The Shorebird Phenomenon

Photo Credit: Patricia Gonzalez

The life history of shorebirds is a chronicle of life on the edge of survival. They seem to heed an irrefutable biological imperative that keeps them moving at a heart pounding pace along the longitudes of the Western Hemisphere. June on their arctic breeding grounds offers many hours of daily sun. When it turns cold in North America, they seek habitats in extreme southern latitudes where it’s summer again. Having double the daylight hours of North America gives shorebirds that much more time to feed and store energy. Food is the constant, driving motivation: for hemispheric migrants, departure time for the next journey is never far off. In their global pursuit of food and breeding grounds, home is nowhere, yet everywhere. As a result they are difficult to track, monitor, and ultimately to protect.

While most shorebird species recovered from the wholesale slaughter at the hands of market gunners in the late 19th and early 20th centuries, the recovery was by no means complete. For many species, the trend has reversed, and a global network of scientists and volunteers is scrambling to understand the gauntlet of natural and human induced perils these intrepid migrants encounter in hopes of stemming their decline. For some species, it is a race against time.

Danger In Numbers
Species like Red Knots and Dunlin that concentrate in large numbers in a single area are vulnerable. Loss of a critical staging area could mean the destruction of a whole flyway population of shorebirds. Example: The 30,000 Red Knots feeding on horseshoe crab eggs in Mispillion Harbor, Delaware Bay are highly vulnerable to human alteration of this resource, or even a catastrophic storm.

Plight of the Red Knot

With annual Red Knot migrations spanning 20,000 miles between the outermost reaches of North and South America, it seems miraculous that an estimated 83% survive to return and breed the next year.

Red Knots arrive in Delaware Bay weighing 90 to 100 grams. There they fatten up to 220 grams (sometimes 240, at which point they can barely fly). 

Increasing body weight by 180% is only the beginning of the amazing physiological and anatomical transformation these birds undergo. While fattening up on horseshoe crab eggs, they triple the size of their liver and double the size of their flight muscles, which eventually serve as a source of protein. To get down to flying weight, they reduce their organ size and resorb parts of their gut. 

The fuel these birds take on in Delaware Bay isn’t entirely depleted on their flight to the arctic nesting grounds. They may arrive to 80% snow cover and the threat of a late spring snowstorm. Insects haven’t emerged yet, so food resources are scarce. To get through the first week or two of pairing, nesting, and egg laying, they have to pack in a lot of energy in the form of fat.

The need to carry in enough energy to successfully breed amplifies the importance of abundant food in Delaware Bay, the Red Knot's jumping off point for their last leg to the Arctic. To make the required departure weight their timing is critical; they may have one to three weeks to make that departure weight. If they don’t make departure weight and attempt to breed, the net effect is increased adult mortality.

A whole-flyway approach is crucial to the birds’ survival – from their Delaware Bay stopover to the arctic breeding and South American over wintering grounds.