Biotic and abiotic factors drive circadian migration patterns in Northern pintails across the Eurasian flyway

Migratory birds crossing large ecological barriers can exhibit significant plasticity in their circadian movement patterns, e.g., shifting from nocturnal to continuous flight over seas. But substantial plasticity in the circadian patterns of active migration has also been reported in other birds, such as most intra-Palearctic dabbling ducks, that do not face major ecological barriers en route. Yet, the factors driving such behavioural variation in these species remain poorly understood.

We investigated how biotic and abiotic factors influenced circadian movement patterns and seasonal migratory schedules during pre-breeding migration of GPS-tracked Northern pintails (Anas acuta) tagged in late winter in northern Adriatic wetlands (Italy) and migrating across Eurasia towards their breeding areas. We classified behaviours from GPS data and estimated the probability of being in active migration as a function of biome, altitude of the sun above the horizon, average ground temperature, and orthodromic distance from the breeding site.

While showing a consistent preference for conditions of low (and decreasing) sun altitude throughout migration (n = 22 pintails, n = 25 complete migration events), we observed a gradual shift towards diurnal flight along the movement path, with the level of diurnality varying according to the specific biome encountered and, likely, to the potential exposure to predators in forested areas, across latitudes. Stopovers were longer in the first half of migration and shorter and more frequent as pintails neared the breeding grounds (located mainly in the Russian Arctic). Although active migration segments were covered at very high speeds, the overall migration pace was consistently slow, suggesting a tendency to follow seasonal cues en route.

Our study provided evidence that circadian migration patterns and seasonal migratory schedules in pintails were continuously adjusted in response to the environmental conditions encountered during migration. This variation was not solely due to the steep reduction in dark hours as individuals migrated northeast, nor to the increasing urgency to reach the breeding grounds. Rather, it likely resulted from an adaptation to ecological contexts favouring daytime migration, counteracted by the necessity of waiting for environmental conditions to become suitable for breeding.