Martins & Swallowers: The Marsh’s Aerial Acrobats
Purple martin colony
Spring is, quite literally, in the air. While fall migration in Cape May is arguably more famous, with the hundreds of raptors, monarch butterflies, and songbirds gathering and waiting at Cape May Point for the perfect conditions to cross the Delaware Bay, spring migration holds many special moments as well.
Whether it be the pairs of ospreys returning to their favorite nesting platforms on the creeks; the scores of red knots scouring the beaches along the Delaware Bay for horseshoe crab eggs; or the many species of colorful warblers traveling north from the tropics, the arrival of migratory birds from their southern wintering grounds comes with great anticipation each year. Every spring, we at The Wetlands Institute make special preparations to welcome back two aerial acrobats to the marsh, and this year was no exception.
On March 23, we pulled our two “gourd racks” out of the shed and gave the plastic gourds a thorough cleaning before lining each one with a generous layer of fresh dry pine needles. Once these racks were ready, we raised them onto a pole about 20 feet off the ground to create a kind of skyline hotel and awaited the arrival of the first purple martin (Progne subis) “scout.” Scouts are typically the oldest martins in a population, returning to previous nesting sites; they can be male or female birds. Over the next several weeks, more adults arrive, followed by the subadult fledglings from the previous year. This year, the first scout arrived at The Wetlands Institute on April 3. There are more than a dozen now, and our hope is that by mid-May, more purple martin pairs will fill the rest of our specially prepared gourd nest cavities to complete their nests and lay eggs. Purple martins use the pine straw we provide and add twigs, mud, and finally green leaves before laying an average of four to six small white eggs.
The purple martin is the largest member of the swallow family in North America, and represents one of many types of birds that nest communally. Like other swallows, they catch insects as their main food source, swooping and diving about the marsh to grab dragonflies, moths, beetles, flies, and other flying insects – often at altitudes of 500 feet or more (higher than any other swallow). Unfortunately, mosquitos are not a major part of their diet, but we can thank martins for keeping the biting fly populations at bay. On the East Coast, purple martins nest almost exclusively in human-made nesting structures, while Western birds use natural nest sites like woodpecker holes in dead snags. Hundreds of years ago, well before Europeans arrived in the Americas, Native Americans used hollowed gourds to attract martins, probably recognizing their value as natural insect control. This, along with a loss of natural nest cavities, likely led to this shift in nesting behavior.
While purple martins nest in colonies, preferring to construct their nests in close proximity to one another, tree swallows (Tachycineta bicolor) like to build their nests with a little more privacy and elbow room. At The Wetlands Institute, we maintain nine or 10 tree swallow boxes throughout the marsh, mounted on poles approximately 6 feet off the ground and set about 30 feet apart. We fit each pole with a PVC pipe below the box to deter predators like raccoons and rat snakes. This season, the TWI research team replaced five older, smaller boxes with new ones constructed to the ideal dimensions to attract tree swallows.
Once a pair of tree swallows selects a suitable nest box, they collect fine dried grass to build a tidy cup nest inside. When the cup is complete, the pair collects feathers to line the cup before laying four to seven white or slightly pink eggs. For most of the year, the tree swallows’ diet is similar to that of purple martins – predominantly insects, though they supplement with calcium-rich foods like small fish bones, pieces of clamshell, and eggshells. Tree swallows can also digest bayberries and other small fruits in colder weather when insects are scarce – something no other swallow can manage. This ability allows them to winter farther north than any other American swallow and also to return to their nesting grounds far ahead of other species. Tree swallows spend their winters along the Gulf Coast, Florida, the Caribbean, Central America, and the Baja Peninsula.
During the latter half of the 20th century, tree swallows and purple martins, along with eastern bluebirds and other cavity-nesting birds, experienced significant declines with the increase in human development and the introduction of invasive species like the European starling and house sparrow. Today, dedicated efforts to manage nesting boxes/gourds for swallows and martins are helping these species recover. Since 2016, nests at The Wetlands Institute have fledged more than 425 purple martin hatchlings and almost 40 tree swallows.
Both purple martins and tree swallows are well known for their mass migrations in the fall, and the marshes along the Maurice River here in South Jersey are considered the largest staging area for purple martin migration in North America. In mid-August, hundreds of thousands of purple martins gather in the marshes, gorging themselves on insects to fatten up for their long migration to South America. Each evening at dusk, the birds fill the sky above the marsh, swooping and diving in what seems like an orchestrated ballet and then, all at once, they descend into the reeds to take shelter for the night. These collections of martins are so large they can be seen on weather radar. They perform this aerial circus act for a couple of weeks before leaving in smaller, but still impressive, groups to head toward their winter roosting grounds in Brazil.
Tree swallows also congregate in flocks numbering into the hundreds of thousands to migrate en masse toward their wintering grounds – covering power lines, wires, groves of trees, cattail marshes, utility towers, or wherever else they can find a suitable roost. On Seven Mile Beach, large numbers of tree swallows feed in the dunes in September in preparation for their long flights south. This strategy of “safety in numbers” is employed by many migrants, including several species of blackbirds and waterfowl, and observing a coordinated migratory flock of tree swallows or purple martins is one of the great natural spectacles we can witness.
Though the season is just beginning, both purple martins and tree swallows are busy establishing territories and scoping out their respective nest cavities, preparing themselves for the business of raising the next generation. Before long, the skies above the marsh here will be filled with young birds taking to the air on newly minted wings, honing their flying skills, building strength, and fattening up for their first epic migratory journey. With a little luck, they will be back next spring – and we here at The Wetlands Institute will once again be ready to welcome them home.