Shorebirds of several different species – most prominent among them a group of red knots – gather on Delaware Bay
beaches during the spring migration stopover at Mispillion Harbor /Delaware DNREC photo: Rob Robinson
Restored Mispillion Harbor Drew Bay’s Earliest Red Knot Flocks During Stopover for Fueling Flight to Arctic Home
Early in the migratory stopover period, observers with the collaborative monitoring effort known as the Delaware Shorebird Project counted nearly 2,000 red knots in Mispillion Harbor alone on May 11, while flocks on the New Jersey side remained minimal. Counts at the harbor regularly topped 1,000 birds and peaked at 3,000 birds on May 20. Red knot flocks stayed in Delaware throughout the season, a break from recent years.
Weight data, collected from 377 individuals in capture events between May 10 and May 30, showed a steady increase throughout the season, with average individual weights by date at or above those of the historical red knot data. During the early and mid-season, weights were particularly high relative to previous seasons, exceeding historic weight averages by date – with 10 out of 35 birds caught on May 21 exceeding 180 grams (g), the estimated minimum departure weight for red knots.
Later in the season, weights became closer to the historic average, as Delaware’s group of early arrival red knots apparently fattened and departed (per capture weights and flagged bird resighting data) and skinny, late-arriving birds continued to filter in. In the final catch of the season on May 30, the disparity between the lightest and heaviest birds was nearly 120g, with new birds arriving even as many birds had already departed the stopover to continue their migration.
Counts on the Delaware coast had been declining over the past several seasons even as model estimates showed the red knot stopover populations to be relatively stable in the Bay, according to “Stopover Population Estimate and Migration Ecology of Red Knots C. c. rufa at Delaware Bay, USA, 2025,” by Dr. James E. Lyons of the U.S. Geological Survey. The birds have concentrated along the New Jersey shore, where factors, such as large stretches of protected roosting marsh adjacent to foraging beaches, likely make it overall preferable red knot habitat – a disparity that may be growing as sea level rise and development limit and degrade roosting habitat in Delaware. Additionally, under typical conditions, the coastline topography and sea temperatures make the shallower New Jersey coastal waters warm quicker and reach the threshold temperatures for horseshoe crab spawning earlier. New Jersey recorded its highest red knot counts in recent history this spring; Delaware’s counts rose alongside them.
A colder-than-average spring shifted Bay-wide spawning timing in 2026. Delaware’s Mispillion Harbor, a restored habitat designed to support horseshoe crabs and shorebirds, warmed earlier than many other Bayshore locations and became one of the first areas with suitable horseshoe crab spawning activity. As a result, some of the season’s earliest red knot flocks settled in Delaware.
“These findings indicate that complex factors such as migration timing, habitat conditions, animal behavior, and the availability of suitable feeding and roosting areas interact with overall horseshoe crab abundance in determining where red knots concentrate each spring,” said Delaware Shorebird Project leader and DNREC Coastal Waterbird Biologist Kat Christie. “While horseshoe crab spawning density in Mispillion Harbor was not noticeably different this year than years where red knots were largely absent from the shorebird flocks there, the red knot response to it was like night and day.”
Delaware Shorebird Project partners – including DNREC, the U.S. Fish and Wildlife Service, the U.S. Geological Survey and the Wash Wader Research Group – capture and band birds using uniquely coded leg flags that allow researchers to track individuals throughout their migration. During the 2026 season, researchers in Delaware flagged 343 red knots, recaptured several dozen more, and recorded nearly 4,000 red knot resightings, while related work occurred in New Jersey and other critical shorebird sites. Observations included birds banded in Delaware, New Jersey, Argentina, Brazil and Canada.
The project also flagged 511 ruddy turnstones, 288 sanderlings and 193 short-billed dowitchers, while deploying satellite transmitters on eight ruddy turnstones as part of the final year of a three-year tracking effort. In total, researchers collected more than 22,500 flagged shorebird observations during the migration season.
Mispillion Harbor has long been Delaware’s most important shorebird feeding area, providing sheltered beaches, calm waters and abundant horseshoe crab eggs. The 2026 season demonstrated the critical value of Mispillion Harbor as a resilient foraging habitat during challenging environmental conditions. Continuing collaborative research into shorebird habitat use in and around Mispillion and in New Jersey could provide managers with further guidance for replicating this success elsewhere and helping ensure red knots have suitable foraging and roosting options in Delaware Bay regardless of the annual conditions.
In a year which could have been potentially disastrous for red knot with delayed horseshoe crab spawning, Mispillion Harbor seems to have provided a safe haven for these birds and the horseshoe crabs they depend on. DNREC biologists are eager to continue to document the complex factors impacting migratory shorebird flocks in order to improve Delaware’s shorebird habitat and ensure the Delaware Bay continues to do its part in the astonishing annual cycle of these hemisphere-navigating birds.
For more information about the Delaware Shorebird Project, visit the de.gov/shorebirds webpage or contact the Coastal Waterbird Program at deshorebirds@delaware.gov.
About DNREC
The Delaware Department of Natural Resources and Environmental Control protects and manages the state’s natural resources, protects public health, provides outdoor recreational opportunities, and educates Delawareans about the environment. The DNREC Division of Fish and Wildlife conserves and manages Delaware’s fish and wildlife and their habitats, and provides fishing, hunting, wildlife viewing and boating access on more than 75,000 acres of public land owned or managed by the Division of Fish and Wildlife. The DNREC Division of Watershed Stewardship develops and implements innovative watershed assessment, monitoring and implementation activities.. For more information, visit the website and connect with @DelawareDNREC on YouTube, Facebook, Instagram, X or LinkedIn.
Media Contacts: Michael Globetti, michael.globetti@delaware.gov; Nikki Lavoie, nikki.lavoie@delaware.gov
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