Differences in weather conditions associated with the onset of pre-breeding migration of Scolopax rusticola (Eurasian Woodcock) and Gallinago gallinago (Common Snipe)

Understanding the mechanisms that influence the onset of migration is of crucial importance given ongoing climate change. The relative contribution of endogenous and external conditions on the decision to start migration is still not well understood because there is contrasting results in the literature while it is difficult to tell if this is due to species differences or methods (e.g., the timescale considered). Here, we evaluate the effect of individual characteristics, photoperiod and weather conditions on the migration onset of 2 species, using data from 106 Scolopax rusticola (Eurasian Woodcock) and 87 Gallinago gallinago (Common Snipe) fitted with transmitters. We considered 2 different timescales: (1) a long timescale being the migration period known for these species and (2) a short timescale being the last 6-days preceding departure. As expected, we show that the photoperiod is the factor most closely related to migration onset at the long timescale for both species, and it is clearly more influential than weather conditions. We also found that some weather conditions can fine-tune woodcock decision during the last 6-days preceding departure, especially wind and humidity. However, no weather conditions correlate with migration onset of G. gallinago. We discuss hypotheses that could explain this intriguing difference between these 2 species of Scolopacidae and suggest that the molt schedule of G. gallinago may result in a trade-off between choosing the best environmental conditions to molt and the best conditions to migrate. Such overlap of competing demands could be increasingly frequent under climate change and would be particularly challenging for migratory species. This study outlines the importance of considering the other life-history events requiring high energetic demands when investigating the effect of weather conditions on migration onset.