In 2015, the USFWS listed the rufa Red Knot as "Threatened" based on a series of significant declines in the number of birds observed at the critical stopover site of Delaware Bay, and the principal wintering areas in Tierra del Fuego, including Bahía Lomas, Chile, and Rio Grande, Argentina. The loss of Red Knots first occurred in Delaware Bay where numbers estimated by aerial survey peaked at 91,000 and fell to less than 50,000 by 2001 then finally to a low in 2005 of 15,000. Using band-resighting data, Baker et al. (2004) demonstrated that a decline in rates of mass gain in Delaware Bay was associated with a reduced apparent survival rate, potentially linking the population declines to failing foraging conditions in Delaware Bay. In a subsequent study, Duijns et al. (2017) used digital VHF telemetry to demonstrate that Red Knots leaving Delaware Bay with low body condition have reduced migratory performance, and potentially a lower likelihood of breeding and surviving through to fall migration. Thus, the number and condition of red knots leaving the bay is crucial to understanding methods of restoring these shorebird species.
The recent inability of shorebirds to gain sufficient weight before flights to Arctic breeding areas is linked to a dramatic decline in horseshoe crab (Limulus polyphemus) egg densities. In the 1990’s, egg densities were estimated on most Delaware Bay beaches at over 100,000/m². By 2000, densities had fallen to less than 5,000 egg/m² and have remained near that level to date. The decline in eggs was a consequence of a dramatic increase in horseshoe crab harvest from around 100,000 in the early 1990’s to 2.5 million by 1998. Although harvests have since been curtailed, the current population of horseshoe crabs is still only a third of carrying capacity. Horseshoe crabs are currently at about 37% carrying capacity according to ASFMC ARM models. Numbers of female crabs are estimated at about 4.5 million but the carrying capacity of the bay is estimated at over 12 million. Thus, the number of horseshoe crabs and their productivity also important in understanding progress towards shorebird recovery.
For over 20 years our team has conducted a series of research and monitoring studies designed to track population recovery metrics in response to conservation actions. These studies primarily focus on monitoring the population of red knots and foraging conditions on Delaware Bay, however our project also includes similar data on the other three species (ruddy turnstones, sanderling and semipalmated sandpipers) to fully understand the progress towards the recovery of shorebirds and the stopover.
The Delaware Bay Shorebird project includes work to:
- Monitor the recovery of red knot and other shorebirds (ruddy turnstone, sanderling) on the Delaware Bay migration stopover; monitor mass gain and year-to-year trends of shorebirds; estimate stopover population size by two methods: bay wide aerial and ground survey, and mark-and-resighting methods
- Monitor horseshoe crab egg densities (an index of shorebird foraging conditions)
- Protect critical habitats to improve foraging conditions for migratory shorebirds.
When asked to describe the ecological conditions of any one year of our 23 years of work on Delaware Bay, Humphrey Sitters, one of the first biologists to understand the value of Delaware Bay to shorebirds would respond “every year is unprecedented.” And so it seemed until this year. Most years begin like all the others but ...