Migrant Knots on the Atlantic Coast May 2007
Tom Virzi, a PhD candidate at Rutgers working on Oystercatchers, called to tell us that there were 1,600 knots near the Fish Factory near Forsyth Refuge. It was a fortuitous call. Humphrey, who has focused on the red knots using the Atlantic Coast at Stone Harbor, as well as Clive and I were confused by the lack of knots in Stone Harbor. In fact Jim Fraser of Virginia Tech found very few knots on the Atlantic Coast of Virginia as well. Over the last four years we have gradually unraveled the mystery of the Atlantic Coast knots.
Originally we thought they were actually knots coming from the Delaware Bay that went to the ocean coasts and marshes to seek new prey because of the decline in horseshoe crab eggs. But recently we published the more likely scenario that the Atlantic birds represent the southeast US wintering knots, primarily the ones from Florida. In other words, the knots coming to the Delaware Bay shore are mostly birds from South America, who make an arduous and long migration that leaves them depleted and in need of recharging on the bay. They are also the main segment of the red knot population and the group that has fallen by over 70% in the last ten years. The Florida knots in contrast, fly a shorter distance and appear to focus more on mussel spat and small clams all along the Atlantic Coast. But good spat and clam areas shift. Some years Stone Harbor is good, sometimes Virginia is good. This year it appears that the area around the Fish Factory is good.
Clive, Mark, Sue Rice visiting from USFWS and I hopped in the small aluminium boat with some trepidation. The wind howled at 25 knots and we were uncertain if we could find the area that Tom described in nasty weather. We did and within a few hours we had thoroughly scanned the flock for alpha-numeric flags. The distribution paralleled our resightings in the Stone Harbor knots, a disproportionate number of birds caught and flagged in Florida and the Atlantic Coast. The presence, however, of some Chile, Argentine and Delaware Bay flagged birds tells us we don’t have this figured out completely.
The distinction between these populations is key to understanding the plight of the red knot. The key indicator on the Delaware Bay is the ability of shorebirds to gain weight. Many factors complicate the assessment of weight gain besides the availability of horseshoe crab eggs especially when you are using captured birds. For example if you make several catches and see a clear gain in the flock weights you can depend that practically all birds are gaining weight. But if a new flock arrives from South America then the flock weight will decline dramatically because you have the influx of new low weight birds. Individual birds may have gained weight but the average weight of birds caught will have fallen.
Despite these and other complications the assessment of weight-gain is key to monitoring the health of the Delaware Bay stopover. For years the ability of birds to gain weight declined and large numbers of birds left the bay without sufficient energy to get to the Arctic or breed successfully. This caused the population to decline. Now that the population is so low, many of the remaining birds are gaining weight simply because there are fewer birds competing for the depleted resources. Understanding the distinction between South American and Florida wintering helped us understand the vital importance of horseshoe crab eggs to red knot, especially the long distance migrants.
In this blog we will present the weight gain data that we use to assess the birds’ progress. Each data point is an average weight for the birds caught that day. Despite all the complications it is clear why the birds are here.
Mandy Dey and the count plane flying overhead
A dense swarm of gnats at Fortescue