RESULTS AND DISCUSSIONMcIntyre, Blodwyn M. 2000. The breeding bird populations of Sweet Briar College.
Naturalist-in-Residence Report Number 1. Sweet Briar College, Sweet Briar VA 24595.Introduction Methods Results and Discussion Conclusions Literature cited
Table 2 shows the percent of land holdings by land cover type and the number of plots in each of the five types. Figure 1 shows the locations of the 50 plots according to zone and land cover type. Table 3 provides a listing and tally of all birds included in the survey.
Table 2. Land Cover Types as a Percentage of Sweet Briar Land Holdings, Number of plots for each land cover type.
Land Cover Type
% Of Sweet Briar Land Holdings
# Of plots
Pine Stands
4%
2
Fields
21%
10
Main Campus
2%
1
Sanctuary
14%
7
Hardwood and
Mixed Forest Stands
59%
30
Table 3. All bird species surveyed and totals of each bird species counted. The fourteen species that were selected for mapping and statistical analysis are listed in bold. Species Observations Turkey Vulture
16
Black Vulture
0
Osprey
0
Red-shouldered Hawk
0
Red-tailed Hawk
0
Broad-winged Hawk
0
Sharp shinned Hawk
0
Kestrel
0
Pheasant
0
Bobwhite
3
Killdeer
1
Woodcock
0
Rock Dove
16
Mourning Dove
14
Yellow-bellied Cuckoo
33
Great-horned Owl
0
Barred Owl
0
Nighthawk
0
Chuck's Will's
0
Whip-poor-will
0
Swift
0
Ruby-throated Hummingbird
2
Belted Kingfisher
0
Red-headed Woodpecker
0
Red-bellied Woodpecker
40
Downy Woodpecker
24
Hairy Woodpecker
17
Pileated Woodpecker
27
Flicker
3
Yellow-bellied Sapsucker
0
Eastern Wood Peewee
75
Acadian Flycatcher
30
Willow Flycatcher
3
Great-crested Flycatcher
1
Eastern Phoebe
13
Kingbird
2
Horned Lark
0
Purple Martin
0
Tree Swallow
0
Cliff Swallow
0
Barn Swallow
28
Rough-winged Swallow
0
Blue Jay
63
American Crow
176
Fish Crow
0
Great Blue Heron
0
Little Blue Heron
0
Green-backed Heron
0
Canadian Goose
8
Mallard Duck
0
Wood Duck
0
Carolina Chickadee
52
Tufted Titmouse
79
White-breasted Nuthatch
8
Red-breasted Nuthatch
0
Carolina Wren
26
House Wren
3
Beckwick's Wren
0
Blue-Grey Gnatcatcher
0
Eastern Bluebird
16
Wood thrush
75
Veery
0
Robin
21
Catbird
4
Mocking bird
32
Brown thrasher
3
Cedar Waxwing
0
Loggerhead Shrike
0
Starling
25
White Eyed Vireo
0
Red Eyed Vireo
123
Yellow throated Vireo
5
Warbling Vireo
1
Summer Tanager
3
Scarlet Tanager
37
Cardinal
118
Blue Grosbeak
0
Rose-breasted Grosbeak
0
Indigo Bunting
121
Dickissel
0
Rufus-sided Towhee
24
White-throated Sparrow
0
Song Sparrow
27
Chipping Sparrow
19
Field Sparrow
4
Grasshopper Sparrow
11
Vesper Sparrow
8
House Sparrow
3
Red Winged Blackbird
0
Meadowlark
5
Common Grackle
25
Brown-headed Cowbird
7
Baltimore Oriole
0
Orchard Oriole
0
House Finch
4
Purple Finch
0
Goldfinch
14
Redstart
1
Ovenbird
18
Louisiana Water thrush
1
Common Yellowthroat
15
Yellow Breasted Chat
0
Blue Winged Warbler
0
Parula
2
Yellow Warbler
1
Yellow-throated Warbler
0
Pine Warbler
9
Prairie Warbler
9
Cerulean Warbler
0
Black and White Warbler
0
Prothonatary Warbler
0
Worm-eating Warbler
8
Kentucky Warbler
7
Hooded Warbler
7
Chestnut-sided Warbler
0
Black-throated Green Warbler
0
Black-throated Blue Warbler
0
Blackburnian Warbler
0
Magnolia Warbler
0
Golden-winged Warbler
0
Nashville Warbler
0
Swainson's Warbler
0
Palm Warbler
0
Species accounts. Statistical data, distribution maps, and data interpretation are given below for the fourteen individual species analyzed.
