Throughout migration, birds make important decisions including where to go, how long to stay, and when to leave, which in turn contributes to the fitness of the individual. The decision of when to migrate requires forfeiting existing conditions, enduring conditions aloft, and assuming risk regarding an unseen destination. Previous studies have failed to identify strong effects of weather on migration, likely because the methods used to quantify timing of departure were inadequate. We monitored specific dabbling duck (Anatidae) departures from a 14,431-ha wetland complex in central Illinois from 1 Oct to 31 Dec 1996-1998, 2003, and 2006-2009 using weather surveillance radar to identify and enumerate emigrating ducks. We used standard weather variables known to influence avian migration to develop candidate models explaining the timing of dabbling duck departures (0 [no departure] or 1 [departure]). We then modelled relationships between the timing of departures and weather and parameterized models to understand the magnitude of the effects of weather variables on individual emigration events. Over ourstudy period, ducks departed on an average of 30% of nights each year, with a total of 216 departures out of all 723 nights. The best approximating model of migration timing captured all the model weight (wi= 1.0; R2= 0.42) and included variables accounting for favorable direction of winds aloft, precipitation, and cloud cover. Based on parameter estimates for the best model, ducks were more likely to depart with following (favorable) winds, no precipitation, and less cloud cover. Our results suggest conditions associated with favorable flight conditions may in some contexts be equally or more important than conditions associated with the dynamic environment immediately occurring at a stopover. However, 58% of the variation in timing of departure was unaccounted for by our best model; thus, many aspects of departure decisions by migrating ducks remain unknown. For example, direction of winds aloft was a key factor in departure decisions in our models, but how ducks perceive or sample wind conditions aloft prior to departure is not understood. Experimental research and individual-based models may elucidate the ultimate factors driving departure decisions in migrating avifauna.
|Original language||English (US)|
|State||Published - 2017|