Breeding Bird Assemblages in Wooded
Patches in Vineyard and Undeveloped Oak
Woodland Landscapes in Coastal-Central
California1
William D. Tietje,2 Jodi Isaacs,2 Kata Bavrlic,2 and Steven Rein3
Abstract
Widespread conversions of oak (Quercus spp.) woodland grassland complexes to row-crop
agriculture in central coastal California have resulted in highly altered landscapes with mostly
unknown consequences for faunal diversity and the health of these communities. To begin to
fill this information gap, in 2002 we established study sites in vineyard and in undeveloped
oak woodland-grassland landscapes in Santa Barbara and San Luis Obispo counties. In
wooded patches on each of the study sites, we sampled structural elements of vegetation and
surveyed breeding bird point count stations. We summarized the data by 95 percent CI. Row
crops, mostly vineyards, comprised two-thirds of the vineyard landscape, compared to twothirds woodland in the oak woodland-grassland landscapes. With the exceptions of duff and
canopy cover, vegetative attributes were similar in the wooded patches on both landscapes.
Breeding bird assemblages were also similar in the two landscapes. Of the 10 most detected
bird species, nine were common to both landscapes. Of the six species unique to vineyard
landscape, three are species that the literature usually describes as adapted to open habitats. In
the oak woodland-grassland landscapes, three of the five unique species we recorded may be
sensitive to fragmentation. The concepts of patch size, isolation, and source-sink dynamics
should be considered in a longer-term study before these preliminary results are used as an
indication that native avifauna can persist in small, isolated treed patches in highly altered
landscapes.
Keywords: Bird abundance, bird diversity, California oak woodland, habitat management,
land use, landscape ecology.
Introduction
Vineyards expanded rapidly the past two decades in north- and central-coastal
counties of California (U.S. Department of Agriculture/Weights and Measures 2001,
Merenlender 2000), often resulting in a mosaic of small residual wooded patches
surrounded by open agricultural development and associated roads and support
facilities. Wildlife biologists, resource managers, planners, and agriculturists wonder
about habitat fragmentation and possible loss of biodiversity in these developed
landscapes. They ask, “to what extent do woodland fragments provide usable
foraging, roosting, and nesting habitat for wildlife . . . to what extent do they retain
and contribute to the maintenance of wildlife diversity?”
1
An abbreviated version of this paper was presented at the Sixth California Oak Symposium: Today's
Challenges, Tomorrow's Opportunities October 9-12, 2006, Rohnert Park, California.
2
Department of Environmental Science, Policy, and Management, University of California, Berkeley,
CA 94720.
3
Statistics Department, California Polytechnic State University, San Luis Obispo, CA 93407.
447
GENERAL TECHNICAL REPORT PSW-GTR-217
To date, no study has assessed avifaunal assemblages in residual wooded
patches surrounded by row crops, compared to avifaunal assemblages in undeveloped
California oak woodland. Little scientific basis therefore exists to guide growers and
local planning agencies. This paper presents preliminary results on natural vs.
induced habitat fragmentation on California oak woodland avifauna.
Study Areas
Study Area Selection
To study bird assemblages in residual patches (vineyard landscape) and undeveloped
oak woodland (oak woodland-grassland landscape), we used geographic information
systems (GIS) to divide the Los Berros area east of Nipomo in southwest San Luis
Obispo County and part of the Santa Ynez Valley in Santa Barbara County, into
levels of vineyardization (Merenlender and Heaton, unpublished data). The algorithm
treats the counties as a grid composed of 25 m2 cells. It calculates the level of
vineyardization as a function of the amount and distribution of vineyard landscape
(mostly vineyards) within 2,000 m of each cell and averages the values for all cells.
The algorithm produced a map with “contour” lines indicating varying degrees of
vineyardization of the landscape expressed as percent vineyardization values. Using
this map, we then selected two vineyard landscape (treatment) and two oak
woodland-grassland landscape (control) study areas—one in San Luis Obispo County
and three others in Santa Barbara County (fig. 1).
