Shaw and Winker • SPRING REFUELING IN MIGRANT PASSERINES 
577 
TABLE 1. Sample sizes and quantified variables with means (SD) for 13 taxa captured during spring 2003 and 2004 in 
Ihe Siena de Los Tuxtlas, Mexico. (Hooded Warblers are for 2003 only). 
Empidonax spp. 
Gray-cheeted Thrush (Cathants minimus ) 
Swainson's Thrush (C. uslulalus) 
Wood Thrash (Hylocichla mustelina) 
Gray Catbird (Dumetella carolinensis) 
Magnolia Waibler (Dendroica magnolia) 
Kentucky Warbler (Opvromis formosus) 
Hooded Warbler (Wihnnia citrina) 
Worm-earing Warbler (Helmitheros 
ivrmivanim) 
Yellow-breasted Chat Helena virens ) 
Ovenbird (Seiunis aurocapilla) 
Painted Bunting (Passerina ciris) 
Indigo Bunting (P. cyanea) 
Mean lime 
of capture 
Mass, g 
Wing chord, mm 
Tail, mm 
Fat score 
35 
mi 
11.8 (4.91) 
66.5 (7.75) 
55.5 (5.07) 
0.8 (0.9) 
58 
1157 
27.1 (3.14) 
98.8 (3.78) 
69.1 (4.52) 
1.4 (0.9) 
323 
1147 
30.8 (3.40) 
95.5 (5.68) 
66.2 (3.52) 
2.1 (1.2) 
120 
1118 
47.6 (5.99) 
100.0 (14.73) 
67.7 (3.39) 
2.1 (1.5) 
38 
0929 
35.1 (2.62) 
86.9 (2.65) 
89.8 <3.831 
1.3 (1.1) 
39 
1212 
7.9 (0.83) 
57.5 (2.07) 
47.2 (2.11) 
0.9 (1.0) 
136 
1151 
13.4 (1.67) 
65.0 (2.60) 
46.8 (2.41) 
1.7 (1.4) 
171 
1132 
11.0 (2.93) 
61.9 (3.98) 
53.8 (4.23) 
1.8 (1.3) 
78 
1124 
13.7 (1.82) 
66.7 (2.91) 
47.7 (2.36) 
2.5 (1-6) 
43 
1042 
26.4 (2.81) 
73.6 (2.53) 
71.4 (3.76) 
2.4 (1.3) 
68 
1125 
18.5 (1.81) 
72.7 (2.62) 
51.6 (1.83) 
1.3 (1.1) 
31 
1013 
15.8 (1.89) 
69.4 (2.67) 
53.8 (2.46) 
1.4 (1.3) 
159 
1111 
15.2 (4.57) 
65.7 (4.94) 
50.0 (3.39) 
1.4 (1.2) 
Warbler {Wilsonia citrina) had a significant 
difference between years (r = 1.99, P = 0.013). 
We selected 2003 for analyses for the Hooded 
Warbler to avoid confounding analyses with 
apparent between-year differences; 2003 had the 
larger sample (2003:171 vs. 2004:36). Twelve 
migrant species and the genus Empidonax had 
sufficient numbers of captures for analyses (n s 
30; Table 1). We pooled data for Least Flycatcher 
(£• minimus ) and ‘Traill’s’ flycatchers (E. 
alnorum and E. traillii) to increase sample size 
sufficiently for Empidonax flycatchers to be 
included in analyses (this may have included a 
few Empidonax virescens , although wc had no 
■’Pccimens). The Empidonax group is hereafter 
referred to as one of the study ‘species,* One ol 
°ur goals was to contrast migrants at this site in 
spring with the same site in autumn: thus, we 
followed Winker (1995a) in analytical assump¬ 
tions and in including all first captures, which tor 
some species may include a small number ot 
locally wintering birds. We reduced the likelihood 
°f including these in three ways. (1) Early-season 
captures of overwintering species were banded 
and excluded. (2) The site is small, most of these 
^cies are territorial in winter and, if step one 
missed a few birds, they were numerically 
damped by passage migrants. (3) We focused 
netting during 2004 on the period of greatest 
migrant movement through this area. 
Our method of quantitatively estimating mass 
gains among stopover migrants assumes that birds 
^raging in a suitable environment will have a 
diurnal mass increase from food intake and fuel 
deposition with subsequent loss at night due to 
fasting, nocturnal metabolism, and excretion of 
undigestible material. Non-foraging individuals, 
or those in an unsuitable environment, will show a 
diel or 24-hr mass decrease. We created a 
‘condition index' for each first-time capture by 
dividing mass by wing chord and multiplying this 
value by a constant (100). Wing chord has been 
shown to be an appropriate proxy for size (Winker 
et al. 1992a, Winker 1995a). We then regressed 
condition index on time of day of capture (first 
captures only) to examine whether there was 
diurnal increase in condition, averaged across all 
sampling days. This technique follows Winker et 
al. (1992a) and Winker (1995a) and examines 
species-level trends. Body mass is the most 
important variable, and inclusion of a size-related 
variable in the condition index somewhat corrects 
for size differences among individuals. (Linear 
regressions of wing chord on time of day showed 
no significant relationships for any species 
examined.) Mass has been shown in fat-free mass 
studies to cone late with fat carried (Rogers 1965, 
Roeers and Odum 1966). The underlying assump¬ 
tion is that condition index is correlated with 
amount of fuel carried. 
We used fat scores, gauged from visible 
furcular and abdominal fat. to corroborate evi¬ 
dence for trends observed in condition indices 
(Winker 1995a, Dunn 2002). Fat scores are 
somewhat qualitative and subject to variation 
between observers, and are not suitable for 
