Shaw and Winker • SPRING REFUELING IN MIGRANT PASSERINES 
585 
Winker (1995a) suggested the majority of 
captures at this site during autumn were not birds 
arriving from a trans-gulf flight, but were likely 
birds moving south along the coast to arrive in 
Los Tuxtlas. Birds from this same site in spring 
were likely reversing Winker's (1995a) proposed 
route and following the coast northward before 
perhaps making shorter overwater crossings of the 
northern Gulf or avoiding overwater flights 
altogether. Eleven of our 13 study taxa occurred 
in substantial numbers in Rappole ct al.’s (1979) 
study of a stopover site in southern Texas in 
spring. This suggests some individuals ol species 
thought to be exclusively trans-gulf migrants are 
moving northward along the Gulf Coast, perhaps 
crossing few areas or only short distances over 
open water. Another possibility is that trans-gulf 
migrants are moving eastward along the southern 
coast of the Gulf of Mexico and making the 
crossing by way of the Yucatan Peninsula. Both 
possibilities allow for arrival of migrants at 
observation points in the southeastern United 
States either by land or water (Stevenson 1957, 
Guuthreaux 1971, Rappole et al. 1979. Wang and 
Moore 1997). Sampling migrant abundance along 
the Gulf Coast from the Isthmus northward would 
provide valuable information about departure and 
arrival points for trans-gulf migrants. 
Fueling Strategies .—Migration routes and dis¬ 
tances to breeding areas failed to explain variation 
in mass gains among species either within or 
between seasons. Species-level comparisons tor 
that tend to spend more time on the site were also 
likely to show higher diel gains than those 
remaining only a few hours (Tables 3, 6). We 
found a significant positive correlation between 
the percent of recaptures occurring after 24 hrs 
(Table 6) and the slopes of the condition index 
gains. These two findings may provide better 
insight into interspecific patterns of refueling in 
Los Tuxtlas thau routes, distances traveled, or 
even seasonality through a complete migration 
cycle. 
Current hypotheses regarding migration 
through Middle America need refinement. Species 
thought to be exclusively trans-gulf migrants 
were, on average, carrying insufficient resources 
to accomplish a Gulf crossing from Los Tuxtlas 
and may often not make such crossings. Refueling 
rates at this site are not higher in spring, and 
considerable variation occurs among species and 
between seasons. The total amount of fat carried 
and time spent at the site appear to be the best 
predictors of diet refueling gains. 
ACKNOWLEDGMENTS 
The University of Alaska Museum and Friends of 
Ornithology provided financial support for this study. Eliut 
Hurtado and Felice Griffiths provided field assistance. 
Patricia Escalante at the Universidad Nacional Autonoma 
dc Mexico, Instituto de Biologia. Colleccion Nacional de 
Aves kindly assisted with permits and logistics. A. B. 
Johnson provided helpful discussion, and Erica Dunn, Abby 
Powell, and Ron Barry provided excellent advice and 
comments on the manuscript. 
our distance estimates to breeding areas are 
complicated by two factors that we cannot 
address: the origins and destinations of captured 
individuals are unknown, and both will likely 
affect fattening strategies. However, most varia¬ 
tion in our data may have a proximate explanation 
in the relationship found by Dunn (2001): that 
individuals arriving at a site with lower mass gain 
more than those arriving with more substantial 
reserves. Dunn (2001) proposed that individuals 
arriving at a stopover site with sufficient resources 
only need a place to rest and maintain reserves, 
and they may not have substantial net gains even 
m ideal habitat. The significant negative relation¬ 
ship we found (Fig. 3) between a species-level 
estimate of fat levels and the amount of fuel a 
species gained (on average) in a day at this site 
suggests this relationship may scale up to the 
species level and help explain single-site patterns 
among species. A second and perhaps not 
unlinked relationship that we found is that species 
LITERATURE CITED 
Alerstam, T„ M. RosfiN. J. Backman, P. G. P. Ericson, 
AND O. HELLCiREN, 2007. night speeds among bird 
species: allomelric and phylogenetic effects. PLoS 
Biology 5:1656-1662. 
ANDRLE. R. F. 1964. A biogeographical investigation of the 
Sierra de Tuxtla in Veracruz, Mexico. Dissertation. 
Louisiana State University. Baton Rouge, USA. 
Blem, C. R. 1990. Avian energy storage. Current 
Ornithology 7:59-113. 
BRiSKlfc, J- V. 1994 Least Flycatcher (Empidonax mini¬ 
mus). The birds of North America. Number 99. 
Chu.d, G. 1. 1969. A study of nonfat weights in migrating 
Swainson’s Thrushes (Hytoeichla ustulata). Auk 
86:327-388. 
ClMPRICH, D. A. AND F. R. Moore. 1995. Gray Catbird 
(Dumelella carolinensis ). The birds of North America. 
Number 167. 
DiRZO, R. AND M. C. Garcia. 1992. Rates of deforestation 
in Los Tuxtlas, a neotropical area in southeast Mexico. 
Conservation Biology 6:84—90. 
Dunn, E. H. 2001. Mass change during migration stopover: 
