Symposium Participants: Alex Jahn
Partial Migration
“Irrespective
of the final result…a deeper understanding of the
partial bird migration system will reveal interesting aspects
of the evolution and coexistence of different life-history
solutions within populations.”
- Per Lundberg (1988)
Differential migration, in which conspecifics exhibit different migratory patterns such as distance and timing of migration, has been well documented in a number of migratory species (e.g., Ketterson and Nolan 1976). Under the umbrella term of differential migration, therefore, exist various intraspecific modifications to the migratory theme, each possibly reflecting species-specific evolutionary adaptive strategies to seasonality of limiting resources. As such, partial migration is generally considered to be a specialized case of differential migration (Alerstam and Hedenström 1998), in which some individuals of a population migrate away from the breeding grounds, while others are resident at or near the breeding grounds.
Migration generally occurs as a response to changes in climate and biotic resources, and is characterized by parameters (e.g., timing, energetics) which are shaped by evolutionary selective pressures associated with environmental changes in space and time (Gauthreaux 1982). What this means for the persistence of partial migration is that climate or resource availabilities must be highly variable from year to year but not so extreme to eradicate one of the alternative behaviors (but see Kaitala et al. 1993).
Such a stable condition would favor migratory behavior in some years and sedentary behavior in other years, as Dhont (1983) found for the partially migrant Stonechat (Saxicola torquata) in Europe. In this species, the number of overwintering birds on the breeding grounds was positively correlated with the previous winter’s temperatures, illustrating the strong selection pressures that at least some migratory bird species are under during harsh temperate winter conditions. Overall, birds are characterized by their ability to respond rapidly to variation in resources or climate. In fact, the genetic basis for migration exhibits a high potential for rapid evolution (Berthold 1991).
Plasticity of response to seasonal environmental variables has even been found within individuals across time. Age-dependent switching, for example, from a migratory to a sedentary condition has been found in European Blackbird (Turdus merula) (Schwabl and Silverin 1990). Furthermore, plasticity in migratoriness from year to year is exhibited by such species as Starling (Sturnus vulgaris) (Kessel 1953), a mockingbird (Brackbill 1956), Song Sparrow (Nice 1937) and Silvereye (Zosterops lateralis) in Tasmania (Mees 1974). However, switching between strategies appears to occur primarily in one direction: migrants tend to become sedentary more often than the reverse (Nice 1937, Lundberg 1988).
Interestingly, results from several European studies show that migrants tend to exhibit lower reproductive success than sedentary individuals in partially migratory populations (e.g., Schwabl 1980, Adriaensen and Dhondt 1990). Such data support the idea by some investigators that those which migrate may often be individuals occupying lower tiers in an intraspecific social hierarchy and are forced to eak out a living by migrating.
Literature Cited
see link
