The Wilson Journal of Ornithology 123(3):575-587, 2011 
SPRING STOPOVER AND REFUELING AMONG MIGRANT 
PASSERINES IN THE SIERRA DE LOS TUXTLAS, 
VERACRUZ, MEXICO 
DAVID W. SHAW'- 23 AND KEVIN WINKER 1 
ABSTRACT.—The narrowing of the North American continent at the Isthmus of Tehuantepec creates an important 
geographic bottleneck for songbirds on their northward spring migrations. The Sierra de Los Tuxtlas. in the northwestern 
portion of the Isthmus, provide an ideal location from which to address questions of resource use and fuel acquismonduring 
migration. We operated mist nets during morning and evening to capture passerines during spring migration in _ an 
2004. Seven of 13 taxa had significant diurnal increases in body condition (an index of size-adjusted mass): Swainson s 
Thrush (Catharus ustulaius), Wood Thrush (Hylocichla musteline,). Magnolia Warbler (Dendroica magnolia), Kentucky 
Warbler [Oporomis formosus}. Hooded Warbler (Wihonia ciirina ), Worm-eating Warbler [Helmut,eras vernuvorum), and 
Ovenbinl (Seiurus aurocapilla). All of these species, except Ovcnbird. also had a significant increase in tat score. Indigo 
Buntings [Passerina cyanea) had a significant increase in fat score but not in condition index. A comparison wit autumn 
migration at this site showed overall similarities in percentage of species gaining mass and in the amount game . ut t ere 
were seasonal differences within species. There was no relationship between increase in body condition and a mainlan 
versus trans-gulf migratory strategy. Received 13 November 2007. Accepted 19 March 2011. 
Migration places intense physical demands on 
birds. One of the main adaptations enabling birds 
lo make long-distance seasonal migrations is fat 
deposition (Blem 1990, Rogers 1991). Food 
resources at stopover locations are likely critical 
and, because the geography of Middle America 
causes a relatively rapid latitudinal decline in 
available space for lundbirds migrating south in 
autumn, competition for food resources may be 
high. Land availability for these birds during 
s pring migration increases rapidly north of the 
Isthmus of Tehuantepec. Several studies have 
addressed refueling and stopover ecology during 
autumn on this isthmus and farther south in 
Middle America (Galindo et al. 1963: Galindo and 
Mendez 1965; Rogers and Odum 1966; Child 
l%9; Winker 1995a, b; Johnson and Winker 
2008), but there are few reports of work 
conducted in spring (e.g., Galindo et al. 1963, 
Wilson et al. 2008). 
Oar field site in the Sierra de Los Tuxtlas in the 
northwestern portion of the Isthmus of Tehuante- 
Pec. Mexico, provides an ideal location from 
w hich to study refueling strategics and stopover 
ecology among migrant songbirds. The ecology ot 
migrating birds in this region is only beginning to 
be understood (Rappole and Warner 1980; Rap- 
Pole 1995; Winker 1995a. b). This site was used 
'University of Alaska Museum, 907 Yukon Drive. 
Fairbanks. AK 99775, USA. 
'CurTent address: Alaska Bird Observatory. 418 Wedge- 
w ood Drive, Fairbanks, AK 99775. USA. 
Corresponding author; e-mail: dshaw@alaskabird.org 
previously to investigate refueling by autumn 
migrants (Winker 1995a), which permits direct 
comparisons of seasonal refueling strategies. Our 
objectives were to gather data on fat levels and 
mass gains among the common, noctumally 
migrating passerines passing through Los Tuxtlas 
during spring migration to understand seasonal 
and geographic patterns of fuel deposition and 
route selection in this region. Specifically, we 
collected data during two migratory seasons to 
address four questions. (I) What quantify of fat do 
spring migrants carry through this region? (2) Do 
they show a net gain in fuel during stopover? (3) 
Arc there differences in how this site is used for 
refueling between spring and autumn? (4) What 
can be inferred about route selection in this 
region? 
METHODS 
Study Area.—'We conducted fieldwork in the 
Sierra'de Los Tuxtlas (Fig. 1) in southern 
Veracruz, Mexico, 90 km southeast of Veracruz 
city. This range of mountains is in the northwest¬ 
ern portion of the Isthmus of Tehuantepec and is 
isolated from the Sierra Madre Oriental by 
extensive lowlands. The Los Tuxtlas region 
encompasses -4,200 km 2 and is dominated by 
Volcan San Martin and Volcan Santa Marta, each 
reaching >1,500 m in elevation. The Gulf of 
Mexico is a short distance from the mountains to 
the north and east. Habitat in the region was 
formerly dominated by the farthest north neotrop¬ 
ical evergreen rain forest but, due to deforestation, 
575 
