On the Move: New Discoveries About Bird Movements

For as long as I’ve been birding, it has been common wisdom that there are diurnal (daytime) and nocturnal migrants. Examples of diurnal migrants include hummingbirds and swallows — it’s hard to imagine birds in these families journeying during the night. The nocturnal migrants include the songbirds, such as warblers, thrushes, tanagers, and vireos. I’d assumed there were hard and fast rules about which birds moved when, but recent observations by birders are turning these notions on their heads.

Richard Crossley, a British birder and the author of innovative bird field guides for both the United States and Great Britain, has devoted the last few years to studying visible migration, particularly spring migration. He moved to the Tejon Pass area of Los Angeles County for the sole purpose of documenting hitherto unknown migration patterns, where he discovered that songbirds are indeed making the expected nocturnal movements across the desert at night. But at dawn, and for the few hours afterward, these birds drop down and begin to feed, particularly in the chaparral. Crossley’s big discovery is that birds don’t necessarily stop and feed during the morning in order to refuel — most keep moving, concurrently migrating and feeding. Often when they run out of vegetation in which to forage, they take off again, some gaining great height to continue their migration during daylight hours. His thinking is that birds will take advantage of favorable conditions to get to their breeding grounds in the shortest time possible. 

Closer to home, birders have noticed a similar phenomenon, but here the visibility of diurnal migration is even more dependent upon favorable weather conditions, stiff northern winds being the main criteria. Because our mountains trend in an east–west direction, if winds are blowing from the north, birds following the coast at night will drop lower in altitude, presumably where the winds are weaker. At dawn, as they reach the Santa Ynez Range, they drop down and begin to feed. Observations here mirror Crossley’s theory that these low-flying birds will keep moving, flying up sheltered canyons, stopping to feed in trees as they climb, and then skipping over the mountain passes.