Elevated testosterone advances onset of migratory restlessness in a nearctic-neotropical landbird
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The primary cue for initiation of spring migratory restlessness (Zugunruhe) in landbird migrants is photoperiod. Gonadal hormones are known to have a role in the regulation of migratory disposition; however, the extent of their effect is not well understood. We examined the impact of exogenous testosterone on the onset of migratory restlessness in gray catbirds (Dumetella carolinensis). Catbirds were stratified by sex and randomly assigned to two rooms; individuals in one room were photoadvanced to initiate migratory restlessness and the other room was maintained on a 12:12 light/dark photoperiod. Each room had three groups (n = 10/group); males with testosterone implants, males with empty implants, and females. We predicted that in the photoadvanced room males with testosterone implants would initiate migratory activity earlier than empty-implanted males. We found that in the photoadvanced group, testosterone-implanted males initiated migration 2 weeks prior to empty-implanted males, and 3 weeks prior to females. In the non-photoadvanced males, the testosterone-implanted males initiated migration at the same time as the corresponding group in the photoadvanced room, while the empty-implanted males and females did not exhibit Zugunruhe. Our results illustrate that elevated testosterone can advance the onset of Zugunruhe, even in the absence of an extended photoperiod. Additionally, the onset of migratory restlessness observed in the photoadvanced, non-testosterone-implanted males and females further supports the importance of photoperiod as a cue for migratory restlessness. An interesting observation was the intersexual differences in the onset of migratory activity in gray catbirds, a species not previously known to exhibit protandry.
KeywordsTestosterone Zugunruhe Dumetella carolinensis Migration Photoperiod Migratory restlessness
This study would not have been possible without the help from numerous individuals. Field assistance was provided by E. Miller, V. Morley, B. Veeneman, and L. Wilson. Assistance with the birds in captivity was provided by A.J. Williams, E. Cohen, R. Felix, J. Freels, S. Glowinski, M. Hughes, J. Jawor, A. Johnson, M. McWhorter, E. Moore, K. Paxton, J. Rich, J. Smolinski, and N. Young. In addition, E. Ketterson, Z. Nemeth, G. Demas, and J. Atwell provided useful discussion and comments on earlier versions of this manuscript. This project was funded by a National Science Foundation Collaborative Research at Undergraduate Institutions [Garvin and Moore 0418715].
All authors declare that the present study complies with the current laws and ethical standards of animal research in the USA. The research received approval from the Institutional Animal Care and Use Committee protocols 06011201 (FRM) and 04RBMG1 (MCG). Birds were captured under the Garvin's Federal (SCCL 827915) and State (Ohio #245) Scientific Collection Permits.
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