# Migration Strategy and Diet Affect the Metabolism of Passerine Birds During Endurance Flight

## TL;DR

This study shows that the way birds migrate and what they eat affects how they use fat and other energy sources during long flights.

## Contribution

The study reveals how migration strategy and diet influence metabolic profiles in passerine birds during endurance flight.

## Key findings

- Long-distance night-migrant frugivores with high fat loads are best adapted for fat use during flight.
- Diet and migration timing influence metabolite levels, with frugivores showing higher fat use.
- Day-migrating insectivores and granivores rely more on protein or glycogen than night migrants.

## Abstract

Bird migration varies greatly in overall distance and length of single flight bouts. Therefore, we expect that metabolic adaptations may also differ widely among migrants. Endurance flight is mainly fuelled by fat and complemented by protein. The proportions of lipids and protein accumulated before flights, and spent during flight, depend on food type. A fruit diet facilitates fattening more than a proteinous arthropod or seed diet. Adaptations to maximize lipid use during flight vary with the length of flight bouts. We expect that the type of diet and migration strategy (length of flight bouts, overall migration distance) affects flight metabolism. On a Swiss Alpine pass, we caught 30 species of nocturnal and diurnal migrant passerines out of natural migratory flight and compared them with conspecifics kept inactive. We examined the effects of migration strategy, primary diet, and body fat stores on plasma concentrations of six metabolites of the fat, protein, and carbohydrate metabolism, used as indicators of relative fuel use. During migratory flight, immediate migration strategy (short hops during day or long bouts during night), general migration strategy (long‐ and short‐distance migration) and diet affected metabolite levels, while fat stores had an additional effect. Triglyceride and free fatty acid levels were high in night‐migrants and frugivores. Uric acid and glucose levels were high in insectivores. Glucose, ß‐hydroxy‐butyrate, and glycerol were less dependent on day/night‐migration or diet. The metabolic profiles indicate that migrant passerines vary in the degree of fat use depending on migration strategy, diet, and current fat stores. Day‐migrating short‐distance migrant insectivores and granivores used protein or glycogen to a higher degree than night migrants. Frugivores maximized fat use. Long‐distance migrants favored fat use even further. Hence, long‐distance night‐migrant frugivores with high fat loads appear best adapted for fat use during migratory flight.

We compared the metabolic profile of 30 bird species caught out of their migratory flight. The degree of fat use depended on migration strategy, diet, and current fat stores. Long‐distance night‐migrant frugivores with high fat loads appear best adapted for fat use during migratory flight, compared to day‐migrants, short‐distance migrants, granivores, and insectivores.