Abstract
Seasonal geophysical cycles strongly influence the activity of life on Earth because they affect environmental conditions like temperature, precipitation and day length. An increase in daylight availability during summer is especially enhanced when animals migrate along a latitudinal gradient. Yet, the question of how day length (i.e. daylight availability) influences the activity patterns of long-distance, latitudinal migrants is still unclear. Here, we ask whether migration provides benefits to long-distance migrants by enabling them to increase their diurnal movement activities due to an increase in daylight availability. To answer this question, we tested whether four vastly different species of long-distance migratory birds-two arctic migrants and two mid-latitude migrants-can capitalise on day length changes by adjusting their daily activity. We quantified the relationship between daily activity (measured using accelerometer data) and day length, and estimated each species' daily activity patterns. In addition, we evaluated the role of day length as an ultimate driver of bird migration. 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 slope of this relationship varied between the different species, with activity increasing 1.5-fold on average when migrating from wintering to breeding grounds. Underlying mechanisms of these relationships reveal two distinct patterns of daily activity. Flying foragers showed increasing activity patterns, that is, 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. Our study reveals that long days allow birds to prolong their activity and increase their total daily activity. These findings highlight that daylight availability could be an additional ultimate cause of bird migration and act as a selective agent for the evolution of migration.
Highlights
Seasonal and daily cycles strongly influence the activity of life on Earth
Our study reveals that long days allow birds to prolong their activity and increase their total daily activity
| 2164 Jo urnal of Animal Ecology determining the species-specific daytime activity patterns, we explore whether migratory birds can benefit from longer days by increasing their total daily activity levels
Summary
Seasonal and daily cycles strongly influence the activity of life on Earth. Within these cycles, different environmental conditions like temperature, precipitation, food availability and the number of daylight hours (day length) change regularly. Previous studies have examined individual birds only over short periods, not covering their entire annual cycle in natural conditions This means it is still unclear to what extent variation in day length may influence birds' energy budgets throughout their life. | Journal of Animal Ecolo gy 2163 that birds with different foraging modes will exhibit different daily activity patterns, and these patterns will change during the year to a greater degree for Arctic migrants than for mid-latitude migrants These questions can only be answered when we study the birds' movements and activity patterns during a whole year (i.e. complete life cycle) and compare the sum of their daily activities. Our study covers a range of behavioural features (flight/ground foragers, arctic/temperate breeders, Figure 1; Table 1), providing us with a diverse set of parameters to explore our hypothesis that the activity of arctic migrants will increase stronger than that of mid-latitude migrants (Sockman & Hurlbert, 2020)
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