by: Dr. Larry Niles of Wildlife Restoration Partnerships. Dr. Niles is working in Delaware Bay on behalf of Conserve Wildlife Foundation. He has helped lead the efforts to protect at-risk shorebirds and horseshoe crabs for over two decades.
A migratory stopover for arctic nesting shorebirds must provide each bird the energy necessary to get to the next stopover or to the ultimate destination, the wintering or breeding area. Delaware Bay stands out among these shorebird refueling stops because it delivers fuel in the form of horseshoe crab eggs giving birds options. Our telemetry has shown that Red knots, the species we best understand, may leave Delaware Bay and go directly to their Arctic breeding areas, or stopover on Hudson Bay. The choice of going straight to the breeding area or stop at another stopover may be critical to understanding the ecological dramas now underway on Delaware Bay.
I am reviewing a new paper by Sjoerd Duijns, a student working on the benefits of being a fat shorebird. Still, a draft, the paper analyses data from radio-tagged red knots leaving the Bay in good condition (i.e. fat) and finds they may leave later from Delaware Bay than lighter birds but arrive earlier in the breeding grounds because they can pick the best time to leave. They are also more likely to breed successfully and survive the Arctic breeding season to the following fall. In other words, being a fat knot on Delaware Bay makes life good.
So, in light of this new information, how did the red knots and other shorebirds fare in this year’s Delaware Bay Stopover? One must not be firm, with so many unknowns, but here’s a working biologist’s best guess.
By all accounts it was one of the worst years in recent memory, but with a twist that offers a glimmer of hope.
First, the Bay’s water reflected an unusually cool May and never really warmed to the levels necessary for a really good horseshoe crab spawn until the very end. This caused odd occurrences of crab spawning. For example, crabs bred in greater densities at the southern beaches this year, more than in previous years. The spawn at Norburys Landing, just south of the commercial oyster aquaculture development zone (ADZ), was one of the best this year, and knots and other shorebirds used the area in great number. One can only guess the water temperatures warmed over the wide inter-tidal flats provided just enough to elicit spawning. The same process was true of all the creeks on the Bay.
Laughing Gulls and shorebirds feast on horseshoe crab eggs at Norbury’s Landing just south of the Aquaculture Development Zone. (below)The southern portion of the bay was much more important this year because waters warmed faster on the large inter tidal shelf of this portion of the bay.
Second, the knot numbers never really climbed to the levels of the last three years. I’m guessing this was illusory, a consequence of the count being done on two days at the peak. It’s likely many more birds came to the Bay and seeing many birds for too few eggs, left for better resources elsewhere. Those that left were probably short distance winterers – those from relatively close in Florida and other nearby areas. The Bay’s horseshoe crab eggs would help them too, but they can get by on Atlantic Coast clams and mussels. The long-distance birds are the ones that need the Bay’s resources.
Third, when finally, the spawn got underway, a freak concurrence of wind and tide killed many thousands of crabs, potentially damaging the population and very likely ending any possibility of a really great spawn. The cobblestone road of crabs on the water’s edge. We saw none of that this year. Not once.
The upper graph compares predicted high tides ( in blue) with the actual high tide (in Red). On the night of 26 May. This occurred during the lunar spring tide, the highest in May. Finally, a brief burst of NW wind pushed the abnormally high tide into waves breaking across the beach berm, carrying with it tens of thousands of horseshoe crabs.
In a freak concurrence of wind and tide, waves pushed horseshoe crab over the beach into the marsh by the thousands.
Grim results, but here’s the twist. In good years, knots leave near the 27th of May. One day they jam the beach gobbling up eggs, the next day there gone. In bad years, they linger. In 2003, we caught birds on the June 10th. There’s a cost to this of course, in lower survival and failing production. This year was a new in between. By the time of departure on May 27th, less than a quarter of the knots were prepared to leave. But they hung on until the 30th, blessed with a new flush of horseshoe crab eggs created by a middling spawn and a northwesterly wind churning up the beaches and exposing deeply buried eggs. Did the birds gain enough weight?