Study Site Characteristics
All study sites are within the coast live oak phase (Griffin 1988) of the central coast
ranges. Coast live oak (Quercus agrifolia) predominates in the overstory, interrupted
occasionally with a blue oak (Q. douglasii) or valley oak (Q. lobata) tree. When
present, the understory is composed of poison oak (Toxicodendron diversilobum),
toyon (Heteromeles arbutifolia), redberry (Rhamnus crocea), and ceanothus
(Ceanothus spp.). On the woodland floor, wild oats (Avena spp.), bromes (Bromus
spp.), and fescues (Festuca spp.) dominate. Common forbs include deerweed (Lotus
scoparius), fiddleneck (Amsinckia spp), filaree (Erodium spp.), and hummingbird
sage (Salvia spathacea). Woodland patches in the vineyard landscape average 5 to 20
ha and usually occur in areas where the hilly topography limits row crop agriculture.
Wooded patches on the oak woodland-grassland landscape study areas are also more
prevalent on the east- and north-facing hillsides, a general characteristic of California
coastal oak woodland.
Methods
Bird Point Counts
Using a 250-m scaled grid overlaid on a digital map of the study areas, we randomly
selected 15 points, each entirely within a woodland patch in each of the two vineyard
landscape and oak woodland-grassland landscape study sites (total of 60 points).
From 23 April to 26 June 2002, we surveyed birds within 50-m radius plots centered
on the points four times during sunrise to 1000 PST, systematically rotating station
448
Breeding Bird Assemblages in Wooded Patches in Vineyard and Undeveloped Oak Woodland
Landscapes in Coastal-Central California—Tietje
visits among observers by study site, count station, and time of day (Bibby and others
2000, Ralph and others 1995). We recorded the species and number of birds detected
during the timed counts.
A) Vineyard Landscape
B) Oak Woodland─Grassland Landscape
Figure 1―Map showing examples of the land matrices used for the Vineyard
Landscape and Oak Woodland-Grassland Landscape study sites in San Luis Obispo
and Santa Barbara counties. The Vineyard Landscape study sites (A) were chosen in
woodland areas where row cropping, mostly vineyards, predominated. The Oak
Woodland-Grassland Landscape study sites (B) were selected in undeveloped Oak
Woodland-Grassland Landscape.
Landscape-level Vegetation Coverage
To calculate landscape-level vegetative cover, we placed a 100-m grid over a GIS
map of each study area and counted the grid intersections that occurred in treed
patches, grass/chaparral, cropped, and human development (e.g., roads, buildings,
water developments). Approximately 1,600 intersections per each of the four study
sites were adequate to determine percent cover of each vegetation type.
Patch-level Vegetation Sampling
In summer 2006, we randomly chose 24 of the 60 count stations (6 at each site) for
stand- or patch-level vegetation sampling. Because of change of ownership at one of
the oak woodland-grassland landscape sites, we conducted the vegetation sampling in
woodland of similar vegetation composition and structure on an adjacent property. At
10 m from the sample point in the four cardinal directions (N, S, E, W), we used a 1m2 frame to visually estimate percent ground cover (grass/forbs and duff), a density
pole (Griffith and Youtie 1988) to measure understory obstruction (an index of shrub
cover), and a concave spherical densiometer (Lemmon 1956) to measure canopy
449
GENERAL TECHNICAL REPORT PSW-GTR-217
cover. As an index of tree density, we measured the distance from the sampling point
to the nearest tree ≥10 cm dbh and ≥1.5 m tall in each of four quadrants (NE, SE,
SW, NW) centered on the sampling point. We also recorded the species and diameter
breast height (DBH) of these trees. Finally, within 10 m of each sample point, we
counted pieces of coarse woody debris (CWD) ≥10.2 cm diameter and ≥1.0 m long
with ≥30 cm in contact with the ground.
Data Analyses
Because there were only two replicates per treatment in this study, we chose to report
95 percent CI for the mean differences in vegetation structure and between oak
woodland-grassland landscape and vineyard landscape areas, rather than conduct
inferential statistics. Although bird monitoring data was adequate for inferential
statistics, replication was still low. Therefore, we also calculated means and SE for
the number individuals of species for each point count over the four visits and two
replications. Because most bird detections in oak woodland are auditory rather than
by sight (Block 1989), and to limit the inflationary effects on the surveys of flocking,
we also indexed bird abundances on the vineyard landscape and oak woodlandgrassland landscape sites by the percent of visits at which a species was detected.
Hence, we calculated the percents of 120 station visits (15 stations on each of the two
sites × 4 visits per station = 120) in oak woodland-grassland landscape and 120 visits
in vineyard landscape in which the species was detected.
Results
Vegetation
Landscape Matrix
The oak woodland-grassland landscape was dominated by treed patches (65 percent).
