This blog focuses on the conservation of wildlife and rural communities that depend upon them. These blogs are meant to help people who care for wildlife and would like to join in the many efforts aimed at improving conditions for wildlife and habitat. Conserving wildlife not only helps animals but rural communities that rely on hunting, fishing, bird watching, and other outdoor sports to create an economy that overcomes the economic burden of living in important ecological places like Delaware Bay.
Previous Post
A Mad Rush
Nearly all the red knots, ruddy turnstones, and sanderlings have left the bay in a way that suggests a mad rush. We watched 40 groups depart on the evening of May 25 as we ate our dinners on our bayside porch. In total, we counted only a few thousand birds, primarily sanderlings and semipalmated sandpipers, but turnstones and knots also took advantage of the southerly wind to fly north into the fading light of a cloudless day. The wind persisted through the night of May 26, although a low cloud cover obscured the departures. On the morning of May 27, with a stiff westerly wind and clear sky, we found only a few knots and turnstones remaining on our shore. We attempted a catch on May 28, but the tide flooded our net before we could twinkle birds into the catch area, and in a flash, we were done for the season.
A flock of knots, turnstones, sanderlings and semipalmated sandpipers leave Delaware Bay for James Bay as shown by birds tagged with Satelite Transmitters, May 25, 2024. Photo by Joanna Burger
The knots we had tagged with satellite transmitters verified our observations. On early catches, Stephanie Fiegin and Theo Diehl attached 37 satellite transmitters, and this summary map shows the tracks of all the departed birds. The sudden departure left us unable to catch knots just before they left, but earlier catches showed birds in good condition and gaining weight quickly. We did make a late ruddy turnstone catch, and most of those birds achieved a sufficient weight of 155 grams.
The map created May 30 shows the flight of satellite transmitter tagged birds leaving the bay for James Bay. Figure by Theo Diehl and Stephanie Feigin
Weight distribution of ruddy turnstones captured on May 28th, 2024 on Delaware Bay. Figure by Stephanie Feigin.
The good news is that 13,475 red knots left the bay in good condition, and the bad news is that our count is 8,700 birds short of last year’s survey results. World-class shorebird experts have counted shorebirds at the same time every year for more than three decades, one of the longest-standing shorebird surveys in the world. In most places, surveys provide managers with helpful information on the health of stopovers. Unfortunately, the agencies responsible for killing crabs ignore our results.
Yet the agency estimates of knot numbers conducted by the United States Geological Survey (USGS) are of little use to managers. Their “superpopulation method” relies on untested assumptions necessary to adapt an estimation method originally designed for stable populations to a migratory stopover where birds are far from stable. Some birds come and stay for a period, others come and go quickly, while others come some years and not others. The result is large confidence intervals. For example, last year, when we saw and counted 22,266 knots using both ground and boat coats, the USGS estimate was 39,361, an understandable increase because it pretends to measure all birds moving through the bay, whether they stay or not. However, the error in the mean of the estimated number, which spans a range of 16,000 birds, obscures any meaningful change in status, which is no doubt a plus for fishery agencies determined to keep up the killing of crabs.
Table taken from a report by J. Lyons of USGS Red Knot Stopover Population Size and Migration Ecology at Delaware Bay, USA, 2022. The table shows the estimates of red knots based on resightings of flagged red knots. The last column is the results of the earlier counts. The estimate is unresponsive to yearly change and has wide confidence limits making it difficult to make any decisions as to the health of the stopover. By comparision the aerial and ground counts are a snapshot done by professional shorebird biologists conducted the same way every year and provide a useful insight to the health of the stopover.
Assessing the condition of the bay stopover is our work, and the aerial and ground survey numbers shown above describe real changes from year to year, a result of uncertain horseshoe crab spawning. One outcome is birds bypassing the bay the year after low egg densities like they did in 2021. We saw this in the birds with satellite tags attached in Lagoa do Peixe. Most did not come to the bay or never left South America. Earlier geolocator data suggest the same.
Is the bay stopover no longer reliable? Although the birds can find other places, none can guarantee weight gains of over 6 grams/day for all comers. Is reproduction failing because horseshoe crab eggs are good in some years and abysmal in others?
The story of horseshoe crab spawn and egg densities this year illuminates the ecologically complicated relationship between horseshoe crabs and the shorebird stopover. Susan Linder and her hard-working team, including Anh Le, Raina Raphael, and Jared Altomare, will keep surveying eggs until mid-June, so our picture is not yet complete.
