The Wilson Journal of Ornithology 124( 1 ):31 —39, 2012 
ARTHROPOD ABUNDANCE AND SEASONAL BIRD USE OF 
BOTTOMLAND FOREST HARVEST GAPS 
CHRISTOPHER E. MOORMAN, 14 LIESSA T. BOWEN,' JOHN C. KILGO, 2 
JAMES L. HANULA/ SCOTT HORN,' AND MICHAEL D. ULYSHEN 3 
ABSTRACT.—We investigated the influence of aitliropod abundance and vegetation structure on shifts in avian use of 
canopy gap, gap edge, and surrounding forest understory in a bottomland hardwood forest in the Upper Coastal Plain of 
South Carolina. We compared captures of foliage-gleaning birds among locations during four periods (spring migration, 
breeding, post-breeding, and fall migration). Foliage arthropod densities were greatest in the forest understory in all four 
seasons, but understory vegetation density was greatest in gaps. Foliage-gleaning bird abundance was positively associated 
with foliage-dwelling arthropods during the breeding (F 18.5, F < 0.001) and post-breeding periods (F - 9.4, P = 
0.004). and negatively associated with foliage-dwelling arthropods during fall migration (F = 5.4, P = 0.03). Relationships 
between birds and arthropods were inconsistent, but the arthropod prey base seemed to he least important during migratory 
periods. Conversely, bird captures were positively correlated with understory vegetation density during all four periods 
IF < 0.001). Our study suggests high bird abundance associated with canopy gaps during the non-breeding period resulted 
less from high arthropod food resource availability than from complex underston and midstory vegetation structure. 
Received 25 January 2011. Accepted $ August 2011. 
Many bird species, including those of early- 
successional habitats and those of small tree-fall 
gaps within mature forest, select disturbed 
habitats during some portion of the year (Hunter 
et al. 2001). Several studies have documented 
greater bird abundance in forest canopy gaps 
created by natural treefalls (Willson et al. 1982, 
Blake and Hoppes 1986. Martin and Karr 1986) or 
group-selection harvest (Kilgo et al. 1999. Moorman 
and Guynn 2001) than in the mature forest 
surrounding gaps. Some mature-forest breeders 
shift into more densely vegetated habitats between 
breeding and post-breeding periods (Anders el al. 
1998: Vega Rivera et al. 1998, 2003; Pagen et al. 
2000; Viiz and Rodewald 2006). Birds use a variety 
of forested habitats during migratory periods (Petit 
2000. Rodewald and Brittingham 2002), but 
mature-forest edges and early-succession habitats 
may experience relatively greater use (Rodewald 
and Brittingham 2004). Reasons for greater use of 
disturbed habitats by birds during certain periods 
remain speculative, but abundant food and protec¬ 
tion from predators have been proposed (Marshall 
et al. 2003). 
Fisheries. Wildlife, and Conservation Biology Program. 
Department of Forestry and Environmental Resources. 
North Carolina State University. Campus Box 7646, 
Raleigh. NC 27695. USA. 
L'SDA, Forest Service Southern Research Station, P O. 
Box 700. New Ellcnlon, SC 29809. USA. 
USDA. Forest Service, 320 Green Street. Athens, GA 
30602, USA. 
'Corresponding author; e-mail: 
chris_moorman@ncsu.edu 
Arthropod populations also are influenced by 
season and habitat type (Johnson and Sherry 2001, 
Greenberg and Forrest 2003) as well as canopy 
gap size (Shurc and Phillips 1991). It should be 
advantageous for birds to choose sites with the 
greatest resource availability (Martin and Kan- 
1986), and greater invertebrate biomass has been 
positively correlated to bird abundance (Blake and 
Hoppes 1986. Holmes el al. 1986), daily nest 
survival rates, growth rales of nestlings (Duguay 
et al. 2000). and liming of warbler migration 
(Graber and Graber 1983). Studies of experimen¬ 
tal prey removal have not linked decreased prey 
abundance with negative consequences for the 
local bird community (Nagy and Smith 1997, 
Marshall et al. 2002, Champlin ct al. 2009). 
Bowen et al. (2007) documented seasonal shifts 
in relative use by birds of canopy gap and forest 
habitat. They speculated these shifts may be 
driven by seasonal changes in arthropod abun¬ 
dance in gaps. Previous studies have not investi¬ 
gated seasonal shifts in avian habitat use as related 
to resource availability over multiple periods. 
Our objectives were to: (I) investigate whether 
bird use of forest gaps was associated with 
arthropod abundance or vegetation structure, and 
(2) ascertain if shifts in relative use of gap and 
forest understory were related to spatial and 
temporal variation in arthropod abundance. We 
predicted positive relationships between avian 
habitat use and arthropod abundance (i.e.. relative 
bird use of gap vs. forest underston will shift 
based on changes in local arthropod abundance) 
from spring migration through fall migration. 
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