A partial migrant relies upon a range-wide cue set but uses population-specific weighting for migratory timing

Nils Linek,M Teague O’Mara,Paweł Brzęk,J Ryan Shipley,Martin Wikelski,Juha Tiainen,Phillip Gienapp,Ivan Pokrovsky,Jesko Partecke,Jan R E Taylor,Tamara Volkmer,Andreas Schmidt

doi:10.1186/s40462-021-00298-y

Abstract

BackgroundMany birds species range over vast geographic regions and migrate seasonally between their breeding and overwintering sites. Deciding when to depart for migration is one of the most consequential life-history decisions an individual may make. However, it is still not fully understood which environmental cues are used to time the onset of migration and to what extent their relative importance differs across a range of migratory strategies. We focus on departure decisions of a songbird, the Eurasian blackbird Turdus merula, in which selected Russian and Polish populations are full migrants which travel relatively long-distances, whereas Finnish and German populations exhibit partial migration with shorter migration distances.MethodsWe used telemetry data from the four populations (610 individuals) to determine which environmental cues individuals from each population use to initiate their autumn migration.ResultsWhen departing, individuals in all populations selected nights with high atmospheric pressure and minimal cloud cover. Fully migratory populations departed earlier in autumn, at longer day length, at higher ambient temperatures, and during nights with higher relative atmospheric pressure and more supportive winds than partial migrants; however, they did not depart in higher synchrony. Thus, while all studied populations used the same environmental cues, they used population-specific and locally tuned thresholds to determine the day of departure.ConclusionsOur data support the idea that migratory timing is controlled by general, species-wide mechanisms, but fine-tuned thresholds in response to local conditions.

Highlights

Many birds species range over vast geographic regions and migrate seasonally between their breeding and overwintering sites
As a consequence of differences in departure timing of migrants originating from fully and partially migratory populations, we examined the hypothesis of whether migrating individuals of different populations use a similar, species-wide set of cues with either identical or population-specific threshold values
After testing for occurring differences in departure conditions between populations, we investigated the potential influence of these environmental variables on individual departure decisions

Summary

Introduction

Many birds species range over vast geographic regions and migrate seasonally between their breeding and overwintering sites. Deciding when to depart for migration is one of the most consequential life-history decisions an individual may make. It is still not fully understood which environmental cues are used to time the onset of migration and to what extent their relative importance differs across a range of migratory strategies. Facultative migrants such as Pine Siskins Spinus pinus supposedly base migratory decisions mainly on environmental conditions that, e.g. predict food abundance [16]. This irruptive migration (Fig. 1b) can flexibly respond to changing local conditions. It is thought to be less regulated by genetic control mechanisms that would be potentially more rigid in their expression

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Conclusion