This weird thing is actually a piece of prime real estate.

With just a few modifications,

it will become a nice summer home for

this couple and their children.

These are purple martins.

They're native North American songbirds

but here's the thing: on the East Coast they don't nest in the natural world anymore;

They only breed in man-made birdhouses

which is weird, and made me wonder, "How is this even possible?"

It turns out there's this vast network of strangely devoted purple martin landlords.

"Purple martins are totally dependent on man for housing now."

"Oh, dear ... he LOVES purple martins."

Jim grows gourds himself.

He's got this whole purple martin colony in his backyard.

To make room for it he waited for his wife to go to work

and then cut down two of her dog wood trees.

"Much to her displeasure."

"He'll go I don't know how far with anything to make those birds happy."

And he's not alone.

I met Jim's whole purple Martin crew and they're all just as obsessed with these birds.

"I'm really in love with 'em if you want to just tell it like it is."

"We look forward to 'em coming. 'The martins are going to be here in two weeks!'

"Y'know you start that up."

Apparently, these little birds are the first to arrive every year

They fly up from their winter range in South America traveling a whopping 300 miles per day

When they arrive in the states they start scoping out birdhouses

They used to nest in tree snags and rocky crevices

but then, thousands of years ago,

Native Americans started putting up gourds for the martins

and now on the East Coast the birds only breed

and raise their chicks in human built housing

The real finale of purple martin season comes in late summer

when the birds gather in enormous flocks before they migrate back south

For the past 25 years, the largest of these roosts

has been in the middle of a lake in South Carolina on a tiny island.

Hundreds of thousands of birds show up

at least half a million

and I thought, "That's something I would really like to see."

♫ So I packed my lunch ♫

♫ and I gave my boss the slip ♫

♫ Wrote my self this theme song ♫

♫ Going on a field trip - yeah! ♫

It took me eight hours to drive down to South Carolina

but I just kept imagining the amazing spectacle that was waiting for me there

So I made it down to Lake Murray,

hired this guy to take me out on the water,

and at long last reached the island

where purple martins have roosted for the past 25 years.

But the birds weren't there.

How did half a million birds go missing?

And was foul play involved?

After all, purple martins have been in trouble for a long time

and it all started with a play written by William Shakespeare.

Stay with me!

In Henry IV, Part 1, a character named Hotspur

mentions a starling.

And centuries later, Eugene Schieffelin (this New York drug manufacturer)

had the whimsical idea to bring every bird

mentioned by Shakespeare to the United States.

This was 1890.

No one really knew about invasive species.

So when Schieffelin released a hundred European starlings in Central Park

everyone was pretty pleased.

In the last century though,

Those hundred birds have turned into an army of 200 million.

And they are the worst! They're bullies!

They'll violently compete for those natural cavities where purple martins used to nest

so even if the martins wanted to return to nature -- they couldn't

And they'll even invade martin houses,

pecking eggs with their sharp beaks,

smothering babies and killing adults

And there are other predators to worry about too.

Owls.

Snake.

Hawks.

Raccoon.

And another invasive species:

the European house sparrow.

So purple martins have come to rely on their human landlords to protect them.

The landlords have cages to keep away owls,

electrified poles to keep away snakes,

traps to catch starlings ...

but was all that working?

When half a million birds failed to show up at Lake Murray

it got me worried: had the predators killed them off?

Or had they just relocated?

I really wanted to find them or at least figure out what happened

so I asked Julie for help.

We turned to weather radar for clues

and there they were!

The birds were still in South Carolina.

"And see that's huge."

Those expanding rings are purple martins.

Their flocks are so dense they show up on radar

when they leave their roosts in the morning

"Yeah - we need to go there!"

The radar images weren't too precise

but it looked like the martins might have relocated up to Lake Monticello

where I might finally get to see a roost.

"I already see one island that looks hopeful."

"But I think there's like a couple of different little islands out here."

"I wouldn't think it's that one. If any one it's this one."

"And it looks like -- is there a tent on the beach."

"I see people - they're sitting in their chairs."

"They'll probably go purple WHAT?"

"They'll probably think it's that street drug."

"We're looking looking for purple martin - ya heard of it?"

"Have y'all seen the purple martins out here?"

"There's somewhere on this lake. There's a bunch of small little birds."

"You didn't see him last night?"

The birds tend to gather before sunset

so we expected to start seeing the early arrivals any minute.

"Here birdie, birdie, birdie."

That's my worried face.

The sun was setting, we'd been exploring the lake for more than an hour,

and we still hadn't seen a sign of the roost.

"I see nothing!"

"That one looks like it's moving like a purple martin."

"That looks like purple martin!"

"And they are going that way."

"Alright I'm starting to see more."

"Look over those trees, Julie."

"Look at the water." "Oh!"

"I mean not just over the water but they're all up in there."

"We'll just have to see where they go ..."

"If we wanted to find the roost,

we needed to get there before dark.

"Your dream might come true, Adam."

Adam: "Well, we'll see."

"Julie, they're flying dead ahead of us whatever's up there."

"Well..."

"What is - what does that island look like?"

"Oh my god!"

"Is it there?"

"Go that way!"

It's still a mystery why the purple martins left Lake Murray

or how they all managed to gather at this new roost site.

Look at all those birds!

Every single one was born in a house that human built

We've created a world that is so hostile to purple martins

and now a species that we don't fully understand

relies on us for survival

♫ Glide on the air ♫

♫ You swivel and turn ♫

♫ Fast beating heart ♫

♫ What do I gotta learn? ♫

♫ Oh, swallow ♫

♫ What did you swallow? ♫

♫ You swoop so low ♫

♫ You come and you go ♫

"Goodnight, purple martins."

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