Do songbirds incorporate photoperiod in their migratory programme during migration? - A comparison between a long distance migrant and a short-to-medium distance migrant

Abstract

Avian migration is based on a genetically inherited, endogenous programme which encodes the spatio-temporal information necessary for naive individuals to perform their first migration. The mechanism responsible for the temporal aspect of migration includes both endogenous clocks and responses to external cues to generate timing programmes.
Of all the external cues involved, photoperiod has the greatest impact on these timing programmes and can both advance and delay the circannual cycle and, thus, migration. Although there is strong evidence that photoperiod is an important cue for transitioning to a migratory phenotype and initiating migration, it is still largely unknown whether birds use photoperiod during their migration, integrating it into their endogenous programme. This would be possible, since the photoperiod changes predictably with date and latitude and could consequently provide temporal and geographical information.

The aim of this study was to test whether two species of passerine migrants incorporated photoperiod in their expression of the endogenous programmes during autumn migration. I conducted photoperiod manipulation experiments on hatch-year individuals of the European Robin (Erithacus rubecula) and the Garden Warbler (Sylvia borin), monitoring their activity and fuelling.
The two study species expressed different responses to the experimental treatment, highlighting the adaptive differences between a short-to-medium distance migrant and a long distance one. The European Robins were sensitive to photoperiodic cues when it comes to migratory activity, meaning they did incorporate photoperiod in their endogenous migratory programme during the different phases of migration. Their fuelling, conversely, seemed to be controlled by other factors, rather than photoperiod. Garden Warblers were not particularly sensitive to photoperiod during their migration, probably because other external cues, such as the geomagnetic field, are more important for this species. A very interesting observation regarding the Garden Warbler was the exceptionally high fuelling rate that this species exhibited, certainly an adaptive trait for a trans-equatorial migrant, for which the time constraint to reach a distant migration goal is considerable.

These results highlight the fact that different parts of the migratory phenotype can be controlled by different sets of cues and that this also depends on the species’ migration strategy.
This means that, to understand how migratory timing programmes are integrated with specific environmental cues in the various migration phases, species-specific and phase-specific studies are needed. Now more than ever it is important to investigate these mechanisms, since temporal environments are rapidly shifting due to anthropogenic processes, requiring adjustments and posing a serious threat to migratory avian species.

Popular Abstract

And you, do you use photoperiod during migration?

Migratory birds, like other migratory animals, are able to perform their long migrations thanks to complex mechanisms that combine information encoded in their DNA and environmental cues. When a songbird knows that it is time to prepare for migration, it will start fuelling, which means eating a lot and accumulating fat reserves to sustain it during its flight. Once the bird is nicely round and ready to migrate, if it is being held captive by a well-intentioned, but nonetheless annoying scientist (like me), the bird will exhibit a behaviour known as “migratory restlessness”, which shows its instinct to migrate, but impossibility to do so.
One of the most important cues to trigger migration and the related processes in songbirds is photoperiod, that is the annually changing light portion of the day (for friends, “day-length”). Photoperiod changes predictably with latitude and date, thus it could be possible for birds to use it to get information about their current position during their migration too. However, while there is evidence that photoperiod is a strong trigger to start the migration process, it is still widely unknown whether songbirds incorporate it into their migratory programme once migration is under way too, that is to say, whether they keep getting information from this cue and adjusting their migration to it or not.

The aim of my study was to test this on two species of passerine migrants, the European Robin and the Garden Warbler. I performed two experiments of two weeks each, one for each species, at the time of their autumn migration. I divided the birds into two groups: the control group experienced an artificial photoperiod corresponding to the local one of Lund, southern Sweden, while the experimental one was subjected to a photoperiod recreating the one of Toulouse, southern France, at that time of the year. In fancy words, I carried on experiments of virtual displacement through photoperiod manipulation. I monitored the fuelling and migratory restlessness of the birds to see if they were affected by this treatment and, thus, if they kept drawing information from photoperiod and adjusting to it during their migration.
So, why southern France? Believe it or not, it was not to live vicariously through my study species, but because southern France is along the migratory route of both species: for the European Robin, a short-to-medium distance migrant that breeds in Europe and winters in the Mediterranean region, this latitude represents the end point of migration. For the Garden Warbler, a long distance migrant that breeds in Europe and winters in sub-Saharan Africa, southern France is a stop-over site (a place to stop and replenish the fat reserves) before the crossing of the Mediterranean Sea and the Sahara Desert, big ecological barriers without any possibility to fuel.
Since southern France is the end point of their migration, I hypothesised that the experimental group of European Robins would reduce their fuelling and migratory restlessness compared to the control one, if they were sensitive to photoperiod. Since southern France is a stop-over site for Garden Warblers on their way to sub-Saharan Africa, I expected both groups to retain high levels of migratory restlessness and fuelling, with the experimental group possibly fuelling more than the control group, in preparation for the crossing of the sea and then the desert (as I said, no food stops there).

My results indicated that European Robins are indeed sensitive to photoperiod during migration, at least regarding their migratory restlessness, which was significantly lower in the experimental group than in the control (hypothesis confirmed!). On the contrary, their fuelling was not affected and, hence, seems to be controlled by other factors, rather than photoperiod. Garden Warblers were not particularly sensitive to the treatment, meaning that other external cues, such as the geomagnetic field, are probably more important to gather positional information during their migration, which stretches across the equator and happens right around the equinox (basically, the worst conditions to use photoperiod as an indication of latitude). Garden Warblers of both groups also showed a remarkably high fuelling rate, with some individuals eating in a day the equivalent of their body mass in grams and resembling small meatballs by the end of the experiment.
These findings highlight that different parts of the migration process can be controlled by different sets of cues and that this also depends on the species’ migratory strategy.

Supervisors: Susanne Åkesson, Giuseppe Bianco


Master’s Degree Project in Animal Ecology, 60 credits
Department of Biology, Lund University, 2025

• author: D'autilia, Clara
• supervisor: Susanne Åkesson; Giuseppe Bianco
• organization: Degree Projects in Biology
• course: BION03 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9195486
• date added to LUP: 2025-09-11 14:30:08
• date last changed: 2025-09-11 14:30:08