Red knots seem to thrive during Delaware Bay stopover

New evidence suggests Delaware Bay may be one of many coastal stopping points for the red knot, a robin-sized shorebird that has made the lower estuary a seasonal tourist destination.

But researchers still maintain that Delaware Bay is the critical link in the spring migration and horseshoe crab eggs – the preferred diet along the Delaware Bay – are vital to the health of the population.

"Delaware Bay is the epicenter of red knots now but a lot of us think they used to spread out up and down the coast," said Barry Truitt, retired chief conservation scientist at the Virginia Coast Reserve of the Nature Conservancy. Truitt studied the birds as they migrated through the barrier islands along the eastern shore of Virginia for more than a decade.

The birds, listed in January as a threatened species, seemed to thrive on their Delaware Bay stopover this spring with 74-percent weighing in at 180 grams or more prior to their migration to Arctic breeding grounds, said Gregory Breese, Supervisory Fish and Wildlife Biologist with the Delaware Bay Estuary Project. The 180 gram weight is the bench mark for peak breeding success.

"It seemed like a great year," he said.

Some birds weighed much, much more. Breese said.

While Delaware Bay is now considered the most critical stopover location prior to Arctic breeding, "what we see now, may not be the way it always was," Breese said.

In fact, there are many unanswered questions about red knots – from what they were feeding on to historic counts to their stopping places over the last two centuries.

Historic records on red knots are scant but Truitt said there is some evidence that back in the day of market hunting and feather collection, train box car loads of dead birds were shipped out of Massachusetts and there are reports of major hunting and collecting efforts on the barrier islands along Virginia's Eastern Shore.

It is there that Truitt and Bryan Watts, director of the Center for Conservation Biology at the College of William and Mary, set out to count spring shorebirds. The team ran weekly aerial surveys between 2005 and 2013 from the last week in April to the first week in June.

Over the years, they found thousands of red knots and other shorebirds, feeding on the barrier islands.

Their conclusion: "the Virginia barrier islands appear to be part of a terminal staging area for red knots that stretches from New Jersey south through the Outer Banks of North Carolina."

This year, the story got even more interesting. Fletcher Smith, a research biologist with the Center for Conservation Biology spent the spring along the coast of Georgia between Tybee and Cumberland Island.

There, each of the major rivers that empties into the ocean forms a sand bar, Smith said.

What Smith and his team found was that thousands of red knots roosted on these sand flats every evening.

At Little Tybee, there was a peak count of 2,700 red knots and nearly 1,000 each at three other locations, he said.

In all, they estimate 6,000 red knots, possibly more, were feeding in this part of Georgia during the spring migration.

A part of the population may arrive early in Georgia and then move north to Delaware Bay, Smith said.

While in Georgia, the birds feed on three things: horseshoe crab eggs. coquina clams and a bivalve found in the area.

Further north along the Virginia barrier islands, the birds feed on coquina clams along sandy beaches and on blue mussel spat (very young shellfish) along the mud flats, Truitt said.

The spat "are about the size of a rice kernel," he said.

The blue mussel connection could be especially important as temperatures continue to warm. Right now, Truitt said, the mussels spawn and the spat grows and thrives at just the right time for the arrival of the red knots. But once the water warms in June "the blue mussel spat all dies," he said. "It's right when the knots come through."

On a typical day in May, Truitt and Watts counted 2,000 red knot on Hog Island.

And some of the birds that feed in Delaware Bay may fly south to roost on Virginia's barrier islands on a regular basis, Truitt said. Scientists already know the birds fly back and forth between New Jersey and Delaware to feed and roost.

Even though birds are being discovered in other locations, federal officials see no indication the Delaware Bay decline of red knots in the 1990s and early 2000s was the result of birds shifting to other staging areas, said Meagan Racey, a spokeswoman for the U.S. Fish & Wildlife Service. That decline has been linked to over harvesting of horseshoe crabs and a drop in the number of eggs available for both red knots and other species of shorebirds.

And Racey said, federal officials working toward a recovery of the population, are concerned about the climate change impact as the blue mussel range contracts to cooler areas.

"It remains a concern for Virginia birds," she said.

As for Delaware Bay, the numbers moving through were steady for the third straight year, she said.

Bracken Brown, Delaware's shorebird project coordinator, said the season started out slow.

"Initially, I was a little worried," he said. "We weren't seeing the big arrivals."

Now, Brown said, he believes that was because there were abundant horseshoe crab eggs on beaches along the Delaware Bay and that allowed the birds to spread out.

"It was a little frustrating for the researchers," he said.

One morning in mid-May, a team that included state and federal officials along with international experts from the British Trust for Ornithology set out near Bowers Beach to capture birds to assess their health.

For hours, they set up their nets and worked to concentrate the birds on one stretch of beach. Then in an instant, a helicopter flew low over the shoreline and the birds dispersed.

There was no catch, no assessment and no color-coded leg banding for that morning.

Brown said that that early in the season, the birds that arrived weighed in at around 90 grams. Before they left two to three weeks later, they needed to double their body weight to ensure breeding success in the Arctic.

"Imagine the amount of eating you'd have to put in to get there," Brown said. The heaviest bird recaptured prior to heading north weighed in at 220 grams, he said.

By the Memorial Day weekend, some 12,000 birds had concentrated in Mispillion Harbor east of Milford, he said.

And then, with a tail wind blowing from the south, they started their flight north to the Arctic, he said.

Probably the biggest departure, he said was on the evening of May 29.

"You can hear how fat the flock is," he said. Rather than flapping wings, it's almost like a buzz. "They are really pushing."

A few days later, all the birds were gone.

Reach Molly Murray at 463-3334 or mmurray@delawareonline.com. Follow her on Twitter @MollyMurraytnj.