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Though ornithologists talk about spring migration as a single event, not all migratory birds arrive on their breeding grounds at the same time. Different species travel during different weeks and months. And within species there are sex differences: Males typically trickle in before females to claim the best breeding territories in advance.

Spring now arrives earlier in the year than it did a few decades ago due to climate change, forcing birds to adjust their migration timing to keep up. And according to new research, males and females are not responding to earlier springs in the same way—the gap between male and female arrival times has widened. It's another climate-change twist that could potentially throw birds' nesting activities out of sync with the arrival of spring's green-up and insect hatch.

The study doesn’t offer clear answers as to how this sex difference could affect bird species' reproduction or population size. Instead, it shows the complexity of ecological effects of climate change, and the importance of studying female bird behavior. 

“We’ve been concerned about how Neotropical migrants as a whole are reacting to the rapidly advancing seasons. But then to think that there’s this further complication that the males are perhaps keeping up a little bit better than the females are—it just throws another layer of complexity and concern into the whole thing,” says ornithologist and author Scott Weidensaul, who wrote about how birds are keeping up with the shifting spring season in the 2022 issue of ̽����ѡ. “That’s the sign of really good science, when it shines a light on something that you hadn’t thought about before and poses other potentially really interesting and important questions.” 

For their published late last year in Global Change Biology, ornithologists Montague Neate-Clegg and Morgan Tingley from the University of California, Los Angeles analyzed 60 years of bird-banding data from the United States and Canada. Their results confirm a few things scientists already know: that spring migration is happening earlier and earlier, and that male birds, on average, arrive on their breeding grounds before females.

But Neate-Clegg and Tingley also found something new: Over time, the gap between male and female arrival has widened. Across 36 species, adult males have moved up their arrival date by just over five days on average, compared to less than four days for females. Adult male Black-throated Green Warblers, for example, arrived 6.13 days ahead of adult females in 1960, but now arrive 7.45 days before them—a 22 percent change.

That might not sound like much. But then again: “One degree Celsius of climate warming doesn’t seem like a big deal, even though it is,” Neate-Clegg says. Breeding can’t get underway until females show up. If the spring green-up, with its boom of nutritious caterpillars, has already happened by the time females arrive and start laying eggs, baby birds will go hungry—no matter how early their fathers were. It's a problem ornithologists dub a “phenological mismatch," referring to out-of-sync timing of seasonal cycles. 

Although other studies have looked at this mismatch, Neate-Clegg and Tingley’s was the first to dig into how those trends vary between demographic groups across many species. “When I started looking into the literature, it didn’t seem like anyone had really tested this idea on a continental scale across species, so I thought this was the perfect time to do it," Neate-Clegg says.

Why males are adapting better than females is an open question. Neate-Clegg thinks climate change may be reducing the risks of arriving too soon; earlier, milder springs mean males can move up their schedule without much chance of, say, getting caught in a late blizzard. There is also some evidence that males, on average, winter farther north than females. They may be exposed to different environmental cues about when to begin their migration, giving them a head start.

It's also not known whether, or how, this sex gap could affect birds’ reproduction and therefore their population numbers. “Maybe if females are showing up later, that has big-picture consequences for how many babies get produced and how many individuals are in the population,” says Ben Freeman, a postdoctoral fellow at the University of British Columbia who studies how climate change affects birds’ ranges and was not involved in the new study. “The world is changing, and different segments within populations aren’t responding the same way to those changes, and there are plausible evolutionary, ecological, conservation, and geographic range consequences. I think this is the sort of paper that will hopefully spawn a lot of more detailed investigations."

It's also the latest study to draw attention to ornithology’s relative ignorance of female birds. Ornithologists have long given male birds more attention in studies than female birds, often assuming their behaviors are similar. Only recently have studies tried to unpack differences between the sexes—for example, that males and females of some species overwinter in different places. This new research points to how differing male and female behaviors can create different conservation needs.

“There’s a systemic issue where people aren’t thinking about sex-based differences in a lot of aspects of biology,” Neate-Clegg says. “I hope this study gets people to think more about the role that sex plays in ecology, and the fact that we need to think about these differences when we’re trying to understand the effects of climate change.”