PATTERNS OF MIGRATION
Band recoveries, netting records, and personal observations help us to critically examine migration routes and probe deeper into the origin and evolution of these pathways. We are beginning to realize certain deviations occur from the "normal" north and south movements expected in most species. In the previous section on routes, we touched briefly on the fact that some routes are not poleward at all, but in some other direction. We know that many migrants do not stop at the exact localities year after year but they probably do follow the same general course each season. After many years of observations a pattern emerges for that population, species, or group of species. In this section we would like to take a closer look at some of the interesting patterns (or "eccentric routes" as Cook (1915a) referred to them) in migration that birds are annually to travel from breeding to wintering grounds and back again. In many cases, the causative agents are unknown or pure conjecture, but in others, sound biological principles can be put forth that may indicate why a particular species could have evolved the specific pattern it exhibits.