A mountain of fieldwork, fleeting lousy health, and several important visitors made an update difficult, so I will do my best to get to the heart of what we think is happening in the stopover at this critical stage when red knots and other shorebirds are getting ready to leave the bay.
First, the question posed at the start of the season in this blog on whether the early spawn in the first week of May was a harbinger of better conditions or a tragic timing mismatch of bird and crab, the former answer proved true. In the last few days, during the full moon spring tide, horseshoe crabs carpeted the beaches of the lower Cape. Reed Beach, with its naturally deep sands, was so pockmarked by partially buried horseshoe crabs. Humphrey Sitters said, ” It looks like a battlefield”. Part of the reason is the settled water of the last week, where nearly every day brought either slight winds or winds from the east, which produce no waves. Sheltered from strong winds that can create waves, overturning crabs and shutting down spawning, the crabs had excellent conditions for spawning. This week, Susan Linder and her team found egg cluster densities higher than in any of her surveys in the last few years. It’s still uncertain if there are more crabs.
The horseshoe crab spawn on Reeds Beach on May 25th 2024
Unfortunately, not all the news is good. We conducted our annual baywide count in NJ with ground crews at each of the major beaches in the accessible Cape May Penisula Bayshore and by boat along the shoreline north of Reeds Beach to Money Island. Dave Fanz piloted his 16-foot Jonboat with Stephanie Feigin and Alinde Fotjik, while I piloted our 17-foot Boston Whaler with Humphrey and Phillipa Sitters. We counted a total of 13,475 knots on the NJ side and another 400 on the Delaware side, numbers that is well below the count from last year at 22,107 and only half of our count in 2018 of 33,000 knots. The question that arises is: why the decline and is it a real concern?
Results of aerial and ground count survey of Arctic Nesting Sborebirds staging on Delaware Bay from 1982 to 2023.
In a blog I posted last year, I explored the impact of unpredictable crab spawn on red knots, which cannot know what to expect when they leave wintering areas thousands of miles to the south. In 2020, cool weather in the 2nd and 3rd week of May prevented crabs from spawning with water temperature below the 59-degree threshold necessary for spawning. Red knots arriving from wintering areas in Tierra del Fuego and Northern Brazil found few eggs, and the 19,397 red knots we counted left before the crabs spawned, while others simply bypassed the bay. The impact of this unpredictable crab spawn was felt the following year when we counted only 6800 knots.
The numbers improved the next year to over 12,000 who came to the bay and found a well-timed crab spawn. The number climbed again in 2023 to over 22,000, who again found a well-timed crab spawn. So why the decline this year?
Horeseshoe crab surface egg densities 2015-2023. Each line represent a week in May ( 1-4)
One possibility is that more will arrive, a guess made by the British Trust of Ornithology in 2003 with a report that suggested the crash of the red knot population was only a result of a long-term trend of birds arriving late and presumably uncounted. Time discredited this wrongheaded interpretation, a favorite of bureaucrats hoping to shield the aquaculture and fishery industries, as the number of red knots continued to decline to unprecedented lows with no recovery.
Over the past 5 years, Stephanie Fiegin and I, with support from Paul Smith of Environment Canada, Christian Friis of The Canadian Fish and Wildlife Service, and Pam Loring and Wendy Walsh of USFWS, have been conducting research using satellite transmitters. These tiny devices have provided a wealth of locational data and detailed timing of birds traveling along the flyway. Prior to this, Joanna Burger and I, with support from many of the same agencies, tracked red knots using geolocators, small devices that only record light and time, enabling a rough estimation of location and timing.
Results from both devices describe similar outcomes. For example, the map below shows the tracks of the 14 birds with a satellite transmitter attached in Lagoa do Peixe in 2023. Of the 14, only three came to the bay, and the rest found thier way using other stopovers or never left Brazil. The results from this year’s tags appear to be similar.
A summary image of all the tracks of red knots with satellite transmitters attached in 2023 in Lagoa do Peixe Brazil. Map by Stephanie Feigin and Theo Diehl.
Joanna Burger, in Burger et al 2022 describing the reproductive success of red knots using geolocators, found similar results but concluded that the red knots that came to Delaware Bay had greater reproductive success. In other words if a bird comes to Delaware Bay and gains weight by eating horseshoe crabs eggs and leaves fat, she will be well prepared for producing young when she nests in the Arctic.
Sjoerd Duijns’s 2018 paper Body Condition Explains Migratory Performance of a Long-Distance Migrant, concluded that knots leaving Delaware Bay in good condition got to the breeding ground faster, were more likely to breed successfully, and were more likely to be detected in the southbound flight. In other words, the birds in poorer condition when leaving Delaware Bay are less likely to breed successfully or survive.
So the implications of the red knots’ avoidance of Delaware Bay are far-reaching. While fishery agencies may interpret avoidance as verifying their mind-boggling conclusion that red knots dont rely on Delaware Bay’s horseshoe crabs, the more plausible and alarming conclusion is that red knots bypassing the bay or leaving in poor conditions are producing less young and dying at faster rates adding up to a real population decline.
