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Peter Frederick, a research professor of wildlife ecology and conservation at University of Florida, has made many research trips over the years to Seahorse Key, a 150-acre island five miles off the coast of Florida that for decades has been home to thousands of nesting birds. That’s how he knew, from the moment he stepped off the boat on a visit one day this May, that something was very, very wrong. Wading bird colonies are cacophonous, but Seahorse Key was almost silent. “It was just the wind,” he recalls. The birds were gone, having apparently abandoned the island in one sudden mass exodus.

Thick with mangroves, carpeted with spiky cactus, and infested with venomous snakes, Seahorse Key is “a pretty nasty place to be,” says Frederick. But its lack of appeal to humans and other mammals was precisely the reason birds flocked to the island in the first place. Until this spring, it had been home to the largest bird colony on the Gulf Coast of Florida: Double-crested Cormorants, Brown Pelicans, White Ibises, Great Egrets, Great Blue Herons, Snowy Egrets, Tri-colored Herons, Roseate Spoonbills, Little Blue Herons, even a rare pair of Reddish Egrets had all nested there.

Frederick, who showed up with small contingent assembled by U.S. Fish and Wildlife Service biologist Vic Doig, discovered that the birds—every last one of them—had disappeared. “What we saw were a lot of abandoned nests,” Frederick says. There were still eggs in a few of them, and more eggs scattered on the ground, each with the same hallmark: “Nearly all of the eggs that were in nests or on the ground had holes in them,” he explains. A few dead bird carcasses were also lying around (thankfully most of Seahorse Key’s birds survived, and resettled on nearby islands).

It’s not unusual for a group of birds to abandon a nesting spot, says Julie Wraithmell, Director of Wildlife Conservation for ���Ϲ��� Florida. Often colony abandonment is the result of changes in the habitat or in prey resources. But what is uncommon is abandonment of a colony as large and established as Seahorse Key, particularly since the island is home to so many different species with such different diets and foraging ranges. “To have everybody disappear overnight?” says Janell Brush, a researcher with Florida Fish and Wildlife Service. “Something else is going on.”

So what caused thousands of birds to flee from Seahorse Key? Frederick and Doig are trying to figure that out. Here, we round up some of the usual suspects and weigh the evidence.

The Suspect: Raccoons

Raccoons are notorious bird-egg swipers. Even just one or two raccoons showing up on an island and climbing up into a nest to steal eggs, especially at night, can cause adult birds to abandon their nests.

Likelihood: Pretty low. There was at least one raccoon on the island at the time the birds fled, but the holes in the eggs were small, which isn’t this mammal’s style (when raccoons eat up eggs, they crush the shells). Frederick says those holes were probably made post-desertion by a scavenging Fisher Crow or Boat-tailed Grackle.

The Suspect: Lack of Food

Seabirds have been known to abandon their nests if they are suddenly unable to find food in the area.

Likelihood: Slim. If the colony was smaller or less diverse, this theory might make sense. But Seahorse Key had a huge number of species, with varied diets and foraging ranges. The kind of disruption that would cause a White Ibis to abandon its nest, for example, wouldn’t necessarily impact a Brown Pelican in the same way.

The Suspect: Bad Weather

Unlike a problem with the food supply, an extreme weather event would impact all of the island’s birds equally.  

Likelihood: It’s possible, but Frederick doesn’t buy it as a plausible theory. “We did have a big line squall at about the right time for these birds to have abandoned—but none of the lightning strikes appear to have hit Seahorse Key,” he says, and that’s what would have scared the birds off.

The Suspect: Eagles

Eagles, which sometimes feast on chicks, have decimated seabird colonies in other areas, like , where they have rebounded in recent years.

Likelihood: Not likely. For one thing, Florida’s eagle population has remained relatively stable over the years; for another, while historically there haven’t been eagles on Seahorse Key, there is a pair of eagles nesting on Snake Key, a nearby island to which many of the birds fled. And that nesting pair is currently rearing a pair of juvenile eagles, “which are some of the worst," Frederick says, in terms of predation and aggression, so if the birds were escaping eagles, they likely would have avoided Snake Key. Most critically: None of the bird carcasses found on Seahorse Key bore the markers of an eagle attack. When eagles catch their prey, they sink their talons into the back of the skull, crushing it. “We did not see that puncture wound” on the birds’ bodies, Frederick says.

The Suspect: Chemical Poisonings or Disease

The bird carcasses found on the island didn’t have outward signs of trauma, which could suggest they died from ingesting a toxin or contracting a disease.

Likelihood: Almost impossible—the carcasses left on the island were tested for toxins at a national wildlife forensics laboratory, and the tests all came back negative.

The Suspect: Humans

Process of elimination. “What I’m left with is some kind of traumatic, loud disturbance,” Frederick says.

Likelihood: It’s unlikely that humans would have ventured onto Seahorse Key—the snakes and the cacti are pretty effective deterrents—and besides, a single human incursion wouldn’t cause the birds to abandon the island they’ve nested on for decades. However, a human-caused ruckus might have done it. The most likely culprit, Frederick says, is noise from an aircraft. Residents of Cedar Key, about four or five miles away from Seahorse Key, “have reported big uptick in flights over the area,” Frederick explains, “and these are mostly military flights.”

Unfortunately, this theory is hard to prove. “The military is not in the habit of giving out their flight plans,” Frederick says, “so it’s pretty hard to ask them, ‘Did you cause this abandonment?’ ”

The Verdict?

Without more information, the mystery of what caused Seahorse Key’s birds to decamp is likely to remain just that. “The bottom line is: We don’t know. We may never know,” Frederick says.

If there is a silver lining to be found in all of this, Fredericks says, it’s that many of the birds were able to resettle almost seamlessly on a nearby island, thanks to the vast habitat available in Cedar Keys National Wildlife Refuge. “These [islands] are very important sites. They are becoming less numerous thanks to seawater rise and increased human disturbance,” he says. If whatever happened on Seahorse Key were to happen somewhere else, those birds might not be so lucky.