Daylength is an ultimate factor of migration

This blog post is provided by Ivan Pokrovsky, Andrea Kölzsch, Sherub Sherub, Wolfgang Fiedler, Peter Glazov, Olga Kulikova, Martin Wikelski and Andrea Flack and tells the #StoryBehindThePaper for the article “Longer days enable higher diurnal activity for migratory birds“, which was recently published in Journal of Animal Ecology.

In general, animals attempt to live in conditions that fit them best. Species’ fitness results from a tradeoff between different factors like competition, predation risk, parasite pressure, and resource availability. The spatiotemporal variation of these factors determines animal distributions and migration patterns. Migratory movements have been changing during evolution and, more recently, changing faster due to the Anthropocene. To understand the evolution of migratory patterns and predict the consequences of human activities, we need to quantify the link between these different migration drivers and animals’ life-history traits. While the impact of many of these influential factors has been studied in detail, the role of day length has long been underestimated and remains unclear.

Day length changes with latitude and the day of the year. This variability affects animals in their activity patterns, especially those that travel across the entire globe, because their activity may be limited by daylight, which varies strongly in different locations and seasons. Nearly all birds migrate along a latitudinal gradient towards a pole, thereby experiencing a substantial increase in daylight hours during summer. Thus, one can assume that higher latitudes provide benefits to migratory birds by increasing their total daily activity and their foraging time.

We studied how the birds’ activity changed when they flew north compared to when they are in the south. In other words, how their activity changes over the year. To understand the background for these changes, we also measure how daily activity changes during the day with different day lengths. We did this with the help of GPS loggers and accelerometers. We are all more or less familiar with the GPS logger: a device attached to the bird that transmits information about its location. The accelerometer quantifies the movement of the animal similar to fitness apps on our smartphones. Using our devices, we got information about the bird’s movement activity every 10 minutes.

We collected this information from 63 birds during several years from four species: rough-legged buzzards, white storks, greater white-fronted geese, and Himalayan vultures. These species differ in their foraging behaviour: geese and storks are ground foragers, and vultures and buzzards search for food in flight. At the same time, these species experience different amplitudes of the annual daylight changes, as two species migrate to the Arctic (buzzards and geese). In contrast, the other two (storks and vultures) stay at middle latitudes.

Figure 1 Migration paths of 63 birds of four species tracked with GPS/GSM. Two Arctic migrants and two mid-latitude migrants. Circles represent the mean daily locations of each species throughout the year.

We discovered that all four species exhibited longer activity periods during days with more daylight hours, showing a strong positive relationship between total daily activity and day length. The total daily activity of birds increased 1.5-fold on average when migrating from wintering to breeding grounds.

Figure 2 Daily activity as a function of day length.

Underlying mechanisms of these relationships reveal two distinct patterns of daily activity. Flying foragers showed increasing activity patterns, i.e. their daytime activities rose uniformly up to solar noon and decreased until dusk, thereby exhibiting a season-specific activity slope. In contrast, ground foragers showed a constant activity pattern, whereby they immediately increased their activity to a certain level and maintained this level throughout the day.

Figure 3 Activity in relation to sun angle above the horizon and activity patterns of two different forager types, flying and walking.

Our results suggest that year-round patterns of activity of avian migrants with contrasting latitudinal ranges and foraging modes depend strongly on day length. When examining single species for short periods, the dependence on this primary factor may not be detectable, as it could be hidden by the species’ dependence on other extrinsic factors that constrain its lifetime paths, such as local weather conditions or food availability. Considering day length as one of the essential (but not unique) determinants of migration becomes highly relevant when exploring species’ life-history decisions in a rapidly changing world because, even under vastly different environmental conditions, the available hours of sunlight will remain stable. Thus, a thorough understanding of this primary, unifying concept of day length dependency will help scientists and conservationists predict future changes caused by the Anthropocene. This is of utmost importance for vulnerable ecosystems, such as the Arctic, where the role of day length is high, and rapid warming is already affecting terrestrial tundra ecosystems.

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