Susan’s team surveys egg densities in two ways: counting buried clusters of eggs and loose eggs on the surface. The clusters are loose conglomerations of about 4,000 eggs laid by females that typically lay 80,000 eggs in a season. They bury the plum-sized cluster about 6 inches below the beach surface. Counting cluster densities in a square meter of beach provides the most robust yearly comparisons because of the low variation around the mean.
horseshoe crab egg clusters contain about 4000 eggs and females will lay about 80,000 in one season.
Egg densities on the surface are more variable as result of the location of the beach on the bay, the wind direction and speed, and the density of spawning horseshoe crabs. On the New Jersey side of Delaware Bay, we have two very different beaches. The lower beaches, from Villas to about Pierces Beach, are shielded by an extensive low tide flat that at Norbury landing on a spring tide could be nearly a half mile out. These flats help to reduce wave action at the beach where eggs are laid.
Wave action, the result of both wind speed and direction, is the second greatest influence on eggs. A westerly wind will affect all beaches on the cape because it north-south shoreline, while an easterly or southerly wind has no impact at all. A south wind affects the upper bay to Egg Island because of its east-west shoreline, but it is shielded from westerly winds. This variability of shoreline increases the value to shorebirds because it gives them more choices if bay winds begin to blow.
The most crucial influence on egg densities for our project is the number of spawning horseshoe crabs. It is the key lever managers can move to improve the stopover. The goal should be to reach egg saturation, or that point when a spawning female digs as many eggs as she intends to lay. All beaches were saturated before the overharvest, with surface egg densities exceeding 50,000/square meter instead of 5000/square meter the average of the last 9 years.
Horseshoe crabs breed on Moores beach NJ while wind generate waves wash over them.
Egg cluster and surface egg densities in the first and second week this year were outstanding, besting the last nine years by substantial margins. Cluster density and surface eggs differed in the third and fourth weeks. Cluster density was better than previous years but not by much. Surface eggs were better but not by much. Eggs in the third and fourth weeks were about average for the last few years. Why were clusters outstanding, but the surface eggs were only above average?
The conditions this year were unusual. Early May was warm, with bay water temperatures above the threshold of 59 degrees until the second week when they fell below. Hot days returned in the third week and stayed warm for the rest of the season. This change in weather explains the great early spawn when crabs buried so many clusters. A nearly month-long calm sea also helped. We had winds from the east or south and light for weeks, so the crabs easily spawned free from onshore waves, allowing spawning without the interruptions normal in the often tumultuous weather of May.
The larger question is whether more crabs spawned this year. At this point, it appears the egg densities improved, but was this because of an increase in crabs or better spawning conditions? Susan will collect more data until the middle of June, so the picture might clarify.
These histograms describe the density of horseshoe crab clusters and surface eggs on NJ Delaware Bayshore. Each bar is the mean for for weeks 1 -2 week and weeks 3-4 for each year. Although far below the 50,000/eggs square meter seen in 1991, this years 3-4 week estimate of nearly 11,000 eggs/sq is an improvement from densities 10 years ago. Figures by Susan Linder
Is this year different? Yes, because the birds that did come to the stopover left early and in good condition. Most knots left by May 26, a good omen for this year’s production. ¨ Unfortunately, it is also different because we saw 8,000 fewer knots than last year and half of what we saw in 2018 and 2019. The reason is probably as complex as the migratory strategy of shorebirds, but our satellite tags suggest birds are staying in South America and going to other places than Delaware Bay. This suggests the stopover is losing its value to the birds; the population of crabs is too small to ensure reliable abundance throughout the season every year as it once did. For the birds coming to the bay, finding food is a crapshoot.
A sign posted by NJ Division of Fish and Wildlife protecting feeding and roosting shorebirds
The last component of creating a good place for birds to stop is to protect them while they feed. New Jersey Fish and Wildlife posts beaches, and the Conserve Wildlife Foundation of New Jersey fields volunteers to help visitors understand the need for protection. We added to the stewards with a new group of shorebird interns fielded by Citizens United to Protect the Maurice River, and we added new materials to help the visiting public understand the need for protection.
This protection from disturbance should be expanded to Delaware, whose agency’s motto is “educate, not regulate.” But this is inappropriate for the stopover and not suitable for a federally threatened species. Red knots are very sensitive to disturbance, as are all the migrating birds that balloon into fatties that can hardly fly. Joanna Burger and I published on the need for protection a decade ago. Besides stopping ATVs ‘ four-wheel drives, photographers and even birders allow birds to forage right up to the boundaries of observation areas, allowing a greater number of people to enjoy the birds.
It’s a good policy for most people.
See below a video showing female horseshoe crabs digging in thier eggs while trailing males fertilize them on Reeds Beach NJ this year by Mandy Dey.
