In his new book, “No Way Home: The Decline of the World’s Great Animal Migrations,” David Wilcove (a biologist at Princeton) discusses the threats facing (and the importance of) migrating species. He talks about how migrating species perform certain ecological services for humans (apparently nitrogen needed to grow grapes in California is brought up by salmon) and how humans have impacted the migrations of various species (housing developments impinge on stop over habitats for birds, roads cut through migratory routes and cars crush species trying to make their way over, etc). One of the major threats he mentions is climate change.
In an interview on NPR, he discusses the example of how different creatures use different cues for their migration. A bird that migrates down to South America may use daylight as its cue. An insect (that the bird relies on for food) may use temperature change as its migration cue. If these two get off sync, the bird may find that when it returns north, there aren’t as many insects available because they have been triggered by the warmer temperatures and have already come and gone. He mentions flycatchers – they leave their wintering grounds at the usual times but when they arrive, the caterpillars they rely on have metamorphosed and the young of the flycatchers cannot survive. He also mentions the simple fact that certain habitats crucial for migrations may disappear. He advocates for not only the conservation of individual species but for the protection of the migratory way of life.