My head hurts. I’ve spent a few weeks trying to understand what we know, what we don’t know, and what we need to know about one of the most significant challenges facing birds: how climate change is altering the timing of the seasons and, in turn, affecting their migration.

Even for someone like me, an ornithologist who has tracked owls, hawks, songbirds, and other migrants across the globe for decades, the science is confusing and concerning. The awe I feel when I follow a one-ounce Swainson’s Thrush tagged in Alaska all the way to its wintering site in Argentina is tempered by the knowledge that systems birds depend on—predictable weather patterns and favorable winds, food at the right time and in the right quantities, and cues provided by temperature, all varying across thousands of miles of travel—will inevitably change as the world warms.

After talking with experts, all I can say is: It’s complicated. Whether you’re an optimist (who believes migrants will adapt) or a pessimist (who thinks they won’t), there’s evidence to confirm your inclination.

One thing is clear: All around the Northern Hemisphere, almost wherever scientists have enough data to track it, the timing of the seasons is changing. Autumn, when climate signals are naturally more complex, is