In honor of World Migratory Bird Day, we're taking a look at the complex and delicate connection that exists between Atlantic horseshoe crabs and a threatened migratory bird call the rufa red knot.
The small but mighty rufa red knot is considered one of the animal kingdom's most skilled long-distance travelers. One red knot that was banded in 1987 and observed 13 years later in the Delaware Bay had flown about 242,350 miles during that time—farther than the distance from the Earth to the moon.
The Atlantic horseshoe crab is another fascinating animal found in the Delaware Bay. Despite their name, these primitive arthropods are more closely related to spiders than crabs, and their ancestors can be traced back more than 400 million years. A female horseshoe crab can lay more than 100,000 eggs in a single spawning season, and while most of them are buried in the sand, many are uncovered by waves and other crabs. These bead-like green eggs quickly dry out once exposed to the elements, and though they'll never hatch, they still serve an important purpose: they're a lifesaving food source for migrating rufa red knots.
Red knots fly a distance that can exceed 9,300 miles when they make their springtime trip from the southernmost parts of South America all the way up to the Canadian Arctic, where they breed and raise chicks. On their migration north, they make a pit stop on the beaches of Delaware during horseshoe crab spawning season, where the eggs provide them with the necessary fuel to continue their journey.
Though the red knots' diet consists of shelled invertebrates for much of the year, easily digestible foods with little or no shell are essential for their migratory stopover diet—the intense journey is so physically grueling, the birds' stomachs and other internal organs shrivel, rendering them unable to feed on their usual food sources. The consumption of horseshoe crab eggs is essential for the red knots to have enough energy to complete the remainder of the journey north, and to breed when they get there.
A healthy presence of horseshoe crabs, and their eggs, in the Delaware Bay is crucial for the red knots—an estimated 90% of their entire population can be found in the area during spawning season. According to the U.S. Fish and Wildlife Service, commercial overharvesting of horseshoe crabs was a major factor in the dramatic decline of red knot populations in the 2000s.
Thankfully, horseshoe crab harvests are now managed to stabilize red knot populations and help them recover, but the problems don't end there. The quantity and quality of beach habitats available for both species have been negatively impacted by coastal development as well as sea level rise, which is a direct result of climate change.
Effects of climate change also impact the location, timing and severity of storm and weather patterns, which can interfere with the red knots' migratory patterns and cause them to arrive in Delaware either too early or too late in horseshoe crab spawning season to consume the lifesaving bounty. Changing weather patterns affect the horseshoe crabs, too, impacting the timing of their emergence from the ocean to shore to breed.
The good news is that the populations of both red knots and horseshoe crabs have stabilized in recent years—albeit, at lower levels when compared with earlier decades—but the story of these species remains a cautionary example of how the actions of humans can have adverse effects on delicate interspecies relationships.
Effects of human-induced climate change aren't the only challenges facing migratory bird species like the rufa red knot. Every year, more than 1 billion birds in North America die after colliding with buildings, mostly during spring and fall migration. Most bird strikes occur in cities, such as Baltimore; because birds migrate at night, they navigate by the light of the moon and the stars, and they become disoriented by the white and yellow lights in cities. After roosting overnight in city trees, they begin flying from tree to tree when the sun rises in the morning. They collide with buildings when they see the reflection of trees on exterior walls, or they see vegetation inside the building.
Spring migration season is in full swing, so if you find a stunned bird on the ground in the Baltimore area, contact Lights Out Baltimore or Phoenix Wildlife Center to report your sighting. Homeowners who have trees across from their windows can move bird feeders 5 feet or closer to windows, and businesses can turn off all non-essential lights at night to lessen the white and yellow lights that attract migratory birds during their nighttime flights. Businesses can also apply a patterned film—like the one found on the National Aquarium's main building and Animal Care and Rescue Center—to the reflective walls of their buildings to deter bird collisions, and change exterior lighting to cool colors, which doesn't attract or confuse migrating birds.
