Ecology and analysis of die-switching by migratory birds: The use of fruits as a food resource during migration

Many species of migratory passerine birds eat fruit as a substantial part of their diet at some point during the year. Evidence increasingly suggests a large proportion of those species are primarily frugivorous during migratory periods. It has been debated whether these primarily insectivorous birds eat fruits during migration for nutritional reasons, or are merely taking advantage of an easily accessible and superabundant food source. The latter hypothesis has generally been favored, as researchers have been unable to show conclusively that birds are choosing fruits for nutritional content, or that birds favor fruits as a food resource during migratory periods. However, many recent studies suggest that there may be nutritional advantages to eating fruits during migratory periods.;In this dissertation, a comprehensive overview and critical analysis of the current state of knowledge about frugivory by migratory passerine birds is presented. Additionally, new data are presented on the timing and extent of a change in diet from insects to fruits by the Wood Thrush (Hylocichla mustelina) on its wintering grounds at La Selva Biological Station, Costa Rica. Fecal samples and nitrogen-stable isotope ratios in blood samples indicate this species switches from a diet of primarily insects to a diet of primarily fruits during spring premigratory fattening. This switch does not appear to be related to resource availability at this site. A similar change in diet did not take place in Kentucky Warblers (Oporornis formosus). Finally, a theoretical basis for birds switching from a diet of insects to a diet of fruits during migratory periods is proposed. Based on the energetics of fat buildup on different nutritional components of food items, eating lipid-rich foods is the most efficient way to build fat stores for migration. However, if lipid-rich foods are not available, birds should choose sugar-rich foods, such as fruits, over protein-rich foods, such as insects. Energy budgets for fat deposition based on the actual nutritional components of different food items eaten by birds in the wild suggest that lipid-rich fruits are the "best" diet for building up fat stores for migration. Arthropods and sugary fruits are similar in value as food for fat deposition, but eating fruits is probably advantageous since they are more easily located and obtained than most arthropod prey.