Banded Shorebirds on Delaware Bay

By Luke Musher

Since the mid 1990's biologists have been banding shorebirds along the Delaware Bay shore, a body of water that is an incredibly important stopover site for hundreds of thousands of shorebirds including Red Knot, Ruddy Turnstone, Sanderling, Semipalmated Sandpiper, and many other species.  The main reason that the bay is so critical is the coincidence of shorebird migration and the spawning of horseshoe crabs. Many shorebirds double their weight during their time spent at Delaware Bay, building important fat stores for migration by consuming the crab eggs that become exposed on the beaches, river mouths, and even marshes.

Part of the 1%.  A Semipalmated Sandpiper banded along the Delaware Bay shore in 2010  or earlier that  we recaptured last week.

One method that biologists are using, including the team I work on studying Semipalmated Sandpipers (hereafter SESA), is putting colored flags with unique codes on the birds.  These birds can then be re-sighted by birders.  The recapture rate of these banded shorebirds is roughly 1%, but by flagging the birds biologists can get a 10% return.  Thousands of Red Knots, SESA, Sanderling, and Ruddy Turnstone have been flagged both on Delaware Bay, and on their wintering grounds in South America, and each location has its own flag color.  For example, while woosh-netting at Fortescue this week, we re-sighted a Red Knot with an orange flag.  Although we could not get a combination from it, we knew it had been banded in Argentina by the color of the flag.

One of a few hundred Red Knot present at Fortescue last week, and one of only a few that was flagged.  [Photo by Luke Musher]

Biologists have long pointed out that the shorebirds are feeding almost exclusively on the horseshoe crab eggs while they are here, but certainly many thousands of birds feed elsewhere, away from exposed eggs such as on mudflats in the back bays and marshes at low tide.  The work I am doing with NJ Audubon is continuing what has been done along Delaware Bay for the past two and a half decades, but also supplementing those data with information on what these birds, specifically SESA, are eating.

By taking blood from the shorebirds, we can not only use genetics to find out the sex of each individual we capture, but also fairly accurately evaluate the diet of these shorebirds as they stopover along the bay.  For example, horseshoe crab eggs and marine invertebrates that shorebirds feed on in mudflats have different isotopic signals.  I wont go into too many details about how stable isotopes work or what they even are (this will most certainly bore you and me and probably stable isotope chemists as well), but I will tell you some basics.

Essentially, you are what you eat.  Organisms are made up of, molecularly speaking, the organisms they consume.  If a SESA eats strictly horseshoe crab eggs for days, its blood plasma will contain fatty acids that match the proportion of stable isotopes that are found in the eggs.  This is important because biologists want to know what will happen if horseshoe crab populations continue to plummet.  Will shorebirds be able to replace a diet of mainly crab eggs with other food sources?

A Semipalmated Sandpiper on Kimble's Beach, NJ just after I bled it.

Needless to say, it is very stressful for shorebirds to be netted, handled, and bled.  They already have a long, highly energy-demanding journey to complete without being consumed by predators such as Peregrine Falcons or Merlins.  They only have a short period of time to build the fat stores they need to get to the arctic, and even begin breeding (early arriving individuals may precede the availability of invertebrate prey in the arctic).  However, the more we can learn about how shorebirds utilize migrations stopover sites and wintering grounds, the more we can help to manage their populations and maximize conservation efforts.

One of the greatest parts of this project is that you, aka birders, can help too!  Many people see flagged or banded birds and report them to the USGS or biologists working in Delaware bay, and are helping tremendously with shorebird conservation efforts. Still, many people see flagged birds and never report them.  Perhaps they don't know how to do so, or don't think it matters.  If you see a flagged bird along the east coast (or anywhere for that matter), try to write down the code and flag color.  It is now incredibly easy to report flagged birds.  Delaware Bay shorebird biologists created a website, bandedbirds.org.  Go on and check it out.  There's lots of great information on the project, the birds, and how to report the flags you see.  So if you're going out this month, or ever, looking at shorebirds, don't forget to look at the legs of the birds as well as plumage and structure, and write down your sightings.  Good birding1