Monitoring the Delaware Bay Shorebird Stopover 2023

Larry Niles – 5/13/2023

We began our 25th year monitoring the Delaware Bay Shorebird Stopover this week with anticipation. Red knot numbers in Tierra del Fuego have increased to around 15,000 from 12,000, according to the survey conducted by Ricardo Matus of Punta Arenas, Chile. It represents the first improvement in several years. Although promising, the new count remains far lower than the 50,000 seen in 2001.

Crabs spawned briefly in mid-April, the earliest in recent memory, but it subsided quickly. A more robust spawn began in early May, and by this week (May 13), egg densities reached about 2251/square meter, a little above average for the last ten years but well above the disastrous years of 2020 and 2022. In both years, the Bay’s water temperature failed to rise above the threshold necessary for spawning, about 59 degrees, until late in the season. In 2020 most shorebirds and nearly all knots had to leave before the spawn got underway and were unprepared for thier final leg of migration to the Arctic breeding areas.

It’s a good start for the red knots, sanderling, ruddy turnstones, semipalmated sandpipers, and dunlin that are only now just arriving. All shorebirds benefit from easily digestible and nutrient-rich eggs, especially Long-distance shorebirds like the red knots of Tierra del Fuego and the many species using the northern shore of South America. They must fly the Atlantic Ocean to reach the Bay, some flying 5 or 6 days nonstop. Eggs on the surface early in the seasons allow all shorebirds to feed upon arrival, and an abundance decreases competition with each other and other species.

The prospects for a better spawn will grow with the New Moon tides starting May 18.

Only small numbers of shorebirds have arrived in the Bay, although they jumped a bit after the southerly winds of the last few days. Most are in the middle Bay at Moores Beach, where we have seen 300 knots, 300 ruddy turnstones, and 800 sanderlings. They were among a 5000-bird flock of dunlin, semipalmated sandpipers, and short-billed dowitchers. This site’s appearance defies its productivity, with the rubble of long abandoned structures strewn all over the intertidal flat. The beach is severely eroded, with marsh muck protruding out from under the sand, which is mostly too shallow for horseshoe crab spawning. Instead, they spawn in good numbers in the creek that drains just north of the Moores Beach road. The site often attracts the first shorebird arriving on this side of the Bay because the crabs spawn in more significant numbers early. This early spurt of spawning is likely a result of the creek’s large marsh drainage, which warms the outflowing water encouraging crabs to breed.

The density of spawning crabs is an important and often overlooked aspect of the crab spawn on the Bay. When crabs spawn, they lay eggs about 6 inches beneath the beach surface. Most often, eggs only reach the surface if another crab digs them up when digging in thier own. Thus the density of eggs at the surface depends on the number of crabs spawning. Too few and all the eggs remain buried. This is the condition of most crab populations because of the people kill them for bait and companies for blood.

But when left to reach the number of crabs a beach can support – or the carrying capacity – then the beach becomes saturated with eggs so that every crab that lays eggs digs up an equal number to the surface. When crabs reach this inflection point, egg numbers skyrocket to the densities the Bay was once renowned for.

Beaches saturated with eggs were once widespread on Delaware Bay before the agencies allowed an overharvest that cut the crabs population back to its existing number. Before the overharvest, the Bay’s crabs produced peak egg densities of over 100,000/square meter. Now we are down to peaks of ten thousand/ square meter, and this depends on ideal conditions. If the water warms too late, for example, as it was in 2020 and 2022, then the density of eggs remains low until after the birds leave for the Arctic.

Determining this inflection point is the focus of another of our research projects. On three study areas along the Bayshore, we erected silt fence weirs where crabs are funneled into a smaller area to simulate a higher density. Susan Linder and her team will sample egg densities in the fenced areas to determine the threshold needed to get to the inflection point. This is the second year of the study.

I write this report after returning from our first day of trapping. Although we came close several times, we could not gather enough knots to catch. We try again tomorrow.