J. Field Ornithol. 82(2):193–201, 2011 DOI: 10.1111/j.1557-9263.2011.00322.x Radio-tags have no behavioral or physiological effects on a migratory songbird during breeding and molt Elizabeth A. Gow 1 , Tyler W. Done, and Bridget J. M. Stutchbury York University, Department of Biology, 4700 Keele Street, Toronto, Ontario M3J 1P3,Canada Received 1 February 2010; accepted 6 December 2010 ABSTRACT. Radio-tags are commonly used in studies of avian behavior and ecology, even though many studies have revealed negative impacts of radio-tags. Other studies of songbirds have revealed no negative effects of radio-tags on parental care, nesting success, or annual return rates. However, such studies have not addressed the potential physiological costs of carrying radio-tags or determined if such costs affect birds during the energetically costly molt period. Simultaneously investigating the behavioral, physiological, and fitness consequences of radio- tags is important for evaluating possible costs. We studied Wood Thrushes (Hylocichla mustelina) in northwestern Pennsylvania and used a repeated-measures design to determine if radio-tags affected parental care behavior and nesting success, and measured levels of plasma lipid metabolites (-hydroxybutyrate, free glycerol, and triglycerides) to evaluate physiological condition. We also examined the possible physiological effects of radio-tags during the energetically demanding period of molt in late summer. Radio-tags did not affect male or female feeding rates, nesting success, number of nesting attempts, number of eggs laid, fledging success, or hatching success, and we found no significant effects of radio-tag use on levels of plasma lipid metabolites of nesting females. During molt, Wood Thrushes with radio-tags had lower levels of -hydroxybutyrate and higher levels of trigylcerides, suggesting they were in better, not worse, energetic condition. Our results suggest that radio-tags do not have negative effects on the behavior or physiology of Wood Thrushes. Similar studies on a wider range of songbirds are needed before concluding radio-tags have little or no impact during the molting period. RESUMEN. Los radio de marcaje no tiene efectos comportamentales o fisiol ´ ogicos sobre las aves paseriformes durante la reproducci ´ on o muda Los radios de marcaje son com´ unmente usados en estudios comportamentales y ecol´ ogicos de aves, a pesar que muchos estudios han revelado efectos negativos por parte de estos radios. Otros estudios en aves paseriformes no han revelado efectos negativos de los radios de marcaje sobre el cuidado parental, ´ exito de anidaci´ on, o las tasas anuales de retorno. Sin embargo, estos estudios no han tenido en cuenta los costos fisiol´ ogicos de cargar los radios de marcaje o determinar si este costo afectan a las aves durante el costoso energ´ eticamente periodo de la muda. Simult´ aneamente investigar las consecuencias comportamentales, fisiol´ ogicas y aptitud reproductiva de los radios de marcaje es importante para evaluar los posibles costos. Estudiamos Hylocichla mustelina al noroeste de Pennsylvania y usamos un dise˜ no de medidas repetidas para determinar si los radios de marcaje afectaron el cuidado parental y el ´ exito de anidaci´ on y medimos los niveles de metabolitos lipidicos en el plasma (-hydroxybutyrate, glicerol libre y triglic´ eridos), para evaluar la condici´ on fisiol´ ogica. Tambi´ en examinamos los posibles efectos fisiol´ ogicos de los radios de marcaje durante los periodos de demanda energ´ etica durante la muda al final del verano. Los radios de marcaje no afectaron las tasas de alimentaci´ on del macho o la hembra, el ´ exito de la nidada, el n´ umero de intentos de anidaci´ on, n´ umero de huevos puestos, ´ exito de polluelos o eclosi´ on de los huevos y no encontramos efectos significativos en el uso de los radios de marcaje sobre el nivel de de metabilitos lipidicos en el plasma de hembras anidando. Durante la muda Hylocichla mustelina con radios de marcaje tuvieron niveles bajos de -hydroxybutyrate y niveles altos de triglic´ eridos, sugiriendo que estaban en mejor, no peor, condici´ on energ´ etica. Nuestros resultados sugieren que los radios de marcaje no tienen efectos negativos sobre el comportamiento o fisiolog´ ıa de Hylocichla mustelina. Estudios similares sobre un amplio rango de paseriformes son necesarios antes de concluir que los radios de marcaje tienen poco o ning´ un impacto durante el periodo de muda. Key words: behavior, lipid plasma metabolites, molt, radio-tag effect, Wood Thrush Radio-telemetry is often used in studies of avian mate choice (Stutchbury et al. 2005, Chiver et al. 2008), habitat selection (Vega 1 Corresponding author. Current address: University of Saskatchewan, 112 Science Place, Saskatoon, SK S7N 5E2, Canada. Email: eliz.gow@usask.ca Rivera et al. 1999), survival (Gruebler and Naef-Daenzer 2008), and dispersal (Anderson et al. 1996). A key assumption when using radio- telemetry is that birds with radio-tags do not differ behaviorally or physiologically from non- tagged individuals (e.g., controls). Assessing behavioral changes of birds with radio-tags is challenging because the behaviors of interest C  2011 The Authors. Journal of Field Ornithology C  2011 Association of Field Ornithologists 193 Journal฀of฀Field฀Ornithology