The existence of partially migratory populations explained by a genetic threshold model

Female blackcap caught and ringed during autumn migration. Photo credit: H.H. de Rooij.

By Marleen M.P. Cobben and Arie J. van Noordwijk

Migration is a very wide-spread behavioural strategy to cope with seasonal changes in environmental conditions. However, many migratory species also feature populations that consist of residential individuals. Partial migration is the phenomenon that a single population harbours both migratory and residential animals. This phenomenon is thought to play an important role in species’ flexibility in migratory behaviour. Theoreticians have shown that the evolutionary stability of such a system can be explained by density dependence and group differences in survival or reproduction.

The heritability of migratory behaviour has been investigated in blackcaps, indicating that a genetic threshold value determines the individual decision to migrate or not. In this study we examine whether such a genetic threshold value can explain the existence and stability in time and space of partially migratory populations in passerines.

We use an individual-based simulation model to investigate a wide range of demographic conditions and monitor individual migratory decisions. The genetic threshold value combined with the local winter survival probability determines migratory behaviour, which makes this a plastic trait in our model.

The results show a decreasing tendency to migrate across the landscape, from fully migratory populations to fully residential populations, with a fairly wide zone of partially migratory populations. This zone occurs under a wide range of parameter settings. Its width across the landscape is determined by the strength of density dependence and the featured dispersal distance, while its location in the landscape depends of the applied strength of density dependence only. Investigating the selection landscape we see that selection on the threshold value only occurs on either side of the zone of partially migratory populations.

With this study we show that the existence of a genetic threshold value determining migratory behaviour, as found for blackcaps, implies a spatially and temporally stable zone of partially migratory populations, even when there are no density dependence or structural group differences applied. The observed selection landscape further shows that genetic diversity for this trait can be maintained across the landscape, implying that migration strategies can be very flexible, also outside the partial migration zone. 

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