In contrast, row-crops (62 percent) dominated the vineyard landscape study areas
(table 1, fig. 1). Tree cover, mostly in small (<20 ha), isolated patches, comprised
only 15 percent of the vineyard landscape study sites. Grass/chaparral vegetation
occurred over approximately a quarter of both study sites (35 and 22 percent,
respectively). Buildings for machinery, irrigation ponds, dirt- or gravel-based roads,
and other infrastructure for row-crop agriculture comprised <1 percent of the cropped
areas. We did not detect these facilities on the oak woodland-grassland landscape.
Table 1―Percent vegetation cover types on the vineyard landscape and oak
woodland-grassland landscape study sites.
Vineyard Landscape
Treed patches
Row crops
Grass/chaparral
Human development
15%
62%
22%
1%
Oak WoodlandGrassland Landscape
65%
35%
Patch-level Structure
Coast live oak predominated in the woodlands (100 percent of trees tallied) on oak
woodland-grassland landscape and in vineyard landscape (98 percent of trees tallied).
450
Breeding Bird Assemblages in Wooded Patches in Vineyard and Undeveloped Oak Woodland
Landscapes in Coastal-Central California—Tietje
With the exceptions of duff and tree canopy covers, all of the habitat elements
measured (grass/forbs, shrub cover, tree distance [index of tree density], Tree DBH,
and CWD) were similar in the wooded patches of both landscapes (fig. 2).
Attribute Mean
Vineyard
Landscape
Vineyard Landscape
s
as
Gr
Oak Woodland
–Grassland Landscape
Oak
Woodland─Grassland
Landscape
/
(
rbs
Fo
%)
ff
Du
)
)
)
)
.)
(%
(%
(m
(no
(cm
ity
ov
ov
D
H
s
C
C
en
DB
CW
py
rub
eD
no
r ee
Sh
e
r
T
Ca
T
)
(%
Vegetation Characteristic
Figure 2―CI (95 percent) comparing selected vegetation attributes in Vineyard and
Oak Woodland-Grassland Landscapes. Percentages of coverage for duff and tree
canopy were greater in Oak Woodland-Grassland Landscape. All other vegetative
characteristics means were not significantly different based on our sampling.
Birds
Species and Individuals
We recorded a total of 73 species of birds in the vineyard and oak woodlandgrassland landscapes. We recorded a mean of 18.8 and 19.0 species, and 80 and 76
individuals, per 4 visits in the vineyard vs. oak woodland-grassland landscapes,
respectively. Based on the 95% CI, the number of species and individuals between
the two areas was similar.
Among the 10 bird species on vineyard landscape and oak woodland-grassland
landscape that comprised the most individuals counted (table 2), nine were common
to both treatments (bushtit, spotted towhee, oak titmouse, dark-eyed junco, western
scrub-jay, California towhee, Bewick’s wren, lesser goldfinch, and blue-gray
gnatcatcher), and comprised 54% and 58%, respectively, of the total numbers of birds
counted. During 120 point count visits in each of the two landscapes (30 stations x 4
visits/station), we detected 35 species more frequently on vineyard landscape (table
2). The difference in detection percentage was greatest for western scrub-jays
(detected 82 of 120 visits in vineyard vs. 52 of 120 visits in woodland-grassland
landscape = 37% difference).
451
GENERAL TECHNICAL REPORT PSW-GTR-217
Table 2―Species most abundant by landscape type (species included were those that were detected 5 or more
times). We calculated total count as the number of individuals identified over all 120 visits to point count stations.
Mean and standard deviation (SE) are based on the four visits to 15 stations at each of the 2 replications of each
landscape type. We counted 2,394 individuals in the Vineyard Landscape and 2,276 individuals in the Oak
Woodland-Grassland Landscape.
Vineyard Landscape
Species
Bushtit
(Psaltriparus minimus)
Spotted Towhee
(Pipilo maculatus)
Oak Titmouse
(Baeolophus inornatus )
Dark-eyed Junco
(Junco hyemalis)
Western Scrub Jay
(Aphelocoma californica)
California Towhee
(Pipilo crissalis)
Bewick's Wren
(Thryomanes bewickii )
Blue-gray Gnatcatcher
(Polioptila caerulea)
Lesser Goldfinch
(Carduelis psaltria)
California Quail
(Callipepla californica)
Hutton's Vireo
(Vireo huttoni)
House Finch
(Carpodacus mexicanus)
House Wren
(Troglodytes aedon)
Nuttall's Woodpecker
(Picoides nuttallii)
Cliff Swallow
(Petrochelidon
pyrrhonota)*
Orange-crowned Warbler
(Vermivora celata)
Anna's Hummingbird
(Calypte anna)
Wrentit
(Chamaea fasciata)
Ash-throated Flycatcher
(Myiarchus cinerascens)
Oak Woodland-Grassland Landscape
Count
Mean
SE
237
30
17.99
219
27
217
Count
Mean
SE.