Ovenbird Seiurus aurocapillus
Figure 3. Ovenbird distribution
Habitat Variable Basal Area 0.012
Large diameter standing dead trees Number of species of trees Forest type Canopy cover Average tree diameter Maximum tree diameter Canopy height Density of ground cover
Ovenbirds were significantly correlated with nine habitat variables, more than that of any other species analyzed. This analysis suggested that numerous factors contributed to the selection of a breeding site. The species appeared exclusively in hardwood and mixed forest stands and, in most cases, were observed in forest interior locations. In particular, Ovenbird favored mature, hardwood forests containing a high density of trees, a diversity of tree species, and standing dead trees. The negative correlation with ground cover corresponded with the species’ status as a ground feeder. Given the complexity of breeding habitat preferences, Ovenbird could be described as a specialist species. Figure 3 indicates the plots where Ovenbirds were located. The statistical analysis suggested that old hardwood forest stands with little human disturbance provide the combination of characteristics necessary to support Ovenbird breeding habitat. Given these criteria a very limited number of locations at Sweet Briar are suitable habitat for Ovenbird.
Wood thrush Hylocichla mustelina
Figure 4. Wood thrush distribution
Habitat Variable Large diameter downed logs 0.013
Slope
The Wood thrush breeding habitat sites were correlated with only two variables, large diameter downed logs and slope, suggesting that the species is not especially selective. The Albemarle County data reflected the same trend. This lack of selectivity contrasts the data for Ovenbird and the several other forest interior species. Figure 4 demonstrates the distribution of Wood thrush across the many land cover types at Sweet Briar. The distribution suggests that this species is more adaptable to human presence and disturbance than are most other forest dwelling Neotropical migrants. Many observations of the species were in fields close to forest edges.
Worm-eating Warbler Helmitheros peregrina
Figure 5. Worm-eating warbler distribution
Habitat Variable Slope 0.001
Canopy cover Broadleaf evergreen shrubs Deciduous shrubs
Worm-eating Warblers were located in a few areas with steep terrain. This data confirmed the literature on the species, which lists slope as the strongest breeding habitat determinant. This bird is specific to closed canopy hardwood forests. It nests in low vegetation, often tucking its nest up under shrubs, which confirms the positive correlation with the presence of shrubs.
Pine Warbler Dendroica pinus
Figure 6. Pine Warbler distribution
Habitat Variable Standing dead trees 0.001
Downed logs Pine Warblers were observed in only three locations at Sweet Briar, two in pine stands and one in a mixed stand that was predominantly pine. The later plot contained six hardwood trees, two live pines, 13 standing dead pines and two downed dead pines. Pine Warblers are specific to pine stands. There was a positive correlation between Pine Warbler and Pine forest type, but it was not significant. Instead the significant correlations were with snags and downed logs. One explanation for this correlation may be based on the fact that pine bark beetle has damaged and killed trees in pine stands to the degree that there are many dead trees in these stands, contributing to standing dead and downed logs. Philopatry, or site tenacity may also play a role in this finding since birds often return to the same breeding site in successive years. The habitat of the former nesting site may have been altered, sometimes even to the point that it would be considered unacceptable to the bird presently. Thus, a pine site that was once suitable breeding habitat for a Pine Warbler may have evolved to a hardwood site as the pines died out, but could still be selected as a breeding site by Pine Warblers. In Albemarle County, the same pattern was characteristic. There were high correlations between Pine Warblers and standing dead and downed logs, and less significant correlations between forest type and Pine Warbler presence.
Prairie Warbler Dendroica discolor
Figure 7. PrairieWarbler distribution
Habitat Variable Invasive species 0.003
Basal area
The Prairie Warbler inhabits open woodlands, brushy areas, scrublands and overgrown fields. The strong negative correlation between Prairie Warbler presence and basal area reflected this habitat preference. The strong positive correlation with invasive species was also a reflection of scrub habitat preference. Three of the four plots in which Prairie Warblers were observed are indicated as hardwood sites in Figure 7. These three locations have been recently logged. The southern most point where Prairie Warblers were observed was clear-cut in 1999. The other two "hardwood" sites with Prairie Warbler observations were selectively cut in 1993. This logging activity creates ideal habitat for Prairie Warbler, which typically choose sites with scrub vegetation. Other than former forest stands that had been clear-cut or selectively cut, little scrub habitat exists on Sweet Briar land.
Common Yellowthroat Geothlypis trichas
Figure 8. Yellowthroat distributionThe Common Yellowthroat did not have significant correlations with any of the habitat variables measured. This fact suggested that this species is not particularly selective when choosing a breeding site. The Common Yellowthroat was observed in all land cover types on the Sweet Briar campus. The literature states that its preferred habitat is scrubland. As with the Prairie Warbler, the only scrubland available on Sweet Briar land is on edges between forests and fields and in recently logged forest stands. The fact that scrubland is only found in transition zones between forests and fields may explain the lack of significant correlations with habitat variables.
