How does age and body condition affect migratory restlessness and orientation in Reed Warblers Acrocephalus scirpaceus?

Orientation cage experiments were performed with 260 Reed Warblers on the southern coast of Spain during autumn 1996, in order to test the hypothesis that birds in good body condition should exhibit a higher amount of migratory restlessness and an activity concentrated more strongly towards one direction than birds in poor body condition. Furthermore, we tested whether body condition and age has an effect on the average preferred direction. Birds with small flight muscles decreased their migratory activity towards the end of the season, whereas birds with medium or large muscles did not. Perhaps, among the birds with small muscles, there were more individuals which were going to stay over winter in Iberia. Surprisingly, birds with low fat reserves were active more often than birds with high fat reserves. Among juveniles, more birds were active than among adults. Neither age nor body condition significantly influenced orientation behaviour. We interpret the higher proportion of active individuals among juveniles and birds in poor body condition as an expression of stronger reaction to the stress situation of the experiment compared to the adults and the birds in good condition, respectively. The fact that we did not find any influence on orientation behaviour might be due to stress reactions or other unkown factors which might have outweighted potential effects of age or body condition.

Korner-Nievergelt, F., Liechti, F. & Bruderer, B, Ardeola 49(1), 2002, 29-37