Why warblers face an increasingly treacherous spring migration

North America is home to more than 50 species of warbler, and dozens of them may be winging their way to your backyard as you read this.  

While not necessarily closely related, warblers can be any number of small-ish, highly vocal, passerine birds that arrive each spring in waves. 

“The cool thing about warblers is that if you go into a forest, you might see 10, 15, maybe even 20 species all in the same place,” says Cody Pham, a Ph.D. candidate at University of California Davis in the Global Ecology & Sustainability Lab. “They all look really, really different, and they all sing different songs.”

While each species has its own lifestyle and timing, scientists and birders alike look to arriving warblers as a sign of spring. What’s more, some of these little creatures travel thousands of miles to get here. 

“I think the best way to describe them is to say they’re little birds that have a bunch of personality,” says Pham, who has studied range shifts in warblers in North America.

Unfortunately, climate change may be making it harder for some warbler species to complete the age-old journey.

Warblers’ hemisphere-crossing journey

When warblers show up in our backyards, many of us might not realize everything that’s had to go right for the animals to get here. 

“Most of the year, they’re not actually in North America,” says Kamal Islam, Professor Emeritus of Biology at Ball State University. “We only get to see them for about three months out of nine months.” 

As long-distance migrants, many warbler species spend the winters in Central or even South America, not to mention all the time it takes to fly back and forth between hemispheres. The birds don’t fly directly but choose stopover sites along the way where they can rest, feed, and refuel. 

In this way, that one tiny bird doesn’t just require a good supply of insects where it breeds in the U.S. or Canada, but also where it winters in Peru or Venezuela, as well as many places in between. 

“If we want to continue to have these birds return to us every spring, we need to think about all of the areas where these birds stop along the way,” says Islam. 

That means conserving habitat but also ensuring that insect populations remain healthy. Not to mention the elephant in the room…climate change.

A climate-driven mismatch

Islam has spent his career studying Indiana’s cerulean warblers, which are considered endangered in the state, and made some interesting and troubling discoveries. 

Many migrating birds have arrival times that synchronized with local events, such as trees and plants getting new leaves, which in turn is timed with a baby boom among leaf-eating caterpillars. So, it makes sense, evolutionarily speaking, for cerulean warblers to arrive in Indiana with enough time to lay eggs so that when the chicks hatch, there are loads of juicy, protein-rich caterpillars to feed them.

(The surprisingly relatable reason why some birds get cranky.) 

However, Islam has discovered that over the course of 38 years, moths and butterflies have adjusted to a changing climate by laying their eggs 14 days earlier than they used to. Cerulean warblers have changed their ways, too, but arrive just four days earlier than they used to. 

“That's what we refer to as a potential mismatch,” says Islam, who notes that the peak of caterpillar abundance now occurs when the birds are incubating their eggs—not when the young are newly hatched and most in need of wriggling caterpillars.

This is just one way that timing might affect plants and animals, though. And along their migration route, birds must make many calculations. 

“Climate change potentially increases the ‘natural’ conflict in resource exploitation of migrants,” says Janne Ouwehand, a National Geographic Explorer who has studied how pied flycatchers adjust their preparations for spring migration in Africa. 

In other words, a species falling out of step with its breeding season food source might begin migrating from its wintering grounds earlier to compensate. But this could potentially affect how much energy it’s stored up for the journey, triggering negative impacts elsewhere in the life cycle.

Climate change could also cause warblers to lose touch with other species. 

(Do birds have migration buddies?) 

“Because there are so many warbler species, a lot of them overlap in range,” says Pham. “But the thing that we're seeing with climate change based on those predictions of the models is that as the species distributions move, those warblers are going to be separated.”

And while associations between bird species is a relatively new area of study, Pham says one well-documented example of how birds rely on each other is by listening to each other’s alarm calls—a benefit that could weaken or be lost as ranges shift. 

Of course, there will be costs for us too.

“With climate change and habitats and ecosystems becoming less diverse, it's really sad to think about how so many people are going to lose the opportunity to experience nature and experience these wonderful things that can bring a lot of joy to people,” says Pham.

The good news is that warblers are a varied bunch, so even though some species such as the cerulean warblers may be in trouble, others such as the black-throated blue warbler seem to be adapting just fine. 

How to spot warblers this spring

Now that you know how precarious the situation is for some of these birds, you might want to try to see them before they’re gone. And to do that, you’ll want to look for areas on the fringes of where habitats meet—like the edge of a forest. 

Different warbler species make use of different parts of a habitat. Black-throated green and bay-breasted warblers hunt for insects in the forest canopy, while ovenbirds are often found foraging on or near the ground and waterthrushes along streams. 

You’ll also want to keep an eye out for a rainbow of different plumage patterns. Blackburnian warblers have orange throats and black markings, while painted redstarts and red-faced warblers bear bellies and faces painted in crimson. Yellow warblers, yellow-throated warblers, and Townsend’s warblers all sport shades of canary, while tropical parulas and cerulean warblers don feathers of brilliant blue.

“It’s just mind-boggling—the fact that a bird that's only about 4.5 inches like the cerulean warbler makes a 3,000-mile, round trip journey is truly amazing,” he says. “And I think it can open up the public to think about a bigger worldview.”