Optimal departure decisions of songbirds from an experimental stopover site and the significance of weather

Recent models have worked with the assumption that birds try to minimize either time, energy or predation risk during migration, or some combination of these. The few empirical studies available have suggested that time minimization may be the most common strategy. One way of distinguishing between strategies is to study the departure decisions of migrating birds. We supplied migrating European robins, Erithacus rubecula, with food ad libitum in the field and monitored their changes in body mass prior to departure. The overall mass gain rate (k(tot), the ratio of daily mass increase to lean mass) of 10 birds using the feeding station was on average 0.05 (range 0.03-0.09). Departure fuel loads (f(dep), the ratio of fuel mass to lean mass) were on average 0.53 (range 0.35-0.66). There was no significant correlation between f(dep) and k(tot), which indicates a strategy of minimizing the energy cost of transport, rather than minimizing time but other aspects of the fuel deposition pattern suggest that time minimization may also be important. Whereas the behaviour of the robins was difficult to interpret in the light of optimal migration strategies, the importance of weather was striking. The robins selected the best weather conditions (tail wind, high air pressure and no precipitation) within the likely period of departure. (C) 2001 The Association for the Study of Animal Behaviour.