Cowbird Molothrus ater
Figure 9. Cowbird distribution
Habitat Variable Broadleaf evergreen shrubs 0.001
The Brown-headed Cowbird feeds in open fields and is a brood parasite of many Neotropical migrants. It typically inhabits edges of forest and field habitat, where it can lay eggs in nests of forest dwelling Neotropical migrants and feed in fields. At Sweet Briar these birds were observed in five hardwood forest locations (Figure 9). The northern most point of Cowbird observation was located in hardwood forest that had been selectively logged. The point in the mid section of the Figure 9 map was clear-cut in 1999. The two southern points are on the border of Sweet Briar property, in close proximity to fields. The low numbers of Brown-headed Cowbirds observed was a surprising statistic. Given the distribution of its most commonly used host species, Common Yellowthroat and Prairie Warbler, one would expect to see Cowbirds in more land cover types on Sweet Briar’s property. The only habitat variable that was significantly correlated with observations of Cowbirds was the presence of broad-leafed evergreen shrubs. This result is not well understood. The sample size could be too small to see any patterns in the data with respect to important habitat variables.
Eastern Wood Peewee Contopus virens
Figure 10. Wood Peewee distribution
Habitat Variable Standing dead trees 0.044
The Eastern Wood Peewee is a habitat generalist, which prefers forests, but occurs in suburban areas and on forest edges. This study supported these characteristics since only one habitat variable, standing dead trees, was significant to the Peewee. This species is somewhat similar to the Wood thrush in its lack of selectivity in choosing a nesting site and in its tolerance of human disturbance. Unlike the Wood thrush, however, the Peewee was rarely found in land cover types other than forest stands, as seen in Figure 10.
Acadian Flycatcher Empidonax virescens
Figure 11. Acadian Flycatcher distribution
Habitat Variable Basal Area 0.001
Canopy height Maximum tree diameter Canopy cover Average tree diameter Forest cover type Standing dead trees large diameter Number of tree species Slope
Nine habitat variables, all indicating species diverse forest stands with large trees and some standing dead trees were significant to Acadian Flycatcher. These results agree with the literature on the habitat preferences of this species. The Acadian Flycatcher is a specialist selecting forest stands with a closed canopy often associated with water — swamps and ravines.
Yellow-billed Cuckoo Coccyzus americanus
Figure 12. Cuckoo distribution
Habitat Variable Land cover type 0.014
Large diameter trees Canopy cover
This analysis suggested that Yellow-billed Cuckoos preferred forest habitat but were indiscriminant about forest type. In all but two observation locations, Yellow-billed Cuckoos were found in hardwood, pine, and sanctuary forests equally. Three habitat variables were significantly correlated with Cuckoo presence, land cover type, large diameter trees and canopy cover. These results are not consistent with popular knowledge of this species. Cuckoos reportedly inhabit dense thickets, brushy roadsides and willow thickets and are seldom in large woods. It is possible that the lack of these habitat types at Sweet Briar may contribute to the Cuckoo being found primarily in forest stands.
Red-bellied Woodpecker Melanerpes carolinus
Figure 13. Red-bellied Woodpecker distribution
Habitat Variable Hardwood forest type 0.045
Canopy height
As shown in Figure 13, Red-bellied Woodpeckers were broadly distributed in all land cover types. The statistical analysis, however, indicated that this species preferred hardwood forest stands with tall trees. This species is indiscriminant in habitat selection and is very adaptable to human disturbance.
Pileated Woodpecker Dryocopus pileatus
Figure 14. Pileated Woodpecker distribution
Habitat Variable Basal area 0.025
The Pileated Woodpecker, like the Red-bellied, was observed in all land cover types. The significant correlation with basal area suggested that Pileated Woodpeckers selected forest sites with a high density of trees. Comparison of the two maps (Figures 13 and 14) indicated that the Pileated was located in more remote areas than the Red-bellied Woodpecker. This distribution suggested that this species is less adaptable to human disturbance.
Downy Woodpecker Picoides pubescens
Figure 15. Downy and Hairy Woodpecker distributions
Habitat Variable Coniferous shrubs 0.008
Presence of invasive species
Downy Woodpeckers were found in a wide range of habitats including hardwood and pine forests, sanctuaries, cut over forests and fields (Figure 15). The positive correlation with invasive species reflected the presence of Downy Woodpeckers in cut over areas. The strong negative correlation with coniferous shrubs was puzzling.
Hairy Woodpecker Picoides villosus
Figure 15. Downy and Hairy Woodpecker distributions
Habitat Variable Presence of snags 0.026
Large diameter trees
Hairy woodpeckers, in contrast to Downey Woodpeckers, were observed exclusively in hardwood and mixed forests, and primarily in forest interior areas (Figure 15). Many of the Hairy Woodpecker observations were in sanctuaries. This distribution and the habitat variables positively correlated with Hairy Woodpecker, snags and large trees, indicated that this species selected older undisturbed stands.
The Downy and Hairy Woodpeckers offer an interesting comparison. The literature states that these two species occupy similar habitats. Downy Woodpeckers, however, frequently inhabit suburbs, orchards and urban areas. These two species were found in very different locations at Sweet Briar. The Downy Woodpecker appeared more adaptable to human land use and disturbance than the Hairy Woodpecker.
Introduction Methods Results and Discussion Conclusions Literature cited
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