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Shorter Days Altered Migratory Restlessness in Juvenile Robins but Not Garden Warblers

Juvenile European robin showing mottled first-year plumage
Research · Behaviour

Juvenile European robins (Erithacus rubecula) adjusted their autumn migratory restlessness when exposed to a shorter photoperiod simulating southern France, while juvenile garden warblers (Sylvia borin) showed little comparable response. The study, published on 15 July 2026, tested whether young songbirds continue to use day length as a cue after autumn migration has already begun.

Researchers in Lund, Sweden, temporarily kept hatch-year birds under two light regimes. Twelve robins and 11 garden warblers experienced the natural local photoperiod, while another 12 robins and 11 garden warblers experienced the shorter day length corresponding to the same date around Toulouse. The team monitored nocturnal migratory activity, body condition and food intake.

Robins adjusted their migratory restlessness

In the robins, the shorter photoperiod affected migratory activity but not fuelling. Night activity increased slightly over time in both groups, but the increase was weaker in birds under the southern-France light regime. The duration of their nocturnal migratory activity also declined more strongly than in the control group. Food intake did not differ between treatments, and treatment effects on body condition were too small to be biologically meaningful.

Garden warblers fuelled rapidly without the same response

Garden warblers responded differently. Their nocturnal migratory restlessness increased over time in both groups, with no clear effect of the photoperiod treatment on either intensity or duration. They nevertheless accumulated fuel very rapidly, gaining about 5 g during the first week — a 26.2% increase in body mass. The authors interpret this exceptional fuelling rate as consistent with the time pressure faced by a long-distance migrant preparing to continue towards and across the Sahara.

The authors conclude that different components of the migratory programme can respond to different environmental cues, and that the response can differ between species. They suggest that garden warblers may rely more strongly on other information, including geomagnetic cues, during this phase of autumn migration. They also caution that the two species were probably at different functional stages of migration during the experiment, so the contrast may reflect both species differences and migratory stage.

Sources and image credit

  1. Differential responses to photoperiod in juveniles of two migratory songbird speciesPrimary open-access peer-reviewed study in Journal of Experimental Biology, published 15 July 2026. Used for the experimental design, species-specific responses in migratory restlessness and fuelling, and the authors’ interpretation and stated limitation.
  2. Differential responses to photoperiod in juveniles of two migratory songbird speciesLund University publication record confirming authorship, DOI and publication date.

Image: Juvenile European robin showing mottled first-year plumage. Photo: Charles J. Sharp / Wikimedia Commons. Representative image for this story; not presented as an animal or scene from the reported study or event. · CC BY-SA 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

Behaviour

SPECIES IN THIS STORY

Species in this story

European Robin Erithacus rubecula Explore species Garden Warbler Sylvia borin Explore species

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