Flight Morphology, Energetic Condition, and the Stopover Biology of Migrating Thrushes

--Flight morphology affects aerodynamic performance and the energetic demand for migration. We investigated the relationship between flight morphology and energetic condition of Wood Thrush (Hylocichla mustelina), Veery (Catharus fuscescens), Swainson's Thrush (C. ustulatus), and Gray-cheeked Thrush (C. minimus) during migration and the consequences for stopover behavior following spring trans-Gulf passage. Differences in morphological variables were found among Catharus thrushes, and between Wood Thrush and Catharus thrushes. Species with longer migratory distances have longer, more pointed wings and higher aspect ratios. The relatively larger wing area and lower body mass of Gray-cheeked Thrush and Veery result in lower wing loadings. Whereas wing size increased with increasing lean body mass in all species, the interrelations of wing span and wing area with lean body mass were allometrical (i.e. deviated from expected isometric relationship). Larger individuals in each species have disproportionately large wings. Wing size was negatively related to amount of stored fat, which indicated that larger individuals within each species have smaller fat stores remaining after trans-Gulf migration. Species or individuals with relatively long, pointed wings are more efficient migrants, and their energetic demand per unit distance travelled is lower. The consequences vis-a-vis stopover biology are considered. Received 2 April 1993, accepted 12 February 1994. MORPHOLOGY AFFECTS the aerodynamics of migratory flight. If long-distance migration selects for reduced cost of transport, migratory birds should have wings with a high aspect ratio and low loading, thereby reducing drag and the energetic cost of powered flight (Pennycuick 1975, Rayher 1988, 1990). These morphological features and others are interpreted as adaptations for migration (Averill 1922, Dilger 1956b, Greenewalt 1962, Cox 1968, Gaston 1974, Leisler and Winkler 1985, Winkler and Leisler 1985, 1992, Leisler 1990). Since flight morphology affects aerodynamic performance (Tucker 1974, Pennycuick 1975, Raynet 1988, 1990), it may also influence a migrant's energetic condition (fat stores) for migration and behavior during stopover. When landbird migrants arrive along the northern coast of the Gulf of Mexico, energetic condition varies. Some individuals have mobilized their lipid stores during passage and arrive essentially fat free, whereas other birds retain sufficient fat to continue migrating the day of their arrival (e.g. Rappole and Warner 1976, Moore and Kerlinger 1987, Kuenzi et al. 1991). Ener• Current address: USDA Forest Service, 2205 Columbia, SE, Albuquerque, New Mexico, USA. getic condition affects the migrant's likelihood of stopover, length of stay, and foraging behavior during stopover (e.g. Bairlein 1985, Biebach et al. 1986, Moore and Kerlinger 1987, Safriel and Lavee 1988, Loria and Moore 1990). If energy consumption varies in relation to flight morphology, the amount of fat required for migration would not be equivalent across individuals within a species or among species, and some individuals or species might experience reduced energy demand during migration and increased rate of passage. Our objective was to examine the relation between flight morphology and energetic condition (i.e. how might the relationship affect behavior of thrushes during stopover). We hypothesize that variation in flight morphology, both within and among species, affects energetic requirements for migration. Species or individuals with more efficient flight morphology should carry less fat stores during migration. We also predicted that, if variation in flight morphology affects energy demand, behavioral decisions regarding migration timing and length of stopover should be influenced. Morphological variations among and within species are a function of differences not only in migratory behavior, but also foraging behavior and habitat use; in addition, there undoubtedly