J. Field Ornithol. 83(4):412–419, 2012 DOI: 10.1111/j.1557-9263.2012.00393.x Estimating mass change of migrant songbirds during stopover: comparison of three different methods Kyle G. Horton 1 and Sara R. Morris 2 Department of Biology, Canisius College, 2001 Main Street, Buffalo, New York 14208, USA Received 20 January 2012; accepted 1 September 2012 ABSTRACT. Stopover-site quality has often been assessed using changes in the body mass of migrants estimated from individuals recaptured on subsequent days or using regression methods. We compared estimates of mass change using these two techniques to estimates of mass change determined from birds recaptured on the same day. Using spring and fall banding data collected on Appledore Island, Maine, from 1990–2007, we examined body mass changes of the five most common species. Over this period, 18,954 individuals of these five species were captured and banded, with 11.6% of birds recaptured at least 1 d after initial capture and 3.1% recaptured on the same day. Using both regression and same-day recapture methods, all five species had positive hourly mass gains during fall migration; results were mixed for the subsequent-day analysis method. Trends were less consistent during spring migration. Using all three methods of estimating mass change, Red-eyed Vireos (Vireo olivaceus) lost mass, American Redstarts (Setophaga ruticilla) and Northern Waterthrushes (Parkesia noveboracensis) gained mass, and results for Yellow-bellied Flycatchers (Empidonax flaviventris), and Black-and-white Warblers (Mniotilta varia) varied with method. We found similar trends in mass change using the same-day recapture and regression methods. However, we found lower mean mass gain for most species using the same-day recapture method, suggesting that there may be a short-term capture and handling effect. Our results provide additional support for the use of regression models to compare changes in mass of migrating songbirds at stopover sites. RESUMEN. Estimando el cambio de masa en aves canoras migratorias en una parada: comparaci ´ on de tres m´ etodos diferentes La calidad de los lugares de parada muchas veces ha sido determinada utilizando cambios en la masa corporal de migratorios, siendo determinada con recapturas en d´ ıas subsiguientes o mediante m´ etodos de regresi´ on. Comparamos estimados en cambios de masa utilizando estas dos t´ ecnicas para determinar el cambio en masa de aves recapturadas el mismo d´ ıa. Utilizando datos de anillamiento durante la primavera y el oto˜ no tomados desde el 1990–2007, en la isla Appledore, Maine, examinamos el cambio en masa corporal de las cinco especies m´ as comunes. Durante el periodo indicado, 18,954 individuos, de las cinco especies, fueron capturados y anillados, con un 11.6% de las aves recapturadas al menos un d´ ıa despu´ es de la captura inicial y de 3.1% de individuos recapturados el mismo d´ ıa. Utilizando ambas regresiones, y el m´ etodo de captura el mismo d´ ıa, las cinco especies mostraron aumentos positivos en peso por hora, durante la migraci´ on oto˜ nal; aunque los resultados fueron mixtos para el m´ etodo de an´ alisis del d´ ıa subsiguiente. Las tendencias fueron menos consistentes durante la migraci´ on primaveral. Utilizando los tres m´ etodos para estimar el cambio en masa corporal encontramos que Vireo olivaceus perdi´ o masa, Setophaga ruticilla y Parkesia noveboracensis, ganaron masa, mientras que en Empidonax flavoventris y Mniotilta varia, los resultados variaron con el m´ etodo utilizado. Encontramos tendencias similares, en cambio en masa, utilizando el m´ etodo de recaptura el mismo d´ ıa y con el m´ etodo de regresi´ on. Sin embargo, encontramos una ganancia menor de masa para la mayor´ ıa de las especies, utilizando el m´ etodo de recaptura el mismo d´ ıa, lo que sugiere que pudiera haber un efecto por la recaptura y la manipulaci´ on. Nuestros resultados proveen apoyo adicional para el uso de modelos de regresi´ on en aras de comparar cambios en masa en migratorias canoras en lugares de parada. Key words: banding, condition, Empidonax flaviventris, migration, Mniotilta varia, Parkesia noveboracensis, passerine, Setophaga ruticilla, Vireo olivaceus The regular semiannual movement of most North American birds is both energetically ex- pensive (McWilliams et al. 2004) and results in high mortality (Sillett and Holmes 2002).Duri- 1 Current address: Department of Entomology and Wildlife Ecology, University of Delaware, 531 South College Avenue, Newark, DE 19716, New Jersey, USA. 2 Corresponding author. Email: morriss@canisius. edu ng migration, migrants must overcome ad- verse weather conditions (Gauthreaux 1999), endure predation risks (Lindstr¨ om 1989, Aborn 1994), and face inter- and intraspecific com- petition (Moore and Yong 1991). Stopover sites provide migrants with an opportunity to build and restore fat stores (Winker 1995, Parrish 1997, Rodewald and Brittingham 2007, Smith et al. 2007). Using changes in the body mass of migrants, the quality of stopover sites can be evaluated, which may be useful in C  2012 The Authors. Journal of Field Ornithology C  2012 Association of Field Ornithologists 412 Journal฀of฀Field฀Ornithology