Narrator: We’re not the only species that’s come to depend on the horseshoe crab… They’re critical to others, too – like the red knot.

It’s May now in Brazil. The red knots have built up their fat reserves… and they’re now in their breeding plumage.

They’re ready to begin the longest leg… of their epic journey.

They’ll fly for as many as four days and nights, without stopping -- mostly over open ocean. With no landmarks to guide them, they’ll use the moon, stars and the sun.

Their departure is mysteriously synchronized with an annual event four thousand miles north.

Here on the Delaware Bay, the waters have warmed… and the world’s largest population of horseshoe crabs is beginning to spawn.

It’s here that the red knot has always found the fuel for their journey to the Arctic. And the horseshoe crab has always provided that fuel – in the form of eggs.

The spawning season is an annual feast. It’s a windfall for shorebirds – and a great opportunity for biologists.

Migrating flocks have just begun to arrive. If the red knots are on schedule, they should be here soon.

Horseshoe crabs prefer spawning in the cover of night, but this is the height of their short season. It only lasts a few weeks.

They come to shore when the tides are highest to lay their eggs deep in the sand.

Swarmed by hopeful males, the female digs down to bury her clutch.

The males are smaller than the females, but they have a special claw that hooks onto her shell.

Since fertilization is external, even unattached males will father some of the eggs.

When her clutch is finally laid, the female breaks through the sand.

In all the commotion eggs from prior spawns get unearthed. What the crabs accidentally dig up is what the red knots depend on.

As the tide recedes and the crabs retreat, a flock of red knots appears in the sky.

They’ve traveled for 4 days and nights without stopping. They’re exhausted and emaciated.

The flock will find a safe place to roost for the night… and rest.

From all over the world, biologists have come to count shorebirds on the Delaware Bay.

The spring migration offers a rare opportunity to assess populations.

Many species of shorebirds are in decline – but the red knots that winter in South America are by far the most serious.

No one knows how many will make it to the Bay this year.

For the next week, more flocks of red knots trickle in.

Once they’re here, they need to eat.

They only have about two weeks to refuel and set off again. Their breeding is tightly synchronized with the brief Arctic summer.

Their digestive systems have shut down for the long journey. It’s here that the horseshoe crab makes all the difference in the world. Their soft eggs are packed with protein and easy to digest.

Some knots will double their weight in two weeks.

While it’s an important food for other species… for the red knot, it’s essential.

This was a revelation to scientists and a key piece of the puzzle. As red knot numbers dropped, evidence began pointing to the Delaware Bay – and to horseshoe crab eggs.

Ongoing studies began to confirm the connection…there’s been a steady decline in eggs on the Bay.

Figuring out how many eggs there are is a tedious job – but it’s become a critical piece of the puzzle.

Phil: It’s a lot of work. We work about 40 hours a week. We take a sample roughly per 1 meter. We do this for about 4 hours every couple of days then we sort through egg. Then we count the eggs. And that takes 6 or 7 hours. And then we count the birds…

From last year, it looks like the egg densities have dropped from the same beaches and it’s not a good sign. It does worry me. It’s just a beautiful animal. I mean it’s… You know, I hate to see something go extinct.

Print Transcript