Size-dependent costs of migration: migrant bird species are subordinate to residents, but only at small body sizes

Migrant species are commonly thought to be poor competitors in aggressive interactions with resident species. However, no studies have tested if this relationship is widespread. Here we compare the behavioural dominance of closely-related species of migratory and non-migratory birds, testing if migrants are consistently subordinate to resident species in aggressive contests. We compiled published behavioural dominance data involving migrant and resident congeners, gathering additional data on the body mass and migratory distance of each species. Focal species included a diverse array of birds (28 taxonomic families, 12 orders) from around the world. We found that migrant species are usually subordinate to resident species, but that this relationship disappears at larger body sizes. For smaller birds (< 500g), resident species were behaviourally dominant in 83-88% of comparisons; for larger birds (> 500g), resident species were dominant in only 25-30% of comparisons. The relative d...