Shaw and Winker • SPRING REFUELING IN MIGRANT PASSERINES 
579 
TABLE 2. Regression of condition index (I) and fat score (2) on time of capture and migration strategy for 13 taxa 
captured during spring migration in the Sierra de Los Tuxtlas. Mexico. All significant regressions were positive; regression 
slopes are in Table 3. 
Species 
Condition index 
Fat score 
Migration strategy 
r’ 
F 
P 
r 2 
F 
p 
Zmpidonax spp. 
0.071 
2.36 
ns 
0.076 
2.53 
ns 
mainland 
0.005 
0.28 
ns 
0.003 
0.16 
ns 
trans-gulf 
0.038 
12.10 
<0.001 
0.069 
24.37 
<0.001 
mainland and gulf 
Wood Thrush 
0.088 
11.06 
<0.001 
0.067 
8.16 
0.005 
mainland and gulf 
Gray Catbird 
0.003 
0.09 
ns 
0011 
0.40 
ns 
mainland and gulf 
Magnolia Warbler 
0.130 
5.39 
0.026 
0.163 
7.03 
0.012 
trans-gulf 
Kentuckv Warbler 
0.048 
6.59 
0.011 
0.065 
8.98 
0.003 
mainland 
Hooded Warbler 
0.063 
11.33 
<0.001 
0.160 
31.94 
<0.001 
trans-gulf 
Worm-eating Warbler 
0.058 
4.62 
0.035 
0.066 
5.27 
0.025 
trans-gulf 
Yellow-breasted Chat 
0.007 
0.31 
ns 
0.011 
0.44 
ns 
mainland and gulf 
0.057 
4.01 
0.049 
0.037 
2.48 
ns 
trans-gulf 
Painted Bunting 
0.002 
0.05 
ns 
0.014 
0.42 
ns 
mainland and gulf 
Indigo Bunting 
0.009 
1.34 
ns 
0.063 
10.31 
0.002 
trans-gulf 
energetic demand may limit the amount of fat 
individuals can carry and their ability to refuel. Of 
these three species, Magnolia Warblers had a 
significant positive increase in both body condi¬ 
tion and fat score throughout the day, and Indigo 
Buntings had significant positive diurnal gains in 
fat score. 
Diurnal mass gains were estimated for the 
seven species with non-zero trends in condition 
index. Subtracting estimated nocturnal losses gave 
an estimate of average mass gain over a 24-hr 
period. Mass gain estimates varied from 2.8 to 
7-7% of a species’ average body mass (Table 3). 
No species had a net loss in condition. Estimated 
gains were lowest for Swainson’s Thrush (2.8% of 
mean body mass) and highest for Wood Thrush 
(7.7%; Table 3). 
Nine of the 10 species in our study for which 
fat-free mass data were available (mean, sample 
s ize, min-max, and SD) were significantly heavier 
than fat-free mass (Table 4). Only Magnolia 
Warblers were not significantly different. How¬ 
ever. the lack of visible fat does not preclude the 
presence of internal or blood-borne fat (extracted 
*>y Odum, in Dunning 1993), and fat scoring is 
subject to variation (Kremenlz and Pendleton 
1990; Table 5). However, use of fat-free estimates 
based on fat scores provides a second useful 
method for estimating the amount of tat available 
for migration. 
There was no relationship between refueling 
l ev el and migration distance from Los Tuxtlas to 
the center of the breeding range (F = 0.271, P = 
0.62). There was a significant negative correlation 
between average amount of fuel carried and extent 
of refueling observed for the seven species with 
significant diel gains (arcsine transformed data; 
F = 10.31, P = 0.024; Fig. 3). A significant 
relationship was found (r = 0.399. F = 7.289, 
p = 0.021) when percent of recaptures (after 
24 hrs; Table 6) was regressed against the slopes 
of the condition index regressions for all study 
species (Table 3). This indicates that species with 
a higher proportion of individuals spending more 
than 1 day on the site refuel more than those with 
higher rates of daily departure (Table 6). 
Flight capacities were estimated for those 
species for which fat-free mass data were 
available. The average individual of these species 
was capable of between 2 and 7 hrs of flight after 
a single day’s foraging (Table 7). based on net 
daily gains for those species showing significant 
refueling (Table 3). Total flight distances, assum¬ 
ing still air and exhaustion of all fuel reserves, 
would allow a range of 323 to 674 km. depending 
on species (Table 7). These distances are clearly 
insufficient for a trans-gulf flight in still air of 
1,150 km departing from Los Tuxtlas to Galves¬ 
ton, Texas. These distances are substantially 
reduced, from 66 to 425 km, when using flight 
capacity estimates based on the average mass of 
individuals with zero fat score, as are the number 
of individuals capable of completing a trans-gulf 
flight to Galveston (Table 8). The average mass of 
Gray-cheeked Thrushes with zero fat score, 
perhaps due to geographic variation in body size. 
