Glossy Ibis Range Expansion in North America

On a June morning in a coastal marsh anywhere from New Jersey to Maine, a line of dark waders works the shallows, sickle bills probing the mud, plumage flashing maroon and green where the light catches it. A flock of Glossy Ibis is an ordinary sight on the eastern seaboard now, common enough that most birders barely slow down for it. Two centuries ago there was exactly one, a single bird shot in New Jersey, and no reason yet to think the species belonged here at all. This is the story of how an Old World wader crossed an ocean and walked its breeding range all the way to Maine.

One bird, 1817

The first Glossy Ibis (Plegadis falcinellus) known from the New World was a specimen collected in New Jersey in May of 1817. The species is an Old World bird, native across Europe, Africa, Asia, and Australia, and the best evidence is that it reached the Americas on its own, carried on trade winds from West Africa to northern South America sometime in the early 1800s, then worked its way north. For decades it remained a curiosity. Audubon encountered it just once in Florida, in 1832, and as recently as the 1930s it was still a scarce bird even there. How a wader crosses an ocean and gains a foothold is the larger question we take up in Bird Vagrancy and Range Expansion Explained. This post follows the one bird.

The march north, then west

The real expansion came in the twentieth century. Glossy Ibis pushed north substantially in the 1940s, and along the northeastern coast it went from vagrant to fixture within a generation. It was first found nesting in Connecticut around 1971 and in Massachusetts in 1974, at Clark's Island off Plymouth, and from there the breeding range crept up the coast until the birds were nesting north to coastal Maine and, in some years, into the Maritime provinces of Canada. Then the front turned inland. Through the late 1980s Glossy Ibis increased rapidly into Texas, reached the Colorado and New Mexico front range by the early 1990s, and turned up in Nebraska by the end of that decade. That westward push carried it into the range of its near-twin, the White-faced Ibis, and where the two meet they sometimes interbreed, which is one reason a dark ibis out of place deserves a careful look. We cover that separation in Glossy Ibis vs White-faced Ibis.

Not a straight line

It would be tidy to call this a steady climb, but the honest record is bumpier. After the surge, numbers at the northeastern edge have proven changeable rather than ever-rising. In Massachusetts, breeding pairs ran to about 47 in the mid-1990s, climbed past 140 by the late 2000s, then slipped back under 60 by 2018, and colonies routinely shift between islands from one year to the next. The continental breeding population is modest, on the order of 13,000 to 15,000 pairs, and the species' status at any given site is famously hard to predict from one season to the next. The arc is real, a dark wader that went from a single 1817 specimen to a Maine breeder, but it has come with plateaus and local retreats, not a clean upward line. Expansion and decline are two halves of the same restless story.

The same thing happened in Europe

If the North American march were the whole picture, you might chalk it up to something peculiar about this continent. It is not. Over the same broad period, Glossy Ibis has expanded dramatically in Europe as well. Spain in particular has seen its population boom in recent decades, helped along by the spread of rice cultivation, which hands a probing wetland forager exactly the flooded, shallow habitat it needs. And the ocean crossings have not stopped: Glossy Ibis banded in Spain have been recovered in Barbados, having flown the Atlantic in the present day much as their ancestors presumably did two centuries ago. The European parallel matters because it shows the pattern is about the bird and its moment, a wandering, adaptable species meeting new habitat and a changing climate, rather than a quirk of North American history.

Are we seeing more, or just looking more?

One caution belongs on any modern range-expansion story. Part of the sense that Glossy Ibis is everywhere lately reflects weather that pushes birds around and, just as much, the sheer number of birders now watching and logging what they see. A single busy season is a noisy signal. The trend worth trusting is the one measured across decades and many observers, which in this case is genuine, but it is still worth separating the real long arc from the ordinary year-to-year noise. We make that general argument in the pillar, and it applies here as much as anywhere.

One of three

Glossy Ibis is not doing this alone. It is the earliest-arriving of three Old World waterbirds that crossed the Atlantic and colonized North America, each a little further along its own arc. The Western Cattle-Egret made the same crossing later and expanded far faster, sweeping coast to coast in a few decades before its own recent decline, a story we tell in Cattle Egret in North America. The Little Egret is the newest of the three, still a rare but increasingly regular transatlantic vagrant caught at the stage Glossy Ibis passed through long ago, covered in Little Egret in North America. Set side by side, the three birds are almost a time-lapse of what colonization looks like, and Glossy Ibis is the one that has already arrived.