They’re blue, round, and small. And birdwatchers throughout Alabama are hoping to catch sight of one before they head north for the summer.

The cerulean warbler is one of many bird species that migrate through Alabama in the spring and fall. But the brilliant blue birds are hard to spot when they’re here, and they’re becoming even more rare.

“It’s a really charismatic species,” said Thomas Thompson, a graduate student at Alabama A&M University who studies the Cerulean Warbler. “I think it captivates a lot of people, when they hear about it. And if you’re actually fortunate enough to hear one or see one, it’s a pretty awesome experience.”

Alabama is a stop-off point for cerulean warblers during migration. Their spring migration — mid-March to mid-May — is almost over but they’ll be back again in the fall.

Some cerulean warblers will breed in Alabama, in the northern third of the state, but their breeding grounds are becoming fewer and fewer.

You might see them on Chapman Mountain, said Lianne Koczur, science and conservation director with Alabama Audubon, the local chapter of the larger bird conservation society.

Cerulean warblers are around just 4.5 inches long. The males are bright blue with a black necklace, and white throats and bellies. Females are a paler bluish green with hints of yellow on their bellies.

Why are they so hard to spot? In addition to the decline in their population, they are a high canopy bird, meaning they hang out in the tops of trees, Koczur said, so they’re hard to see from the ground. The warblers also like to be in the interior of the forest, in mature trees, Thompson said.

“That’s another reason that they’re kind of poorly understudied is that they’re a cryptic species,” Thompson said. “They’re hard to see, they’re hard to find. More often than not you just hear them and you’re lucky if you get to see one because they’re tiny little birds.”

The warblers begin migrating north in mid-March and are largely past Alabama by mid-May. From there, they settle and breed as far west as Minnesota all the way to the east coast, but primarily in the Appalachian states, according to the Audubon Society’s migration tracker.

Their migration south for the winter begins as early as mid-July, becoming more prevalent in Alabama by September, according to the Audubon Society. From there the birds will settle in South America for the winter, in the Andes Mountains in Peru and up through Venezuela.

Since the 1980s, the warbler’s population has decreased by nearly 70%, according to the Audubon Society. From 1966 to 2005, the bird’s population decreased by an average of 3% every losses year. That’s one of the most dramatic declines of any bird species in recent memory, according to the U.S. Fish and Wildlife Service. It’s one of the fastest declining songbirds in North America, Thompson said.

“It’s kind of sad, because a lot of the birds that I hear singing there, I know they’re singing and there’s no female there to answer the call,” Thompson said. “It’s like taking a glimpse into kind of a disappearing population.”

Cerulean warblers breed in mature deciduous (meaning the trees lose their leaves) forests. This habitat has been lost over the decades, causing the bird’s population to decline. Their winter habitat in the Andes is also in danger of disappearing due to development, the fish and wildlife service said. Cerulean warblers are on the service’s Birds of Conservation list, meaning the bird is one of its highest priorities for conservation.

Because the warblers are so picky about their habitat, that makes them even more susceptible to habitat loss, Thompson said. Some birds can live anywhere, but cerulean warblers require those tall, mature deciduous trees, much of which has been lost. Even within the forest, the warblers like to breed in places where there’s a gap in the tree canopy, because it amplifies their song.

There used to be more locations where the bird could breed in the state. But today there’s just three known breeding populations in Alabama, Thompson said, though he acknowledged there could be populations on private land unknown to researchers. Those breeding populations are in the Sipsey Wilderness Area in the Bankhead National Forest, on Larkin Fork and in the Walls of Jericho tract in Jackson County.

“They used to be considered common. So there was sightings of them all around Birmingham and in multiple counties throughout Alabama,” Thompson said. “There was known breeding populations, and now there’s really only two known breeding populations left.”

There are several other warbler species that breed in Alabama, including Kentucky warblers, hooded warblers and Swainson’s warblers, Koczur said. All of these birds breed here in Alabama but migrate south to Central and South America for the winter.

This story is the first in Beautiful Birds of Alabama, a new series from AL.com featuring some of the state’s most unique birds.