英语听力：自然百科 马蹄蟹与漂鷸（二）

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It's now May in Brazil, the Red Knots have built up their fat reserves and they are now in their breeding plumage. They are ready to begin of longest legs of their epic² journey. They'll fly for as many as four days and nights without stopping, mostly over Open Ocean, with no landmarks³ to guide them. They'll use the moon, the stars and the sun. Their departure is mysteriously synchronized⁴ with an annual event, 4,000 miles north.

 

Here on the Delaware Bay, the water has warmed and the world's largest population of horseshoe crabs⁵ is beginning to spawn⁶. 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 spawning⁷ 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 Knot's on schedule, they should be here soon.

 

Horseshoe crabs prefer spawning at the cover of night, but this is the height of their short season. It only lasts for few weeks. They come to shore when the tides are highest to lay their eggs deep in sand. Swarmed⁸ 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 on to her shell. Since fertilization is external, even unattached males would father some of the eggs. When her clutches are finally laid, the females break through the sand. In all the commotion⁹, eggs from prior spawns¹⁰ get unearthed¹¹. What the crab accidently dig up is what the Red Knots depend on.

 

As the tide recedes¹² and the crabs retreat, a flock of Red Knots appears in the sky. They've traveled for four days and nights without stopping. They're exhausted¹³ and initiated¹⁴. 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 migration¹⁵ 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 seriously. No one knows how many will make it to the Bay this year.

 

For the next week, more flocks of red knots trickle¹⁶ 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 Artic summer. Their digestive system has 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 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. Ongoing¹⁷ 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 in the puzzle.

 

That's a lot of work. We work about 40 hours a week. We take a sample roughly per one meter and we'll do this for about four hours other couple of days, then we're sought to eggs, then we count the eggs and that takes six or seven hours, and then we count the birds. From last year, it looks like the egg densities¹⁸ have dropped from same beaches and it's not a good sign. In Delaware in May, it's just a beautiful animal. I mean, it is, oh, I hate to see some go extinct.

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