Shaw and Winker • SPRING REFUELING IN MIGRANT PASSERINES 
581 
TABLE 3. Change in mass, indicating slope (m), diurnal gain (g) based on a 12.5-hr day, nocturnal loss (g) based on 
4.5% of species’ mean mass, estimated 24 hr net gain (g), and percentage of the species' average mass in the Sierra de Los 
Tuxtlas. Mexico in spring. 
Species 
m 
Diurnal gain 
Nocturnal loss 
Net gain/day 
% of mass 
Swainson's Thrush 
0.00188 
2.243 
1.386 
0.858 
2.79 
Wood Thrush 
0.00465 
5.813 
2.141 
3.671 
7.72 
Magnolia Warbler 
0.00146 
1.046 
0.411 
0.634 
6.94 
Kentucky Warbler 
0.00171 
1.393 
0.601 
0.791 
5.92 
Hooded Warbler 
0.00118 
0.913 
0.488 
0.424 
3.91 
Worm-eating Warbler 
0.00150 
1.253 
0.615 
0.638 
4.67 
Ovenbird 
0.00156 
1.418 
0.831 
0.587 
3.18 
sex ratios, or unknown factors, was 0.7 g less than 
the fat-free mass. The average mass of individuals 
with fat scores of zero for all other species was 
greater than the fat-free mass. 
We examined the proportion of the sample 
population capable of completing a trans-gulf 
flight in still air by calculating the amount of fuel 
necessary lor each species using both techniques 
for estimating fuel content and comparing it to the 
estimated amount of fuel carried by each 
individual (Table 8), Percentages of the popula¬ 
tions of study species capable of the flight varied 
from 0 to 15.4% using a base of fat-free mass, and 
from 0 to 9% using a base of zero-fat score 
individuals. Hooded Warblers had the most 
dramatic difference between the two techniques 
(13.5 vs. 0%), whereas some species varied little 
or none. 
We used ANOVA comparing the standard 
errors of the regressions to ascertain whether 
single-route migrants fattened differently than dual- 
route migrants; this test failed to show signifi¬ 
cant differences between groups (F = 0.144, 
P = 0.932). 
DISCUSSION 
The majority of migrant passerines in our study 
carried substantial quantities of fat. Nine of the 10 
study species that were comparable with Odum 
(in Dunning 1993) had an average body mass 
significantly greater lhan fat-free mass. The 
amount of fuel estimated varied from 7 to 23% 
of live mass. Eight of the 13 study taxa had 
significant diel gains in either body condition or 
fat score. However, neither diel gains nor total fat 
carried (Tables 5, 7) were sufficient for the 
average individual of any species to cross the 
Gulf of Mexico from Los Tuxtlas in still air 
(Fig. 4). A lack of power due to small sample 
sizes cannot fully explain why some but not all 
taxa had significant gains. Species such as 
Yellow-breasted Chat and Gray-cheeked Thrush 
did not have any indication of change in diurnal 
condition or fat score trends despite substantial 
sample sizes (Tables 1, 2). 
Body molt was noted in three taxa; two had 
significant diel gains either in condition index or 
fat score. Molt places additional energetic de- 
TABLE 4. Comparison of fat-free mass (from Odum in Dunning 1993) to mean mass of study species captured in 
spring in the Sierra de Los Tuxtlas, Mexico using two-sample r-tests. 
Species 
Fat free mass (g) 
TuxUas mass (g) 
Difference (g) 
% of live mass 
1 
P 
Gray-cheeked Thrush 
25.30 
27.13 
1.93 
7 
3.78 
<0.001 
Swainson's Thrush 
24.18 
30.79 
6.61 
21 
28.89 
<0.001 
W°od Thrush 
42.21 
47.54 
5.33 
11 
8.37 
<0.001 
Gray Catbird 
31.80 
35.09 
3.29 
9 
6.94 
<0.001 
Magnolia Warbler 
6.92 
7.89 
0.97 
12 
0.75 
ns 
Kentucky Warbler 
11.36 
13.38 
2.02 
15 
13.61 
<0.001 
Hooded Warbler 
8.20 
10.60 
2.40 
23 
22.63 
<0.001 
10.79 
13.62 
2.83 
21 
12.08 
<0.001 
Ovenbird 
15.52 
18.55 
3.03 
16 
10.87 
<0.001 
Indigo Bunting 
12.34 
14.70 
2.36 
16 
14.58 
<0.001 
