Seasonal, Diel, and Spatial Dispersion Patterns of Golden Eagle Autumn Migration in Southwestern Montana

Abstract

In this report we describe three behavioral phenomena of Golden Eagle (Aquila chrysaetos) fall migration: agespecific seasonal differential migration, age-specific diel patterns of migration, and the spatial dispersion pattern of migrating Golden Eagles. Immature individuals of many North American raptor species migrate earlier in the season than adults (see Appendix 2 in Kerlinger 1989). As resource levels decline in the fall, less efficient foragers (Hoffman 1985) or subordinate birds (Gauthreaux 1978) are likely to be the first to seek resources elsewhere. Hoffman and Potts (1985) reported that immature Golden Eagles migrate earlier in the season than adults although Kerlinger (1989) reported the reverse. We examined our data to address whether there was seasonal differential migration between age classes of Golden Eagles migrating along the Bridger Range. Because different travel requirements or behavioral differences between age classes may result in age-specific migration patterns within periods of the day, we also investigated whether immature and adult eagles had different diel patterns of migration. A migrating raptor searching for lift may locate an updraft by flying towards other soaring birds. Such visual interaction among migrants would result in aggregation. This is similar to Kerlinger's (1985) aggregation model for flock formation. While his model applies to species that typically migrate in flocks, other species that do not form discrete flocks may still exhibit nonrandom dispersion due to visual interaction. Alternatively, Golden Eagles may avoid other eagles during migration. Pairs of Golden Eagles space out the nests that they use in a given year more regularly than random (Watson and Rothery 1986) and their territorial behavior may carry over to the migration season. In that case we would observe even dispersion. For aggregation or repulsion due to visual interaction to occur, concentrations of migrants would have to be sufficiently dense that individuals could see each other. Furthermore, aggregations of aerodynamically dissimilar birds would be ephemeral because of different climb and glide characteristics so it is appropriate to focus on concentrations of aerodynamically similar birds, e.g., birds of the same species, when testing for nonrandom dispersion. Golden Eagles passed our lookouts in sufficient densities that if they respond to the presence of other soaring Golden Eagles we should have observed nonrandom dispersion.