It’s hard to say, our last catch of just 33 knots suggests they might have, but an end-of-the-season catch makes a poor assessment. Once birds start leaving, the ones behind could be the light birds not ready to leave, or the heavy birds waiting for better weather. We won’t really know until the fall counts in the southbound stopover or the winter count in Tierra del Fuego.
This, our 21st season of intense research and conservation on Delaware Bay by all accounts will be like no other. Throughout all of it, the team of scientists and volunteers remained inspired, energetic and resourceful. In this one month, we conducted more scientific investigation and conservation than most projects do in an entire year. Whatever the outcome of this year’s stopover season, our team can look hopefully to the north and know that all that could be done for the birds was done.
Those of us that were paid for our time sincerely thank those who volunteered their time including; the stewards that manned the closed beaches helping hundreds of people understand why closures were needed; the volunteers in the banding team who endured long hours of preparing equipment, making bands, sewing nets and keeping cages and of course counting, catching and processing birds; the volunteers who doggedly pursue opportunities to resight flagged birds to estimate numbers and yearly survival; the volunteers that provided meals every single night, a welcome relief from a hard day’s work; and finally, the volunteers that went out all over the Bay to save horseshoe crabs in weather both good and bad. We all did our best. God help the birds and horseshoe crabs.
Our banding team on a catch at South Reeds.
Dr. Larry Niles has led efforts to protect red knots and horseshoe crabs for over 30 years.
Our latest catch of red knots and ruddy turnstones two days ago (May 27) suggests 2017 to be one of the most challenging years for our 20 years of work on Delaware Bay. It challenged the birds for certain.
For example, as of two days ago (May 27), the average weights of red knots remain mired in the mid 160’s when it should be in the 180-gram range. This seems a minor difference but to red knots it means a flight through the cold and often inhospitable north country of Canada and dropping out of the sky never to be seen again or landing and never attempting to breed. We really don’t know for sure what happens to ill-prepared shorebirds, except they are less likely to be seen ever again. In 2017 most birds will be ill prepared.
This histogram of the red knot catch made on May 27, 2017 shows the range of weights and the number of birds in each weight category. The average of 168 is far lower than the 180-gram threshold necessary for a successful flight to the Arctic.
This season also challenged our understanding because it lies so far outside the norm. To be sure the cause of this dramatic scarcity of horseshoe crab eggs springs from the cold weather this May. We started with a good crab spawn in the first week of May, when water temperatures rose somewhat faster than normal, a consequence no doubt of one of the warmest winters on record. Then cold and wet weather dogged the Bayshore until the day of this post. Today, temperatures will rise no higher than 70 degrees Fahrenheit and the prospects for warmer weather are unlikely for the rest of the week. The cold air temperatures forced down the bay temperature in the second week and although it has gradually improved it is still low by normal standards.
The Delaware Bay water temperature as measured by the bour off Lewis Delaware. The Bay warmed early until the second week of May when cold weather chilled the Bay.
All of this led to generally diminished horseshoe crab eggs especially on the beaches, the mainstay of most stopovers of the past. This year crabs mostly spawned in the creek mouth and outer creeks shoals, mostly spurred to spawn by tidal waters warmed by the movement in and out of the small estuarine systems. Water moves in with the tide and out again to the creek mouths twice a day warming the shoals.
The Importance of Tidal Creek Mouths and Shoals
A shoal at the mouth of BayCove creek and the crab spawn on Reed creek.
The creeks also accumulate sand leaving them loose and perfect for crab spawning. Even in good years, egg densities in the creeks top all beaches, no matter where they occur. The sands of the shoals loosen by the waves of the Bay and release more buried eggs than those buried in the beaches, making more available to the birds. The creek mouths on the Cape May peninsula from Green Creek to Moores Creek saved the birds from an even worse fate this year and the best were those that benefited from the restoration of Reeds, Cooks, Kimbles and Pierce Point Beaches. As it happens a major portion of the Bay’s knots stayed in this area throughout the entire month but flying widely in search of pockets of good spawning in other places. This year Norbury’s in the south and Goshen in the north stood out, and extended the core stopover area.
