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2. Shorebirds Find Enough Food

by Larry Niles
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 Shorebirds find Enough

More red knots came to Delaware Bay this year and most found enough horseshoe crab eggs to fuel the last leg of their 10,000-mile flight to breed in their Arctic home. The birds got the assist from a well-timed and robust horseshoe crab spawn. The weather cooperated, causing only short interruptions in spawning.

Odd then that both populations, the red knot, and horseshoe crab, remain at the same number as previous years. Yes, In 2019, we saw more red knots and horseshoe crab eggs on the bay beaches, yet the knots and crabs numbers remained the same. The reasons behind this contradiction give hope for both species.

 

Precarious Numbers

Many shorebird species, including the red knot, hang precariously at stable but low numbers. In the 1980’s we counted well over 90,000 red knots and a million shorebirds of six different species. All changed after the over-harvest, horseshoe crab egg densities plummetted 80% by the in the 2000s. Shorebirds died as a result of too many birds chasing too few crab eggs.

Still, they kept coming to the Bay. Their numbers remained stable but precarious, and the future looks unpredictable. Horseshoe crabs face the same circumstance. Their number of crabs haven’t changed since fishers foolishly took to many to use low-value bait in the late ’90s and early 00 “S. The population fell by two thirds. Crabs can’t breed until nine years old, so it takes time to rebuild. They haven’t recovered. Baywide egg densities plummeted, leaving birds desperate.

 

New twist of an old survey 

Nevertheless, our experience this year offers a hopeful twist to this long conservation story. I can explain by first looking at the counts of red knots on the Bay. There are two. The first, a 36-year-old survey by people observing shorebirds from the venerable Cessna 172 coordinated with ground and boat counts. I flew these counts early on and can attest they demand a strong stomach.

The flight, led by Guy Morrison along with Joe Smith and Stephanie Feigin, takes two hours to comb the curved coastline of Delaware bay. The plane flies at only a few hundred feet so observers can count red knots and ruddy turnstones roosting or feeding below. Tens of thousands of shorebirds can fill the air while speeding by at 80 mph, so counting becomes difficult to even the trained eye.  

The second method of estimating numbers comes from a statistical model using the ratios of marked and unmarked birds ( marks being the flags with a unique combination of numbers and letters). Each bird’s unique flag code tells us when and where biologists banded the bird and all the places observers resighted her. On Delaware Bay, observers search flocks every day, recording unique flag codes and a regular ratio of flagged to unflagged birds. Statisticians, from USGS and USFWS, plug the data in a complicated mathematical model and an assessment of population size emerges.  

 

Guy Morrison, Stephanie Feigin, and Joe Smith survey shorebirds on Delaware Bay in 2016 from a Cessna 172 flying just a few hundred feet high Red knots lift to flight allowing accurate counting.

 

Each method describes a part of a bigger picture

Both methods of estimation have faults. Observers typically undercount and can’t see birds that have already left the Bay or have yet to arrive. The surveys take place only two times a month while birds arrive and depart according to their needs. Some only pass through and unlikely to be counted. Some stay for weeks. 

The mathematically estimated number of red knots, counts all birds, short or long stays. But the models require a lot of data, evenly collected throughout the stopover period and along the Bayshore. Without good data, the estimates can go awry. 

We failed to do the work in NJ in 2015. The agencies modeled a number using only resightings from one place in Delaware and then only at the end of the season. Magically the red knot population jumped 25% in number. The following year, with data from both NJ and DE collected throughout the period, the amount returned to the 2014 estimates. The method depends on well-collected data to work well.

Which count is the most reliable?  In all but 2015, the estimated number remained around 45,000 birds,  while the visual count hovers at about half the estimated number.   The counter, including this author,  assumed the model estimate high while the mathematicians thought we failed to see 20,000 birds. 

 

A Clearer Picture Emerges

But over the last three years, a different picture emerged.  In 2017, we observed 17,000 red knots, a 30% decrease in the visual count from the previous year.  Then in 2018, the observed number shot up to just over 32,000 and remained about the same in 2019. Yet the model estimate stayed the same in all three years. We started to think twice about our surveys, could we have missed all these birds?.  

This year we found that both surveys correct but measuringing different  We found the observed number reflects the number of birds staying in the Bay and thus finding enough eggs to keep them in place. If egg numbers go down as they did in 2017, then the observed number falls. Birds come to the Bay, find insufficient eggs, and move on. The model estimate still counts these disappointed birds. The observers may or may not. 

