Body condition and feather molt of a migratory shorebird during the non-breeding season

Abstract

Migratory shorebirds have some of the highest fat loads among birds, especially species which migrate long distances. The upland sandpiper Bartramia longicauda makes long distance migrations twice a year, but variation in body condition or timing of feather molt during the non-breeding season has not been studied. Molt is an important part of the annual cycle of migratory birds because feather condition determines flight performance during migration, and long-distance movements are energetically costly. However, variation in body condition during molt has been poorly studied. The objec tive of our field study was to examine the timing and patterns of feather molt of a long distance migratory shorebird during the non-breeding season and test for relationships with body size, fat depots, mass, and sex. Field work was conducted at four ranches in the Northern Campos of Uruguay (Paysandú and Salto Departments). We captured and marked 62 sandpipers in a 2-month period (Nov–Jan) during four non-breeding seasons (2008–2012). Sex was determined by genetic analyses of blood samples taken at capture. Molt was measured in captured birds using rank scores based on published standards. Body mass and tarsus length measurements showed female-biased sexual size dimorphism with males smaller than females. Size-corrected body mass (body condition) showed a U-shaped relationship with the day of the season, indicating that birds arrived at non-breeding grounds in relatively good condition. Arriving in good body condition at non-breeding grounds is probably important because of the energetic demands due to physiological adjustments after migration and the costs of feather molt