Rare birds evolving before our eyes: Are invasive pest snails helping an endangered species recover?

Amy Bennett Williams
Fort Myers News-Press
An endangered snail kite hunts for apple snails at Harns Marsh in Lehigh Acres recently. The rare birds are usually found in the Central Florida,  Lake Okeechobee and the Everglades area but a colony has established itself in the Harns Marsh area and they feed mostly on the invasive exotic apple snails. A University of Florida study published in 2017 says the birds evolved to develop a larger beak over a short span of 10 years so they could consume the larger invasive exotic apple snails. As scientists continue to research the changes and map the birdsÕ genome, some say it may be saving the species from extinction.
  • While the process is often thought to occur slowly, these birds have evolved quickly
  • Scientists are mapping snail kites’ genome to determine exactly how it’s happening
  • A graph plotting bill size of sail kites for each year showed them increasing in size
  • Since the snail invasion, kite numbers have gone up

A decade ago, a groundbreaking study of endangered everglades snail kites startled many who study birds, invasive species and evolution.

It showed the rare Florida raptors were adapting rapidly to the invasion of exotic apple snails in their wetland homes by evolving bigger bills to handle bigger mollusks. But though the evolutionary process is often thought to occur slowly, over millennia, these birds, some of which live in Lee and Collier counties, had changed in a matter of generations.

Now, scientists are mapping snail kites’ genome to determine exactly how it’s happening. They’re also looking at changes in the birds’ nesting, which seems to be increasing from generally raising just one brood a year to adding another.

An endangered snail kite hunts for apple snails at Harns Marsh in Lehigh Acres recently. The rare birds are usually found in the Central Florida,  Lake Okeechobee and the Everglades area but a colony has established itself in the Harns Marsh area and they feed mostly on the invasive exotic apple snails. A University of Florida study published in 2017 says the birds evolved to develop a larger beak over a short span of 10 years so they could consume the larger invasive exotic apple snails. As scientists continue to research the changes and map the birdsÕ genome, some say it may be saving the species from extinction.

Snail kites are one of Southwest Florida's iconic species, says avid birder, Fort Myers pediatrician Jose Padilla-Lopez. "Beautiful red eyes, bright orange legs and very sharp curved beak. It’s prized by birders because it’s uncommon and in the U.S. unique to South Florida."

In fact, he says he's quite irritated that no local sports team has taken its name.

One of Padilla-Lopez' favorite observation spots is Lehigh Acres' Harns Marsh, where the exotic snails are flourishing.

"In the past we had to travel to the Everglades to see the kite, now we have it closer. With a little luck you can see them hovering over the marsh hunting for snails or perched in low shrubs over the reeds," he wrote in an email. "They like large open freshwater marshy areas."

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Did you know? Endangered snail kite released in Lehigh Acres

The seed for the initial paper, "Rapid morphological change of a top predator with the invasion of a novel prey," published in Nature Ecology & Evolution, was planted in the early 200s, when scientists noticed the kites, which normally ate the native Florida apple snail almost exclusively, starting to dine on the much larger exotics.

“Historically, they’ve just essentially eaten that one species of prey,” said principal researcher Robert Fletcher, an associate professor at the University of Florida. But soon after the exotics showed up, scientists began noticing them trying to tackle the exotics – often with little success.

“They were dropping the snails quite frequently and it was unclear that they could really manage the sheer size of the exotic apple snail. … it was just a very challenging thing for them to eat.”

Dropped Invasive exotic apple snails shells are shown in a pile under a tree at Harns Marsh recently.

And for the snail kites, whose numbers were in freefall, finding something to eat had become a stiff challenge, as populations of the native snails dropped as well.

“But then, as time progressed, we started to see the numbers increase again, with very consistent but gradual increases, and we know they were using areas with the exotic snails,” Fletcher said. “So the question became: Are the exotic snails actually helping snail kites increase their numbers? How are they doing this, and what could be going on?”

Fletcher challenged his students to think outside the box: “What if their bills are getting bigger? And they laughed me out of the room. They thought that was the most ridiculous idea and not reasonable at all to assume it was happening. And yet we kept seeing those increases,” he said. “Finally I just said to  them: Humor me. Just take a look at our data."

Grad student Christopher E. Cattau crunched the numbers.

“He came into my office one day and he sets a piece of paper down on my desk, doesn’t say anything, just pushes it over to me,” Fletcher recalls. “And I flip over the piece of paper.