Oak Titmouse
272
34
9.75
8.58
Bushtit
231
29
16.86
27
22.52
Blue-gray Gnatcatcher
189
24
10.81
168
21
13.62
Dark-eyed Junco
167
21
5.96
163
20
7.78
Spotted Towhee
159
20
7.40
137
17
4.32
Hutton's Vireo
140
18
6.39
128
16
9.06
Bewick's Wren
128
16
4.17
121
15
16.45
Lesser Goldfinch
124
16
8.05
106
13
11.85
California Towhee
85
11
2.72
89
11
5.62
85
11
2.39
82
10
6.30
82
10
7.80
74
9
9.04
Western Scrub Jay
Pacific Slope
Flycatcher
Orange-crowned
Warbler
66
8
5.78
73
9
8.10
Anna's Hummingbird
48
6
4.38
61
8
4.41
Wilson's Warbler
44
6
4.17
58
7
5.52
40
5
2.83
40
5
2.33
House Wren
Warbling Vireo
(Vireo gilvus)
40
5
5.42
37
5
2.72
Nuttall's Woodpecker
37
5
2.67
35
4
3.34
House Finch
35
4
5.40
30
4
5.70
27
3
1.77
30
4
3.45
Wrentit
Purple Finch
(Carpodacus
purpureus)*
Ash-throated
Flycatcher
24
3
3.25
23
3
1.25
American Crow
Violet-green Swallow
(Tachycineta
thalassina)*
19
2
3.96
17
2
2.95
Wilson's Warbler
(Wilsonia pusilla)
Turkey Vulture
(Cathartes aura)
White-crowned Sparrow
(Zonotrichia leucophrys)
27
3
8.75
24
3
6.23
California Thrasher
(Toxostoma redivivum)
21
3
1.51
452
Species
Breeding Bird Assemblages in Wooded Patches in Vineyard and Undeveloped Oak Woodland Landscapes in CoastalCentral California—Tietje
Vineyard Landscape
Species
Song Sparrow
(Melospiza melodia)
Bullock's Oriole
(Icterus bullockii)
Mourning Dove
(Zenaida macroura)
American Crow
(Corvus brachyrhynchos)
Brewer's Blackbird
(Euphagus
cyanocephalus)
Northern Flicker
(Colaptes auratus)
Pacific slope Flycatcher
(Empidonax difficilis)
Barn Owl
(Tyto alba)
Lawrence's Goldfinch
(Carduelis lawrencei)*
Red-tailed Hawk
(Buteo jamaicensis)
American Kestrel
(Falco sparverius)*
European Starling
(Sturnus vulgaris)*
Lark Sparrow
(Chondestes grammacus)
Black-headed Grosbeak
(Pheucticus
melanocephalus)
Lazuli Bunting
(Passerina amoena)
Yellow-rumped Warbler
(Dendroica coronata)
Black-chinned
Hummingbird
(Archilochus alexandri)
Black Phoebe
(Sayornis nigricans)
Great Horned Owl
(Bubo virginianus)*
Ruby-crowned Kinglet
(Regulus calendula)*
Western Tanager
(Piranga ludoviciana)
Total Count
Oak Woodland-Grassland Landscape
Count
Mean
SE
19
2
1.06
17
2
1.46
15
2
14
Species
Count
Mean
SE.
California Quail
12
2
1.77
12
2
1.07
3.00
Northern Flicker
Brown-headed
Cowbird
(Molothrus ater)*
11
1
1.85
2
2.19
Lark Sparrow
10
1
1.39
14
2
3.15
Black-chinned
Hummingbird
9
1
1.13
11
1
1.19
9
1
1.64
11
1
1.41
9
1
0.99
10
1
1.28
9
1
1.46
10
1
1.49
8
1
1.69
10
1
1.04
8
1
1.41
7
1
1.73
8
1
1.85
7
1
1.46
7
1
1.25
7
1
1.46
7
1
1.13
6
1
1.16
6
1
6
Lazuli Bunting
Townsend's Warbler
(Dendroica
townsendi)
Western Tanager
American Robin
(Turdus migratorius)
Black-headed
Grosbeak
Western Wood-Pewee
(Contopus
sordidulus)
7
1
1.25
1.39
Bullock's Oriole
Steller's Jay
(Cyanocitta
stelleri)*
White-breasted
Nuthatch
(Sitta carolinensis)
