# Effects of daylength manipulation on migratory activity and fuelling in a long-distance nocturnal songbird migrant

## TL;DR

This study shows how changing daylength affects the migration behavior and fat storage in young birds.

## Contribution

The study experimentally tests how artificial daylength changes influence migratory restlessness and fat accumulation in juvenile birds.

## Key findings

- Treatment birds delayed their migratory restlessness onset compared to controls.
- Treatment birds initially increased but later reduced fuelling, resulting in lower overall fat accumulation.
- Photic cues significantly regulate migratory activity and fuelling in juvenile birds.

## Abstract

Migratory birds have evolved a multitude of physiological and behavioural adaptations to reach their population-specific wintering areas during their first migration. The endogenous program encodes distance, direction and fuelling, and involves species-specific adaptations leading naïve migratory birds along highly diverse routes. While daylength has been extensively studied in relation to the onset of migration, its potential role in the transition out of the migratory phenotype remains largely untested. Here we study, by experimentally increasing the daylength in autumn simulating a temporal shift earlier in the season or a latitudinal displacement toward the wintering area as experienced later in the season after the autumn equinox, what effect a substantial photic treatment has on overall level and diel pattern of activity and fuelling in juvenile Eurasian reed warblers (Acrocephalus scirpaceus) migrating to tropical Africa. The treatment group experienced a 2h increase in daylength in the evening, while the control group was held in the local photoperiod in southern Sweden. The controls showed strictly nocturnal migratory restlessness starting immediately after sunset, while the treatment birds responded by delaying the onset of nocturnal migratory restlessness following the artificially delayed sunset, without changing the level of activity. Treatment birds increased fuelling initially, but then reduced it after one week in captivity, resulting in a lower fuelling rate as compared to the controls by the end of experiment. The reduced fuelling suggests treatment birds possibly interpreted the diel period as arrival to the wintering area. The results confirm the importance of photic information in regulating phenotypic expressions of migratory activity and fuelling in juvenile birds.

The online version contains supplementary material available at 10.1007/s00359-025-01772-3.