There was a graph, plotting bill size of sail kites for each year.

"And every year, it was just inching up –  a little bit bigger, a little bit bigger, a little bit bigger," Fletcher said. “I was stunned. Even though I’d told him to look at it, I wasn’t really expecting it to show any pattern.”

An endangered snail kite hunts for apple snails at Harns Marsh in Lehigh Acres recently. The rare birds are usually found in the Central Florida,  Lake Okeechobee and the Everglades area but a colony has established itself in the Harns Marsh area and they feed mostly on the invasive exotic apple snails. A University of Florida study published in 2017 says the birds evolved to develop a larger beak over a short span of 10 years so they could consume the larger invasive exotic apple snails. As scientists continue to research the changes and map the birdsÕ genome, some say it may be saving the species from extinction.

Questions remained, though – questions Fletcher and his team are still exploring.

“It was quite clear the morphology had changed very rapidly. But it’s a whole other challenge to understand how could that be.”

Using measurements taken during routine banding of the birds' weight, bill size and leg length, the scientists began to see that not only were the kites getting bigger, “but the effects on bill size were above and beyond their mass. The birds had disproportionately larger bills for their weight.”

They were exciting times, Fletcher says, “And really neat for a lot of different reasons: It was giving us some hope on their ability to respond.

For Fletcher, there was a personal connection as well.

“When I got into this field, one of the things I read very early on that I found incredibly fascinating was “The Beak of the Finch,” a 1995 Pulitzer Prize-winning book about South American finches that showed similar rapid evolution spurred by environmental changes.

“It was the most amazing story in science I’d ever read about as a young scientist and to see the same thig happening here … wow.”

Since his paper was published, researchers have learned that bill size across the birds’ range varies in proportion to the length of time the exotics have been there.

“So the largest-billed birds are up in the region that have had exotic snails the longest, and in areas where there haven’t been exotic snails so long, there tend to be smaller bill sizes,” Fletcher said.

Plus, they’ve learned since the study came out is that not only do bigger bills confer survival advantages on individuals,  “it turns out female snail kites appear to prefer males with bigger bills as well,” Fletcher said. “So the more we look at it there’s probably more benefits in term of bill size than we even showed in that article.”

What’s next?

“Now one of the things we’re trying to do is map the genome of snail kits to understand which genes are responsible for bill size. If we can do that, we’ll have a much better understanding of how these changes occurred.”

The irony that an endangered native species’ survival may have been helped by an invasive pest isn’t lost on Fletcher.

“It is a real challenge. This snail has been labeled one of the worst invasive species in the world because it can have such an impact on ecosystems. One of the ongoing questions  for those working on the recovery of snail kites is how do we balance this knowledge that exotic snails – at least over the short term – have benefitted the snail kite population and should we be trying to manage for exotic snails or not? It’s something we discuss quite a bit,” he said. “We don’t want to promote exotic snails but at the same time, they probably are not going away any time soon.”

Invasive exotic apple snail eggs are seen at Harns Marsh in Lehigh Acres  recently.

The fact is, since the snail invasion, kite numbers have gone up.

“So there’s a lot of hope and a lot of  discussion about whether snail kites are OK now. But we really don’t know how long this will last.”

Often, Fletcher says, exotic populations explosions are followed by drops.

“So we’ll be trying to look closely to see how this plays out over time.”

See snail kites

The Audubon Society chapter of Hendy and Glades counties is planning two escorted birdwatching driving trips Saturday and March 20th to Stormwater Treatment Area 5/6, where two weeks ago, members spotted seven snail kites, says the chapter's veteran birder Margaret England says. The migratory waterfowl (ducks) have been the highlight of the escorted driving trips the past two months and Swallow-tail kites are arriving from Brazil to breed but we rarely see them at STA5/6. With the STA5/6 trips we practice social distancing."

Register online: www.hendrygladesdaudubon.org."

An endangered snail kite hunts for apple snails at Harns Marsh in Lehigh Acres recently. The rare birds are usually found in the Central Florida,  Lake Okeechobee and the Everglades area but a colony has established itself in the Harns Marsh area and they feed mostly on the invasive exotic apple snails. A University of Florida study published in 2017 says the birds evolved to develop a larger beak over a short span of 10 years so they could consume the larger invasive exotic apple snails. As scientists continue to research the changes and map the birdsÕ genome, some say it may be saving the species from extinction.