Effects of photoperiod on the migratory behavior of songbirds during spring and autumn migration

Abstract

Photoperiod, the relative duration of daylight in a 24-hour diel-cycle, is an environmental cue that influences physiological processes in birds, regulating life-history events such as reproduction and migration. Here we study whether variations in photoperiod affect migratory behaviors, including activity, orientation, and body mass, across bird species during 2023 spring and autumn migration. Our experiments involved monitoring activity, orientation and body mass in birds. During spring, we focused on species with polar and subpolar breeding ranges, while in autumn we focused on European robins. In both experiments, the experimental group experienced continuous daylight. For spring, we found that under a 24-hour photoperiod polar birds increased nocturnal activity and kept a stable body mass, suggesting reliance on alternative environmental cues to navigate towards breeding grounds compensating for a delay in their migratory route. For subpolar birds, we found no significant change in nocturnal activity, although an increase in body mass indicated ongoing reverse migration, possibly due to the long photoperiods being perceived as unsuitable for breeding. Similarly, during autumn we found a decrease in the nocturnal activity and a shift in orientation for birds after treatment. For the standardized body mass, the control birds increased their body mass, suggesting energy storage for migration, while treatment group birds maintained a stable body mass despite reduced food intake, suggesting energy use different from during migration. In summary, our study shows that continuous daylight can change the nocturnal activity level, trigger a shift in orientation, and change in body mass and food intake of birds during migration. Understanding each one of the phases of the migratory journey and the implications of changing photoperiod on each one of them is essential to understand bird responses to changing environmental conditions due to climate change.

Popular Abstract

Effects of photoperiod on the migratory behaviors of songbirds during spring and autumn migration

Maria Laura Mahecha Escobar
Supervisors: Susanne Åkesson & Nicholas Per Huffeldt

1 Migratory birds make incredible journeys across the globe to escape the winter in their home regions and find resources in different parts of the world. Birds use various cues to navigate the world, whether on departure, during the journey, or upon arrival, as each clue gives them an idea
of conditions at different sites. Birds also use a magnetic map to help them navigate, but other cues like day length give them clues as to how far south/north they are and whether they should stop to
breed or winter.

2 But, what happens when climate change? Climate change is changing the temperature, resource availability, and ecological conditions that these birds prefer, causing them to move further and further north from where they were previously found. However, climate change cannot change the amount of daylight in places, so if birds use both tracks, how might they adjust to these changes?

3 First, we need to know what these light changes do to birds while they are migrating because it is at these moments that they are using these cues. Specifically, we asked what would happen with birds moving above the polar circle that have never experienced 24 hours of continuous daylight during summer. We investigated precisely these responses during the spring and autumn migration of 2023. In spring, we exposed birds that breed above the polar circle (polar birds like willow warblers) and birds that breed below the polar circle (subpolar birds like robins) to light regimes identical to the natural ones and then changed the conditions to more extreme ones, that are only found above the polar circle and correspond to 24 hours of continuous light. During the autumn, we did a similar experiment with robins. However, we had two groups of birds; one followed the regular light schedule, and the other had 24 hours of continuous light.

4 In both spring and autumn, we measured migratory activity as the time the bird was flying/moving during the "night" period. We also checked how the bird was orienting before and after the experiments and how heavy it was during the experiment. In spring, we studied this for 7 days, while in autumn, we continued for 15 days.

5 During spring, our polar birds (including willow warblers) found that under constant light, they were more active during the "night" period, flying towards the north, and getting heavier with the experiment. This means that when we did the experiments, the birds relied on other cues to know where they were and were late in their migration so they could compensate for the time lost.

6 During spring, our subpolar birds, including robins, were as active as at the beginning of the experiment. They were also going south from the start, and their orientation did not change
under 24 hours of continuous light. They were heavier, though. We think this could indicate that the birds were going in the opposite direction from the usual migration, meaning they were expressing
reverse migration. Could this be related to the 24h light? We think that some of the responses are because of the light, but most came from the beginning of the experiments.

7 In autumn, we only studied robins. The robins that we kept under constant light for 15 days were less active at "night", and they switched their orientation from eastward to northward. This makes us think that the robins were changing their migratory behavior due to continuous light and slowing the migration. Plus, they kept the same mass while eating less food, which means they used the energy from food more efficiently but did not accumulate fat for migration.

8 Both experiments in spring and autumn, showed the complexity of the migratory behaviors and the different responses we can get from each of the behaviors we tested during the study period. For
polar birds, the influence of light at this migration stage resembles an enhancement for getting fast
to their reproduction places. In contrast, it makes no difference for subpolar birds if they are engaged in an apparent reverse migration. In autumn, the continuous light slows down the
migratory behavior of robins. These results are not definitive for what could happen to the birds
under a climate change scenario, but they give us an idea of where the birds might have more challenges adjusting to new light regimes.

We want to emphasize that there is an open field for addressing questions about how climate change can impact the migration of birds and that there is still a lot we need to study to understand the complete process. Nevertheless, we are happy that our contribution to this field open up for further studies of songbird adaptations.

• author: Mahecha Escobar, Maria Laura
• supervisor: Nicholas Per Huffeldt
• organization: Degree Projects in Biology
• course: BION02 20231
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9175957
• date added to LUP: 2024-10-03 12:05:26
• date last changed: 2024-10-03 12:05:26