Fruit consumption in migratory passerines is limited by water ingestion rather than by body water balance

Many insectivorous passerines become frugivorous during migration. Because water may facilitate the digestion of dry fruits, some passerines may benefit from staging in stopover sites that offer access to drinking water. In autumn, water consumption by blackcaps Sylvia atricapilla staging in Israel was found to induce a shift from insectivory to frugivory. We tested two alternative hypotheses concerning the mechanism facilitating consumption of the relatively dry fruits which are common in this region: 1) water intake facilitates the passage of fruits within the digestive tract when these two resources are simultaneously ingested, and 2) improved body water balance allows the consumption and ingestion of large amounts of dry fruits. Blackcaps were subjected to five treatments that included temporal separation of water and fruit consumption, as well as subcutaneous water injection to maintain balanced body water in water‐deprived birds. Fruit consumption rate was measured daily. We found that only simultaneous provisioning of water and fruits significantly increased fruit consumption rate, implying that drinking water directly improves fruit digestion within the digestive system. Furthermore, the fuel deposition rate increased with increased fruit consumption rate. These results emphasize the importance of water availability for the ecology and conservation of migrating passerines.