Weather significantly influences the migratory behaviour of night-migratory songbirds tested indoors in orientation cages

Each year, millions of birds migrate from their breeding grounds to their overwintering sites and back. The success, duration and effort of such a long journey depend strongly on the meteorological parameters the birds encounter en route. The behaviour of free-flying migratory birds and those studied outdoors in orientation cages has been shown to be influenced by local weather factors. However, only a few, mostly descriptive, studies have analysed whether meteorological parameters should be considered when testing captive migratory birds indoors in orientation cages. Here, we used a statistical approach to analyse how meteorological parameters affect orientation performance in a night-migrating songbird, the garden warbler (Sylvia borin), when tested indoors in Emlen funnels. Our results show that the activity and directionality of the tested birds is influenced significantly by local atmospheric pressure. Together with previous studies, these results suggest that weather conditions have an impact on orientation behaviour even during indoor experiments. The findings emphasize the importance of mixing up and/or randomizing different conditions on any given night. Ideally, during any given test night, the same number of birds should be tested in each condition involved in a given study. Thereby, the documented weather effects will influence the results of all experimental groups equally.