Hermit Warbler
(Dendroica
occidentalis)*
5
1
1.06
1
1.49
Red-tailed Hawk
5
1
0.92
5
1
1.06
Yellow-rumped
Warbler
5
1
1.41
5
1
0.74
5
1
0.92
5
1
1.77
5
1
1.41
Total Count
2,276
2,394
** Species unique to either vineyard or oak-woodland with a total individual count (from 120 visits) greater than four.
453
GENERAL TECHNICAL REPORT PSW-GTR-217
Unique Species
Of the 73 species we recorded five or more times, six were unique to vineyard
landscape and five to oak woodland-grassland landscape (table 2). Of those unique
species in vineyard landscape, cliff swallow, American kestrel, and European starling
favor more open woodland habitat and are often associated with human development.
In contrast on oak woodland-grassland landscape, of the five unique species, brownheaded cowbird, Steller’s jay, and hermit warbler may be sensitive to habitat
fragmentation.
Discussion
We detected similar bird assemblages in the wooded patches on our vineyard
landscape and oak woodland-grassland study areas. This preliminary finding, if born
out by further study, agrees with the conclusion that effects of agriculture on avifauna
are complex and depend on particular species and ecological variables (Ormerod and
Watkinson 2000). In contrast to temperate deciduous forests that have been
fragmented the past century by agriculture, California oak woodland-grassland
systems have been naturally patchy for millennia due to the influence on vegetation
pattern of vegetation management during Native American occupation (Anderson
2006), soil fertility, seasonal rainfall, and plant intra- and inter-specific competition.
Avifauna may have preadapted to an anthropogenically created patchy woodlandcropland system that has increased the past century. Therefore, some concepts of
patch dynamics developed from studies conducted in eastern deciduous contiguouscanopy forest of the United States (Manson and others 1999; Demaynadier and
Hunter 1998), may not so strongly influence bird assemblages in California oak
woodland (e.g., patch size, edge vs. interior [Vreeland and Tietje 2004], isolation,
source-sink populations). Importantly, however, the results of this study are
preliminary, since they are based on only one season of point counts, only two
treatments and two controls, and a small amount of data. Patch dynamics are not
considered in this study. More study is needed to determine whether native fauna can
or cannot persist in highly altered California oak woodland environments.
Patches of woodland surrounded by row crop development are increasingly
becoming a common and ecologically important feature of California oak woodland
landscapes. In addition to the implications of this study in California central-coastal
oak woodland, a study by Reynolds and others (this volume) in two north coast
counties found no evidence that the extent of vineyard development affected
avifaunal nest success at the landscape scale. The authors conjectured that highly
modified oak woodland landscape, if of sufficient size and properly managed, can
support a diverse and productive avifauna. It is therefore important to work with
landowners on management prescriptions that maintain and even restore and enhance
landscape structures that maintain native bird assemblages. Ranches and farms often
have available the materials and labor to provide some protection and restoration of
“odd areas,” including residual patches of habitat in which trees, shrubs, downed
wood, and other habitat elements can be left in place without interference with
agricultural production. Success is most likely when stakeholders are engaged
collaboratively in the process (Ormerod and Watkinson 2000). A landowner can
oftentimes take advantage of assistance programs provided by local government
agencies. For example, Cooperative Extension, the Natural Resource Conservation
454
Breeding Bird Assemblages in Wooded Patches in Vineyard and Undeveloped Oak Woodland
Landscapes in Coastal-Central California—Tietje
Service (NRCS), California Department of Forestry, and Land Conservancies are
often eager for cooperators to engage in vegetation and wildlife enhancement, and
demonstration projects. Such efforts will help maintain the rich avifauna native to
California oak woodland.
Acknowledgments
The University of California Integrated Hardwood Range Management Program,
Agreement IHRMP 00-4, supported this research. We thank Hampton Farming, Santa
Barbara County, the UCSB Sedgwick Reserve, and the owners of private ranchland
in southern San Luis Obispo County for permitting access to study sites. Special
thanks to Maria Loper for analyzing the data and preparing the tables and figures for
the manuscript. The Cooperative Extension Office, County of San Luis Obispo,
provided logistical support.
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