Chen et al. • SEARCH BEHAVIOR OF MIGRATORY SONGBIRDS 
357 
area. The number of birds may increase 10- to 100- 
fold in these situations. Consequently, space for 
each individual to search for food was reduced 
sharply and an increase in social interaction was 
expected (Morse 1970). Migrants moved quickly 
when foraging in mixed-species foraging Hocks, 
and used more “flights" than those that foraged 
solitarily. Morse (1970) found that birds reduce 
their foraging space in proportion to Hock size. 
Moreover, birds in mixed flocks have to make 
continual adjustments to match the overall rate of 
flock progression (Hutto 1988). Thus, migrants in 
flocks would take more flights to keep pace with 
other individuals. During periods of fallout, mi¬ 
grants were everywhere in the plots and it appears 
to be better for a bird to move away from other 
individuals to reduce conflicts (Moore and Wang 
1991). Migrants moved around frequently and more 
flights were made during periods of high migrant 
density than during periods of low migrant density. 
Holmes and Robinson (1981) indicated vege¬ 
tation structure strongly affects foraging behavior 
of perch-gleaners, but had little influence on 
species that sally to catch prey. In general, the 
more a particular type of habitat is used by a bird, 
the greater the bird is affected by that habitat. 
Most warblers are typical, near-surface searchers, 
whereas vireos exhibit some medium-distance 
search. Thus, warblers are more severely affected 
hy environmental factors than vireos. These 
differences are apparent with the models that 
contained three-factor interactions for warblers, 
hut had far less high-order interactions for vireos. 
En route migrants often move among different 
habitats at stopover sites (Hutto 1985, Moore et al. 
1995, Barrow et al. 2000), and lean birds usually 
have a higher search rate, more plastic foraging 
behavior, and use more types of habitat than fat 
birds (Loria and Moore 1990. Wang and Moore 
2005). All transient warblers in our study, except 
Hooded Warbler and Prothonolary Warbler (Pro- 
lonotaria citrea), had a higher search rate than 
wintering Yellow-rumped Warblers. 
Migrants often have high energy demands at 
stopover sites. Marlin and Karr (1990) also found 
dtat migrants increase use of energetically-expcn- 
s * v e maneuvers during early spring and late fall 
migration when insects are limited. Our results 
indicate that morphology, phylogeny, vegetation 
structure, and social status also affect search 
behavior, and thus have a role in shaping 
ex ploitation strategies of migrants at stopover 
s 'tes. Thus, suitable habitat at stopover sites 
should be identified and managed for migrants 
to increase their success during migration. 
ACKNOWLEDGMENTS 
We are grateful to the Stream family in Louisiana for 
permission to conduct research on their property. Permis¬ 
sion to conduct research in Texas was granted by the Texas 
Parks and Wildlife Department through Terry Tourney, 
Candy Abshier Wildlife Management Area. The Stream 
Property Management Co.. Rockefeller Wildlife Refuge, 
Louisiana Department of Wildlife and Fisheries, and 
Tennessee Gas Co. provided invaluable assistance for this 
study. Field work was supported by the Houston Audubon 
Society and the U.S. Kish and Wildlife Service. We are 
especially grateful to David Richard and Terry Tourney for 
use of their facilities and help with logistics. We are 
indebted to E. B. Moser for help with the statistical 
analysis. Clinton Jeske. Robert Dobbs, and two anonymous 
reviewers provided recommendations for improving the 
manuscript. 
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