Trade-off between migration and reproduction: does a high workload affect body condition and reproductive state?

Migratory birds have to invest much energy into flight to reach their summer and winter quarters. Many studies have shown how migration affects body physiology, including the accumulation of energy stores and the reduction of nonessential organs. In spring, the costs of migration may trade-off with preparations for breeding, such as the timing and extent of development of primary and secondary sexual traits. Birds arriving earlier on the breeding grounds often have a higher reproductive success than late-arriving birds, but no study to date has addressed whether and how the flight workload during migration itself influences reproduction. Using a wind tunnel, we investigated the effect of a high workload during long flights on measures of body condition and reproductive state in male rose-colored starlings (Sturnus roseus). We compared an experimental group that flew in the wind tunnel every day and covered a total flight distance of > 4700 km in 49 days with a control group of males that did not fly. All birds had ad lib access to food. After the migration period, individuals from both groups were kept in a common breeding aviary, where they directly competed for nest-boxes and females. Contrary to expectation, birds from the experimental and control group did not differ significantly in the spontaneous seasonal changes in fat score, in breast muscle thickness, in plasma testosterone levels, and in bill and mantle color. Body mass increased more slowly in experimental than in control birds, but it reached the same level soon after the migration period. We did not observe any effect of the experimentally increased heavy workload on behavior during the early breeding phase or on any parameter of reproductive success. We thus failed to find a trade-off between long flight and the development of traits in preparation for breeding or reproductive success. A possible treatment effect might have been obscured by the unrestricted food supply. However, we cannot exclude effects on other life-history stages such as future survival, migration, or reproduction. Our results attest to the strong endogenous control of seasonal physiological changes in preparation for breeding that occur independently of the extreme effort invested in long-distance migration.