001) used Shoestring Acacia streetscapes
landscaped with grass more than Shoestring Acacia streetscapes with gravel landscaping
and Evergreen Elm streetscapes. Conversely, House Sparrows (F 1,194 = 62.98, P < 0.0001),
Rock Pigeons (F 1,194 = 5.67, P = 0.02), Northern Mockingbirds (F 1,194 = 12.26, P = 0.006),
and Verdin (F 1,194 = 5.26, P = 0.02) all used the Evergreen Elm streetscape landscaped with
grass more than Shoestring Acacia streetscapes with grass or gravel landscaping and the
gravel landscaped Evergreen Elm streetscapes. For all other bird species, there was no significant
difference (all F 1,194 < 2.29, P > 0.13) in their use of the grass and gravel landscaped
Shoestring Acacia and Evergreen Elm streetscapes.
Table 3. Nests found in the street trees during the study of five exotic tree streetscapes in metropolitan areas
of Phoenix, AZ, April 2005 through March 2006
Bird Species Number of Nests Number of Active Nests
Mourning Dove 10 2
Inca Dove 6 1
Verdin 3 2
House Finch 2 1
House Sparrow 2 2
Northern Mockingbird 1 0
Unknown bird species 7 1
Figure 3. Proportion of avian foraging guilds
associated with five exotic tree streetscapes
in metropolitan areas of Phoenix, AZ, April
2005 through March 2006.
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2020 No. 39
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Discussion
The five exotic tree streetscapes varied in their use by birds of different species and foraging
guild associations. Several bird species appeared to exhibit clear preferences for one
or more of these streetscapes, whereas other species used all of the streetscapes relatively
equally. These findings are consistent with studies of streetscapes in cities located on other
continents, including Europe (Fernandez-Juricic 2000, Murgui 2007) and Australia (Ikin et
al. 2013, White et al. 2005, Young et al. 2007).
With the exception of House Sparrows, few birds used the Evergreen Elm trees themselves.
House Sparrows used the Evergreen Elms for perching and vocalizing. In addition,
House Sparrows (granivorous ground gleaner) comprised the vast majority of birds that
rested and foraged under the Evergreen Elm trees.
Thornless Mesquite is the only street tree that was frequently used by Inca Doves. Five
nests were found in Thornless Mesquite trees, almost all of which were made by Inca Doves
or Mourning Doves. We suspect the umbrella-like growth form of this tree provides thermal
and protective cover (i.e., shade) to both birds using the trees (e.g., for nesting) and for birds
using the ground below. Moreover, birds in this region are adapted to desert environments
and consequently to desert trees (Green and Baker 2003, Hostetler and Knowles-Yanez
2003, Litteral and Wu 2012), such as the Velvet Mesquite (Prosopis velutina Wooten) and
Screwbean Mesquite (Prosopis pubescens Benth.), which are native tree species closely
related to the Thornless Mesquite.
Growth habits and other characteristics of the trees themselves, in addition to the landscaping
components beneath and adjacent to the street trees, influenced the bird use of these
exotic tree streetscapes in this highly urbanized desert environment. Although the use of
Shoestring Acacia trees by birds was moderate, European Starlings (omnivorous ground
forager), Great-tailed Grackles (omnivorous ground forager), Mourning Doves (granivorous
ground gleaner), and White-winged Doves (granivorous ground gleaner) frequently
used the ground beneath most of the Shoestring Acacia trees. The areas under three of the
four Shoestring Acacia tree streetscapes studied were landscaped with thick stands of coolseason
grasses (e.g., perennial ryegrass [Lolium perenne Linnaeus]) which the birds foraged
on. In contrast, the Shoestring Acacia trees that were landscaped with gravel (only one of
the studied streetscapes) were used infrequently by birds. Thus, we suspect much of the
attractiveness of the Shoestring Acacia streetscapes to birds from a foraging perspective is
likely due to the grass landscaping rather than the trees thems elves.
House Sparrows and Mourning Doves used the Sissoo trees for perching, vocalizing,
nesting, and other activities. Most of the nests observed and monitored during our
study (e.g., those of Mourning Doves, Inca Doves, House Sparrows, and House Finches
(Haemorhous mexicanus Muller) were located in Sissoo trees. Tweit and Tweit (1986)
found that Inca Doves only nest in exotic vegetation and not in native plants in urban areas
within a desert environment. The dense foliage of Sissoo trees likely provides good thermal
and protective cover, thus making these trees attractive to birds for nesting and other activities.
Interestingly, in their native range Sissoo trees are used by a variety of birds, including
Columbids and Passerines (Kaur and Kumar 2018).
Of the five exotic tree streetscapes studied, Mulga streetscapes were used the least by
birds (in regard to abundance) but these streetscapes had the highest avian diversity. This
finding is consistent with studies from Australia (where the Mulga is native) where high
levels of avian diversity are associated with Mulga forests and plantings (Cody 1994, Recher
and Davis 1997). The sparse foliage and upright growth form of this tree appears to
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2020 No. 39
10
provide little thermal or protective cover, thus making it unattractive to birds. No birds were
observed feeding from the flowers or the seeds produced by the Mulga trees. Most observations
of birds associated with Mulga trees were Mourning Doves and Great-tailed Grackles
forging on sparse amounts of seed-producing grasses beneath the Mu lga trees.
Urban bird use of exotic tree streetscapes is influenced by the species of street trees
present as reflected in the composition and diversity of the avian communities observed.
Avian communities found in these streetscapes were comprised of both exotic and native
species. The growth habits and other characteristics of these exotic street trees, in
addition to the landscaping components related to them, influenced the bird use of these
streetscapes in this highly urbanized desert environment. Developers, city planners,
biologists, and landscape architects who are interested in providing suitable habitat for
birds, either native species or all species (including exotic species) while simultaneous
providing a functional, aesthetic streetscape environment in cities (Savard et al. 2000)
located within arid desert environments could select exotic tree species using the findings
from our study.
Acknowledgments
Valley Metro Rail Inc. and the US Department of Agriculture provided funding and support for
this study. We thank T.L. DeVault and three anonymous reviewers for helpful comments on this
manuscript. All applicable international, national, and/or institutional guidelines for the care and use
of animals were followed. Procedures performed in this study involving animals (i.e., observations)
were in accordance with the ethical standards of the USDA/APHIS/WS National Wildlife Research
Center Institutional Animal Care and Use Committee.
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