Horseshoe crabs breeding at Reeds South shoal.
Spawning Starts Again
Then in the last three days all changed. The new moon tides, reaching around 8 feet for four nights in a row, spurred spawning despite the relatively cool water. Today, May 29, the crabs shifted into high gear.
It takes a lot of crabs breeding to bring eggs to the surface on the beaches, there must be enough breeding for one crab to dig up the eggs of another. For the first time, this occurred in the last few night and we finally saw green eggs on the sand. A welcoming sight for the birds, who could barely stand still and gobble up the fat producing eggs. With most of these Arctic nesting shorebirds remaining in the Bay and apparently feeding right into the night, they still might reach the fat gain finish line and only lose a few days reaching the Arctic. The next few days will tell.
Horseshoe crab eggs litter the surface of North Reeds Beach.
A close up of the same beach. The immense number of eggs on the surface of the beach is in part due to the number of crabs spawning and the wave action of the Bay.
Shorebirds and Gulls feeding near dark in the intertidal flats and creek shoals of Reeds, Cooks, Kimbles and Pierce Point Beaches.
Can they though? A good question and just one of the many that have challenged our team’s knowledge of this well-known stopover. With literally centuries of combined experience (many of our team, including this author, are long in the tooth as the Brits would say) we still kept guessing what would happen next throughout the season. Would the birds suffer mighty declines as a consequence of the generally diminished spawn of horseshoe crabs? Or will they build weight in time to get to the Arctic in good condition? This is usually the central question.
Why Are There Fewer Knots?
An equally intriguing question, however, is why have red knot numbers in Delaware Bay declined this year? We estimate shorebird numbers in two ways, direct observation by aerial and ground counts and a statistically derived estimate based on the resighting of birds flagged with unique IDs. Our aerial and ground counts tell us how one year compares to another because we have been doing shorebirds counts by airplane, boat and on the ground since 1981. This year the number fell dramatically.
Guy Morrison, Christian Friis and Joe Smith survey shorebirds by airplane near Egg Island Point.
At the start of our project on Delaware Bay in 1986, we had nearly 100,000 knots on the Bay and nearly 1.5 million shorebirds of all species. The number of knots declined to around 15,000 in the mid-2000’s, then jumped to over 24,000 over the last four years. This year our best estimate is around 17,969, a 5,000-bird decline. Why did this happen?
One must always consider the possibility of a large group of birds dying. But this is not likely.
More likely, some portion of the knot flock came to the Bay, and on finding too few eggs or too much competition, moved on to better places. The ones moving on could have been the short distance migrants, those who spent their winter in Southeast US or the Caribbean. These birds travel a shorter distance and so have a longer time and lower energy needs than those that winter in South America. These long-distance migrants would have a very difficult time gaining weight on anything other than crab eggs (I explained this in a previous post). There are two reasons to believe the short distance birds moved on from the Bay this year.
The first is the discovery of birds banded in Delaware Bay this year and reported elsewhere. I reported on Mark Faherty’s Ebird report of a knot he saw in Cape Cod, that was flagged by the Delaware Bay Shorebird Team on May 16, 2017. The second line of evidence is the 1,300 birds seen by our team feeding on a 10-mile stretch of the Atlantic Coast marsh from Cape May to Stone Harbor. Play this out over the entire coast of New Jersey and other places with sand and marsh, like Cape Cod, and one could easily imagine 5,000 knots using other places.
About 200 knots feeding and roosting on the inter-tidal shoal of Hereford Inlet on the Atlantic coast.
