Carpenter et al. • CERULEAN WARBLER MICROHABITAT ASSOCIATIONS 
215 
(Hamel 2000b, Jones and Robertson 2001, Jones 
et al. 2001. Hamel 2005). Our average Shannon- 
Weiner index for used plots was 0.72 i 0.01 
(maximum 1.0) and indicates a moderately 
complex canopy structure. The disparity between 
our used and unused plots may be due to use of 
crude interval measurements which did not 
accurately distinguish finer complexities at a 
significance level of 0.1. This may also clarify 
why Cerulean Warbler presence was negatively 
related to canopy structure in our modeling. A 
more complex canopy structure at unused plots 
does not necessarily indicate an abundance of 
suitable habitat is available elsewhere in Alabama 
for Cerulean Warblers. The unused plots' lack of 
other microhabitat characteristics (e.g., high 
percent deciduous basal area and fewer but well¬ 
spaced, large diameter trees) identified as impor¬ 
tant to Cerulean Warblers will likely prevent these 
areas from supporting future populations, if 
current conditions persist. 
The importance of canopy gaps to Cerulean 
Warbler territory and nest site selection has been 
supported by some studies (Oliarnyk and Robert¬ 
son 19%. Nicholson 2004) and questioned by 
others (Jones et al. 2001, Hamel 2005, Barg et al. 
2006b). Used plots had significantly larger canopy 
gaps compared to unused plots, and our top model 
indicated Cerulean Warbler presence increased as 
distance to a canopy gap decreased. Cerulean 
Warblers in our study may be exploiting these 
openings as supplemental foraging areas through¬ 
out the breeding season and during post-breeding 
dispersal (Blake and Hoppes 1986, Vilz and 
Rodewald 2006), and to increase vocal deliver¬ 
ance and recognition of neighboring conspecifics 
(Barg et al. 2006b). Cerulean Warblers may also 
be using openings <10 nf created by smaller 
snags, another variable present in our best 
supported model, which may be contributing to 
forest heterogeneity and canopy complexity 
(Oliarnyk and Robertson 1996, Wood et al. 
2006; but see Barg et al. 2006b). Our ordination, 
however, contradicts these findings by disassoci¬ 
ating the principal component representing cano¬ 
py gaps with Cerulean Warblers. A plausible 
explanation is the cleared fields maintained within 
the heavily forested landscape of the Jackson 
County populations possibly influenced compari¬ 
son with the more fragmented landscape surround¬ 
ing unused locations (Carpenter 2007). The appro¬ 
priate size, quantity, and distribution of canopy 
gaps and other small-scale disturbances, as well as 
evidence of whether or not Cerulean Warblers are 
using them, remains unclear in Alabama. 
Our results help clarify the roles of avian 
species assemblages and vegetative characteristics 
in habitat used by Cerulean Warblers. This 
dynamic relationship is further complicated by 
resource availability, predation, competition with 
other organisms, and habitat alteration: none of 
which was accounted for in our study. Future 
Cerulean Warbler research in Alabama and 
elsewhere will benefit by addressing these issues 
in more detail. 
ACKNOWLEDGMENTS 
Funding for this project was provided by Alabama A&M 
University. Alabama Department of Conservation and 
Natural Resources. U.S. Forest Service, and U.S. Fish and 
Wildlife Service. We are grateful to E. C. Soehrcn. A. A. 
Lcsak, C. M. Kilgore, and L. M. Gardner-Barillas for field 
assistance: J. A. Cochran. T. U Counts, and G. M. Lein for 
logistical support: R. L. West and T. M. Haggerty for 
historic Cerulean Warbler records and Breeding Bird Atlas 
data: and W. B. Tadesse and K. E. Ward for suggestions 
that improved the thesis on which this study is based. This 
manuscript benefited greatly from comments provided by 
Jason Jones, C. E. Braun, and two anonymous reviewers. 
This project would have been impossible without the 
cooperation of many private landowners who granted 
access to their property. We especially thank the Cagle 
and Miller families, and the Stevenson Land Company. 
LITERATURE CITED 
ASKINS, R. A., .1. F. LYNCH, AND R. Greenburg. 1990. 
Population declines in migratory birds in eastern North 
America. Current Ornithology 7:1-57. 
Barg, J. J.. J. Jones, M. K. Girvan, and R. J. Robertson. 
2006a. Within-pair interactions and parental behavior 
of Cerulean Warblers breeding in eastern Ontario. 
Wilson Journal of Ornithology 118:316-325. 
Barg. .1. J.. D. M. Aiama, J. Jones, and R. J. Robertson. 
2006b. Within-territory habitat use and microhabitat 
selection by male Cerulean Warblers (Dendroica 
cerulea). Auk 123:795-806. 
BEYER. H. L. 2004. Hawth's analysis tools for ArcGIS. 
Marshfield, Wisconsin. USA. www.spatialecology. 
com/htools 
Beers. T. W.. P. E. Dress, and L. C. Wensel. 1966. 
Aspect transformation in site productivity research. 
Journal of Forestry 64:691-692- 
Bibby. C. J.. N. D. Burgess, D. A. Hill., and S. H. 
Mlstoe. 2000. Bird census techniques. Third Edition. 
Academic Press. London. United Kingdom. 
Blake. J. G. and W. G. Hoppes. 1986. Influence of 
resource abundance on use of tree-fall gaps by birds in 
an isolated woodlot. Auk 103:328-340. 
Blake. J. G. and J. R. Karr. 1987. Breeding birds of 
isolated woodlots: area and habitat relationships. 
Ecology 68:1724-1734. 
