Decision-making in migratory birds at stopover: an interplay of energy stores and feeding conditions

Abstract

Migrating birds make stopovers to rest and fuel to prepare for their next flight. The decision when to continue migration significantly affects total duration of migration and thus arrival timing at the migratory destination. Departure decisions of migrants are therefore important to understand variation in arrival timing. Since the amount of energy can limit flight duration, feeding conditions and energy stores have a significant effect on the departure decisions. Unexpectedly though, various fasting-refuelling experiments controlling for these two parameters and using migratory restlessness as a proxy for departure probability did not find consistent patterns within and across different songbird species of departure decisions. Here we performed a fasting-refuelling experiment on four actively migrating songbird species during autumn, to assess the significance and consistency of the feeding conditions and energy stores on the bird’s departure decision. We found no differences in the departure probability between low and favourable feeding conditions in all species. During the low food phase, however, birds with higher energy stores were more likely to depart than leaner birds. When fasted individuals encountered improved feeding conditions, they significantly increased their energy stores and showed a significant drop in migratory restlessness. This is tantamount to the decision of staying at stopover. The consistency of the patterns seems to be generalizable across species. Additionally, the results highlight the importance of the interplay of feeding conditions, changes in these and the bird’s current energy stores for the stopover decision-making process. Many migratory songbirds travelling thousands of kilometres do so by making several single nocturnal flights interrupted by resting periods on the ground. To decide when to continue migration, birds seem to follow general departure rules. Fat birds continue migration when they do not find food during their rest, while lean birds stay until the feeding conditions have improved. In this study, we show for the first time a generalizable consistent pattern that feeding conditions, changes in food availability and the current energy stores jointly influence the departure decisions of migratory songbirds at stopovers. This is in contrast to former studies showing inconsistent patterns on the reaction of a low food phase regarding the departure probability. Our experiment, therefore, advances our knowledge about the decision-making process of bird migrants and demonstrates the importance of favourable feeding conditions for migratory birds resting at a stopover site.