Effects of body condition and food intake on stop-over decisions in Garden Warblers and European Robins during spring migration

During migration, birds need to optimize their time and/or energy management, especially during stop-overs. Previous studies with caged birds under controlled condition support the notion that departure decisions are condition-dependent, but they did not take into account the availability or the actual intake of food. In the study reported here we investigated whether food intake interacts with condition in influencing migratory disposition in temporarily caged wild migrants. We conducted the study on Garden Warblers (Sylvia borin) and European Robins (Erithacus rubecula) at a spring stop-over site that is reached after a long non-stop flight over the Mediterranean Sea. The birds were held in cages during the day and the first night following capture, and their locomotor activity, food intake, and body mass change were recorded. In both species, food intake was positively correlated with body mass change. Body condition was positively correlated with migratory restlessness (Zugunruhe) in both species and was also negatively related to diurnal activity in Garden Warblers. Food intake was negatively correlated with diurnal activity in Garden Warblers. When only birds with low fat scores were considered, we found a positive effect of food intake on Zugunruhe in Garden Warblers. In general, European Robins consumed less food than Garden Warblers and no significant effects on Zugunruhe were found. Our results show that food intake interacts with condition in affecting migratory behavior during stop-overs. In particular, food intake may induce Garden Warblers with low fat stores at arrival to leave a stop-over site earlier. The large individual variability in food consumption supports the notion that birds vary in their capacity to refuel soon after arrival at a stop-over site. Overall, our work suggests that the decision to stay at the stop-over site or to resume migration depends on the interaction of condition, refueling capacity and rate, and migratory strategy.