Moorman et al. • SEASONAL BIRD USE OF HABITATS AND ARTHROPODS 
37 
gaps than in adjacent forest understory, but 
relative capture probabilities likely remained 
constant across seasons. Different capture proba¬ 
bilities among net locations should not have 
influenced our ability to detect seasonal shifts in 
relative bird use of the lowest stratum (<3 m) of 
v egetation in gaps and forest. Nets sampled only 
vegetation near the ground, and we could not 
address shifts in bird use of the forest canopy. 
Birds may have used gaps in response to 
available fruits. Gap interiors contained early- 
successional fruiting species (e.g.. winged sumac 
|Rhus copallina] and blackberry), while other 
fruiting species such as poison ivy {Toxicoden¬ 
dron radicans) and hawthorn {Crataegus spp.) 
were common at the immediate gap edge (LTB, 
pers. obs.). We observed omnivorous birds eating 
fruits in gaps, including American beautyberry 
(Callicarpa americana). flowering dogwood 
{Comas florida ), grape (Vitis spp.). hawthorn, 
poison ivy, and winged sumac (LTB, pers. obs.). 
Fruit typically is most abundant on the Savannah 
River Site from late summer through early fall 
(McCarty et al. 2002). Some bird species such as 
Summer Tanager and Gray Catbird are known to 
shift to diets higher in fruit during fall migration. 
We did not. however, find a corresponding shift 
in habitat use for omnivorous species such as 
Northern Cardinal, suggesting that birds were 
meeting their nutritional needs without closely 
following seasonal fruit availability (Bowen et al. 
2007), Vegetation matter, including fruit, com¬ 
prised <5% of the crop contents of several of the 
birds most commonly captured during all four 
periods (Moorman ct al. 2007). Fruit production 
within our canopy gaps was relatively low and 
highly seasonal with no fruit available during 
spring, one of the periods of highest bird use 
(Bowen et al. 2007). 
Data from our study suggests bird use of gaps 
ma\ be more closely related to vegetation 
structure, important for various aspects of their 
life history (e.g.. cover for fledglings, foraging, 
and molting), than to arthropod abundance. We 
consistently detected more birds in the gap and 
edge habitats during all periods (Bowen et al. 
2007), and the understory vegetation density also 
was highest in these locations. Dense understory 
vegetation could be associated with increased 
foraging substrate density, thereby increasing 
foraging efficiency, even if arthropod density on 
foliage was equal to or lower than areas w ith less 
complex vegetation structure. Other studies also 
have shown that small canopy gaps wdth greater 
foliage density and understory habitat structure 
tend to support more birds than mature forests 
(Blake and Hoppes 1986. Martin and Karr 1986, 
Kilgo et al. 1999, Moorman and Guynn 2001. 
Wilson and Twedt 2003). Forest-breeding birds 
may use early-successional habitat with greater 
foliage density because of increased protective 
cover, particularly during migration when birds 
move through unfamiliar areas. Dense understory 
vegetation provides birds more perching sites, 
protection from the elements, greater cover during 
molt (Anders et al. 1998; Vega Rivera et al. 1998. 
2003), protective habitat for young during the 
post-breeding period (Pagen et al. 2000), and 
protection from predators (Moore et al. 1995). 
ACKNOWLEDGMENTS 
Financial support for this project was provided by the 
USD A, CSREES National Research Initiative Competitive 
Grants Program (Award # 00-35101-9307), North Carolina 
State University, and the LSDA, Forest Service Southern 
Research Station. We thank the U.S. Department of Energy- 
Savannah River for providing access to the study site and the 
USDA. Forest Service-Savannah River for logistical support 
with a special thanks to J. I Blake and E. G. Olson. We thank 
W. P. Smith for invaluable assistance during early phases of 
the research. R. A. Lancia, D. .1. Robison, and T. R. Simons 
reviewed earlier drafts of this manuscript. We are grateful for 
field assistance from T. B. Champlm. S. M. Junker. K. M. 
Mack, and D. E. Westerman. and for statistical assistance 
from M. L. Gumpertz and S. B. Donaghy. 
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