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GUY RAZ, host:

Yesterday we brought you a story of sex: the annual mating frenzy of horseshoe crabs on the beaches of Delaware Bay. The results of that giant orgy: billions of crab eggs covering the shore.

That's where we start today's installment. It's a tale of gluttony by tens of thousands of shorebirds who fly in to gorge themselves on those eggs. Quite a few human visitors come along, as well.

This year, Louisa Jonas from member station WYPR in Baltimore is one of them.

(Soundbite of birds chirping)

LOUISA JONAS: A lanky man in a floppy hat and windbreaker is hiding behind shrubbery on a beach in Mispillion Harbor.

Mr. NIGEL CLARK (British Trust for Ornithology): I keep myself really low and just literally (unintelligible) forward.

JONAS: Nigel Clark is from the British Trust for Ornithology. He's peering through his scope, trying to read tags on the legs of hundreds of migratory shorebirds feeding in front of him.

Mr. CLARK: Ah, I think I can just see Brazilian-marked(ph) bird. It's amazing on these beaches just to think that we can see birds that we know have been marked in Brazil or Argentina, the very southern tip of South America, right at the southern tip of Tierra del Fuego. That's where the majority of these birds are coming from.

JONAS: Nigel is particularly interested in the red knot, a chestnut-colored bird about the size of a robin.

(Soundbite of bird)

The red knots winter in Argentina but fly 10,000 miles to the high Arctic to breed. To prepare for their journey, they shrink their own digestive organs and turn the tissue into fuel to burn in flight. By the time they look down on the shores of the Delaware Bay, they are really hungry.

Mr. CLARK: When a bird arrives, they are literally skin and bone, very little on them at all. They look very thin, sleek birds running around with extremely long legs, it seems.

JONAS: Delaware is the red knots' most important feeding stop on their trip. They time their arrival to perfectly coincide with the annual horseshoe crab spawning here.

The crabs lay billions of pearly-green, caviar-sized eggs. Many of these eggs would just dry out or wash away, but instead, they become a banquet for the migratory shorebirds.

Mr. CLARK: So the bird is feeding on really the surplus eggs in the population. It really is a case of nature's bounty that they're taking rather than the fact that the birds are stopping the crab's breeding.

JONAS: The red knots only have a few weeks to build up enough fat reserves to fuel their last leg of their journey. They eat so greedily that stretches of sand can be completely hidden by their red breasts and little pecking heads.

Mr. CLARK: By the time they are ready to go, they are like little balls - it's the best way of putting it. They are round. They look like they have short legs because they are carrying so much fat on their breast, and they almost waddle, they are so heavy.

So I thought we should celebrate…

JONAS: Nigel heads back to a beach house not far from Mispillion Harbor. He is one of 20 or so volunteers who gather here every year as part of the Delaware Shorebird Project. There are sleeping bags in every corner, tide and moon charts on the walls. A sign on the oven reads: Do not touch. Drying feathers.

Ms. JACQUIE CLARK: When we (unintelligible) than that, we need to get people out to it very quickly. It's very…

JONAS: Jacquie Clark is Nigel's wife and also works for the trust. She says the bounty of crab eggs in Delaware that's available only in late spring is an example of nature's mysterious synchronicity. The red knots need that fuel to get to the Arctic to breed, and they need to get there exactly when there will be plenty of food for their chicks, and that's only for two short months in the summer.

Ms. CLARK: Hopefully, they'll get the timing just right. It's really critical they arrive just as the snow melts, and they can lay their clutch of eggs really quickly. Then as their chicks hatch, it should be just about the same time that the insects hatch as well. So the chicks get the maximum food.

JONAS: But something's gone wrong for the red knots. Nigel says the population's crashed since the 1990s. The red knot is a candidate species under the Endangered Species Act. Climate change may be affecting when the ground thaws in the Arctic. There may not be enough horseshoe crab eggs in Delaware to fatten up the birds.

The volunteers head off the next morning to see if the birds are gaining enough weight. On the beach at Mispillion Harbor, the team sets up a cannon-net, which uses gunpowder, to blast the net over hundreds of feeding shorebirds.

(Soundbite of birds)

Wings flap, and the birds bounce up and down like popcorn, struggling to escape.

Unidentified Woman: Okay. I've never done it (unintelligible).

Unidentified Man: Okay. There's plenty (unintelligible).

JONAS: The volunteers gently peel back the net and lift the birds out. Sometimes, two or three in a handful and place them into ventilated boxes. The team gets to work weighing, measuring, banding and taking feather samples.

Mr. CLARK: Oh, one more. There we go. There you go, baby.

(Soundbite of birds)

JONAS: The crew returns to the beach house. Nigel and the other volunteers crunch the data with hopes that the birds are plumping up. Soon, they'll have to fly 2,500 miles to the Arctic in five days without stopping.

Mr. CLARK: Red knot in Delaware Bay in the Americas is absolutely intertwined and critical. There is nowhere else in the world where a population of birds is so dependent on such a small area. And if they don't have Delaware Bay, they won't get to the Arctic, they won't breed and the population will die out. It's as simple as that.

JONAS: This year turns out to have been a pretty good year for the red knots in Delaware. But in order for the population to recover, the birds need several good years in a row, not just in Delaware, but in Argentina and in the Arctic.

For NPR News, this is Louisa Jonas.

RAZ: And if you missed the first part of our story, you can hear it at npr.org. You can also see photos of the crabs and of the birds they keep plump.

(Soundbite of music)

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