The stopover season slowly draws to an end. We made a catch Friday and the average weight compared favorably to conditions 13 years ago. In 1998, the average weight of our catch on May 28 climbed to over 185 grams, just two grams more than Friday’s average weight. These weights suggest the flock is ready to lift off as soon as the weather is favorable, perhaps Friday night or definitely by Saturday. See the histograms below comparing the distribution of red knot weights caught a five days after arrival on May 16, to red knots captured just before departure on May 28th. The other species appeared to arrive later in the season this year, but they too are catching up to their own Arctic flight weights.
Weight Distribution of knots near arrival on May 16, 2010
Weight Distribution of Red Knot near departure.
The distribution of birds on the bay changes ever year, and this year is no different. Last year at this time, nearly all the red knots concentrated in Mispillion Harbor in Delaware after being fairly split on each side of the bay during the rest of the season. Many of the knots in New Jersey concentrated at Moore’s Creek. The year before they favored Reeds Beach and Cooks Beach in New Jersey. This year they stuck mostly to Kimbles Creek and the adjacent Pierces Point and Cooks Beach. At higher tides they moved to Reed Beach, with a second group of knots concentrating at Egg Island and west to Money Island.
What makes a good beach for red knots and other shorebirds on the Delaware Bay? We know shorebirds need high densities of horseshoe crabs eggs. But if this were all they needed, Mispillion Harbor, Del., with egg densities topping 100,000 eggs/sq meter, would be packed again this year. It was not. The reason for this seeming anomoly points to the second most important aspect of good shorebird habitat on the bay: safety from predators. According to Kevin Kalaze, the leader of Delaware Fish and Wildlife’s shorebird program, peregrine falcons regularly hunt the shorebirds of Mispillion Harbor, forcing them to find food elsewhere despite the high densities of eggs. Growing numbers of peregrine falcons on the Delaware Bay and Atlantic Ocean coasts are decreasing the suitability of a number of important areas such as Stone Harbor N.J., and Egg Island. The need for peregrine management , like taking away nesting platforms built on the marsh, will likely become an important issue this year.
Shorebird roost at Egg Island
For shorebirds, safety from predators has two components: The density of predators and the vulnerability of prey. A good site must allow birds sufficient reaction time when a predator approaches. High egg densities in places where birds cannot see all around will likely be avoided if they have a choice of similar egg densities in an open beach with low dunes. My experience on the Texas beaches of Padre Island suggests that knots prefer high densities of cochina clams in areas where the dunes were “blown out” by a storm, creating a wide area for birds to see behind the seashore. Likewise, on Delaware Bay it is likely that – if egg densities are similar – shorebirds will prefer safe areas to unsafe areas.
Blow out on Beach at South Padre Island, TX
Another important feature is the proximity of safe nighttime roosts. Nighttime brings great risk to shorebirds and they choose their nightly roosting sites carefully. Most often it is on a spit of sand reaching far into the sea that is safe from the high tide. Stone Harbor Point and Brigantine Natural Area in NJ are good shorebird roosts. We suspect Egg island Point is a good nighttime roost because it is remote, safe from ground predators and safe in almost all tides.
Shorebird Team at Stone Harbor, NJ
This season, with its favorable conditions, allows us to concentrate on more than just horseshoe crabs and their eggs. What other multiple factors combine to make a good habitat for shorebirds stopping over on the Delaware Bay are important issues that warrant our attention.
