BAUCHINGER, U; VAN’T HOF, T; BIEBACH, H; University of Munich (LMU), Germany; Wright State University, USA; Max-Planck-Institute for Ornithology: Testicular development during long-distance spring migration

Long-distance migratory birds are challenged to reproduce immediately after arrival in the breeding area, following the demanding process of migration. We studied gonadal development during spring migration. Garden warblers Sylvia borin were sampled before migration in Tanzania, during stop over in Ethiopia (prior to the flight across the Sahara), and Egypt (immediately after the flight across the Sahara). In addition, we sampled birds after a simulated stopover in Egypt, and birds in breeding condition in the lab. We found that testicular mass increases during spring migration. The first significant increase occurred after the desert crossing and a further significant increase during the recovery period in Egypt. Testicular mass increase was strongly associated with growth of seminiferous tubules and the most advanced stage of spermatogenesis. Even though steroid concentrations generally tend to be low during migration, the plasma concentration of testosterone is significantly positively correlated with the mass of the testes. LH concentration was constant during migration and showed no significant relationship to testicular mass. This is in contrast with numerous laboratory studies indicating increasing LH concentrations when testicular size increases. Experiments that neglect the energetic burden imposed on birds by long-distance migration might mislead with respect to LH levels regulating gonadal maturation. In summary, the present data suggest that time requirements of spermatogenesis may initiate the testicular maturation during spring migration in order to ensure the birds� full spermatogenetic capacity soon after arrival at the breeding area.