Fortunately, horseshoe crab egg densities rebounded in 2018 and 2019. More eggs attracted more birds to stay and fatten up. So in 2017, only 40% of the flock stayed to feed. Nearly 70% stayed in 2018 and 2019. So the first bit of good news is that more birds are staying

But how could this happen when the number of horseshoe crabs has remained unchanged throughout the period?

 

Beach Restoration Taking Effect

The answer may lie along the Bay’s sandy shore. In the two years of very high visual counts, 2018 and 2019, nearly all the red knots in the Bay used one 2 mile stretch. It includes five beaches separated by dead-end roads and small drainage tidal creeks on the northeastern side of the Cape May Peninsula. The American Littoral Society and partners restored these beaches, Reeds, Cooks, Kimbles, Baycove, and Pierce’s Point and continue to so now. This team, of which I am part, has been placing sand, building small oyster reefs to protect the beaches, and experimenting with different sand grain size, dune profiles, and beach berm sizes. We strive for beaches best suited to the needs of horseshoe crabs and shorebirds and resilient enough to add protection for local landowners. That 29,000 red knots, nearly all the Bay’s flock used these five beaches, verifies our approach.

So our work helped overcome the lack of recovery in crab numbers. More crabs came to the restored beaches because they proved more suitable for breeding horseshoe crabs. Higher densities of breeding horseshoe crabs mean crabs digging new nest of eggs, dig up already laid eggs bringing them up to the surface. More crab eggs on the surface attract red knots who stay as long as eggs persist.

 

Above: A comparison of Moores Beach  NJ (Top) and Thompsons beach NJ before and after restoration by American Littoral Society and my partnership.  (Photos by Joe Smith and Larry Niles)

Above: Bags of whelk shell form a low breakwater in Reeds Beach, NJ.  Ultimately marine life, including oysters, will colonize the shell creating reefs protecting the beach from waves during lower tides. (photo by Joe Smith)  Below: American Littoral Society and my partnership built these reefs to test their impact both on the beach and to see if they impeded horseshoe crab spawning.  Below: They did not reduce spawning, instead, the reefs caused sand to settle creating shallow intertidal shoals and slews used by horseshoe crabs to feed and wait for the higher tides to breed. 

 

Chasing Birds

There remains one more part of this ecological puzzle, perhaps in the end, the essential part. Undisturbed, shorebirds will stay to eat horseshoe crab eggs for as long as they need. But it doesn’t always work out that way in nature. Gatherings of shorebirds attract predators like peregrine falcons, merlins, and coopers hawks who attack flocks, keeping them in the air sometimes for hours.

Humans have the same effect. Beach drivers, sport fishers, photographers, and determined birders assume one person cannot harm a flock of thousands of birds?  Few recognize minor but constant disturbance can stop red knots and other shorebirds from eating horseshoe crab eggs even when in high densities.

When informed of their impact, most people willingly give the birds space. In the Delaware Bay, volunteer stewards organized by the Conserve Wildlife Foundation help visitors understand the effect of their actions. If left to roam, people can quickly clear shorebirds from a beach, especially birds gaining weight and flying like their carrying a wet diaper.

Restricting people to observation sites like roads that end on a beach not only prevents people from roaming the beach and pushing away birds.  It also helps birds feel safe enough to feed right up to people observing them. People, especially photographers, still get close views and birds get to feed without interruption. So limiting people to small sites works out to be a win-win for birds and people.

 

Can Shorebirds Hope for a Future?

Is their hope for shorebirds and horseshoe crabs to remain in our future? As a scientist, I cannot wish away my skepticism; our world is indeed on the brink of ecological disaster. Just today, the NY Times reported on an analysis signed by 11,000 scientists practically shouting out the danger we face by ignoring climate change. The world dies while we dither.

Rescuing horseshoe crabs and migrant shorebirds represent a tiny sliver of the immense amount of work necessary to rescue the world from our short-sightedness fueled by greed. But the work here on the Bay provides a glimmer of how good sense finds expression and gives hope for both wildlife and people in an increasingly hostile world.

 

Next post

 

Stewards help people understand why 10 of NJ Beach are closed to people to prevent disturbance to shorebirds while trying to feed on horseshoe crab eggs. Jim May mans his post at Kimbles Beach.
 

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