But this reduction in population also suggests an explanation for the sudden rise in numbers found in 2013. Did the restoration of habitat on the Delaware Bay coast bring back knots that once used the Bay but stopped because of the lack of available eggs? In other words, did the increase in numbers seen in the Bay reflect a return of birds and not a population increase? This year’s loss may simply be a result of those returning birds, leaving once again.
Let’s hope so. At any rate, eggs are now available to all birds in the Bay and we should start seeing them leave for the Arctic. Let’s hope for that too.
Dr. Larry Niles has led efforts to protect red knots and horseshoe crabs for over 30 years.
Four days ago, the shorebirds of Delaware Bay could look forward to a bright future. But in the last week their chances for survival and good production have diminished. In fact, they are as dismal as the cold drizzle pockmarking the murky water in front of our house on Reeds Beach.
The following two graphs tell the story. We captured red knots on May 12 and 16 that showed a normal, although not spectacular progression. Then we made a catch of knots on the 19th and again today on the 23rd, and in total they gained only 2 grams of fat per day. With an average weight of 144 grams at this late date and only 7 days to go before they must leave for the Arctic, their future looks bleak. If current conditions hold, the knots will suffer their worst year in 14 years. Turnstones fared no better gaining less than a gram per day on average.
The graph plots all the average weights of all the catches of red knot our team has made since 1997. The average weight of our catch on May 23 suggests trouble is brewing for these birds.
The graph plots all the average weights of all the catches of ruddy turnstones our team has made since 1997. The average weight of our catch on May 22 suggest trouble for both species.
So why this dismal report? Several factors are at work that were covered in my last post. The water temperature of the bay has only just exceeded the threshold for horseshoe crabs to start breeding in earnest. But the nights are cold and the water temperature remains cold. Last night was the first good crab spawn since the birds arrived and it was lackluster. Although there is some spawning in the creek mouth shoals and the lower beaches near Norburys Landing and Villas, our most productive beaches remain nearly devoid of crabs and crab eggs.
The second problem is still a mystery. At this stage only about half of the red knots have found their way into the bay. Weather patterns in the southern US could have been blocking migration for the last 5 days because adverse winds, poor visibility and rain impedes birds progress and could stop northward movement. The conditions have finally let up, so it possible a new cohort of birds might arrive any day. If so they will face a true food fight with many birds already here and desperate for eggs.
Laughing Gulls, Red Knots, Ruddy Turnstones, Sanderlings and Semipalmated Sandpipers struggle to find horseshoe crab eggs on the shoals of South Reeds Creek. Eggs are scarce because cool water temperatures prevent crab spawning.
However, it may not be the adverse weather but choice that is keeping the population down. Today we learned of a red knot banded on May 16 this year by our counterparts in Delaware that was resighted in Cape Cod by Mark Faherty, the senior scientist for Massachusetts Audubon in Cape Cod. In other words, red knots and other shorebirds are coming to the bay finding too little food or too much competition for the food now available and choose to move on. This is very possible for the short distance migrants because they arrive earlier and have less weight to gain before leaving for the Arctic. Knots wintering in Tierra del Fuego, arrive later and in much worse condition, often times lose muscle mass to get here. They cannot recover that loss and still gain an extra 80 grams on their normal diet of mussels and clams.
We tried to test this second possibility today (May 23) after our red knot catch this morning. Humphrey Sitters, Amanda Dey and I surveyed the intertidal mud flats of the Atlantic Coast from Cape May to Stone Harbor looking closely at the mussel beds. We know that every year red knots, especially short distance birds, use mussels and to some extent clams, rather than come to the bay and feast on horseshoe crab eggs – simply because they can. We found 1700 knots, so it possible even more are spread out further up and down the coast.
In the end, it may turn out to be both explanations. The short distance knots are using the Atlantic Coast and the long-distance birds have yet to arrive. One should remember that the knot is federally listed in both Canada and the US primarily because of the dramatic declines in the long-distance winters. There is still a small window for a successful outcome. The next few days will tell the full story.
Dr. Larry Niles has led efforts to protect red knots and horseshoe crabs for over 30 years.
