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April showers bring May flowers . . . unless they’re already in bloom, that is. This proverb might soon need an update because the onset of spring has shifted in North America, as the leaf-growing start dates of trees and plants have changed by as much as a day each year over the past decade. In the West, spring is arriving later; in the East, it's arriving sooner.

That shift is bad news for migratory birds, many of which follow a strict schedule to get to their breeding grounds in spring. Once they land, they expect to feast on a bounty of insects, which are themselves gorging on the fresh foliage. If the birds miss the peak plant emergence, chances are the best food has already been snatched up—or, if they arrive early, they'll struggle while they wait for it to become available. This isn't just problematic for adults: The birth and survival of their chicks depends on nature’s seasonal buffet, too.

A new study published in confirms the growing disconnect between birds’ internal clocks and the changing seasons. Researchers from across the country studied 48 migratory songbirds, and found that nine (Great Crested Flycatchers, Eastern Wood-Pewees, and Northern Parulas, to name a few) are struggling to keep pace with the onset of blooms. Across all the species they looked at, the gap between avian arrivals and the growth of spring leaves in prime breeding locations has increased by an average of half a day each year.

Scientists have tracked spring bird arrivals for decades, but this research offers a broader perspective across species. “What we were trying to do was for the first time scale this up to get a bigger picture,” says Stephen Mayor, lead author of the study and an ecologist at the University of Florida. “We haven’t been able to do that kind of thing in the past because we just haven’t had good data.”

The new analysis paired more than a decade’s worth of data from the citizen-science website with information from a NASA satellite that tracks the yearly arrival of spring greenery. “A single scientist can’t study the globe, can’t study a continent, so tackling these questions requires a new approach,” says Morgan Tingley, an ornithologist at the University of Connecticut and co-author of the paper. Layering the two data sources showed that certain birds are rescheduling their migratory journeys as spring green-up starts on alternate dates. The question, however, is if they’re adjusting quickly enough, Tingley says. Mayor echoes that concern. “One week per decade can really add up pretty quickly and leave birds out of sync with their environments,” he says.

Tingley is particularly worried about the Yellow-billed Cuckoo, which is already in danger of . He also points out that three of the most popular spring migrants—Rose-breasted Grosbeaks, Scarlet Tanagers, and Indigo Buntings—are facing jarring changes to their calendars.

To further complicate matters, the seasonal shifts are divergent on opposite sides of the continent. “As soon as we put these things up on a map, we recognized that something very different was happening in eastern North America than in western North America,” Mayor says. In parts of the West, spring is arriving later, and out-of-sync birds are arriving before it’s in full swing. Where Eastern birds might miss the big feasts, Western birds may have to tough it out before they have the chance to rebuild their fat deposits after a long migration. “There’s lots of regional variation, but it’s a pretty stark difference,” he says.

Yet timing is only half of the climate change puzzle. Some of the northern breeding habitats that birds are flying to are in terms of temperature or yearly rainfall—a double whammy for struggling populations. “They’re going to have to figure out both where and when they’ve got to arrive,” says Brooke Bateman, the director of program, who wasn’t involved in the Scientific Reports study. “That’s kind of a lot to deal with at one time.”

Because long-distance migrants have to plan their journeys from afar, they may rely solely on environmental cues. “It’s not like these birds have an app on their phone that can tell them the weather in New York,” Tingley says. “We’re changing weather patterns and changing what’s going on without giving birds an ability to respond.”

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