The purple martins of Lake Murray

A late summer trip to Lake Murray to watch the purple martin phenomenon has been on my South Carolina bucket list for more than five years, ever since one of my Fish & Wildlife Service friends told me about this amazing experience just 1½ hours from Greenville. This year the stars aligned when The Nature Conservancy announced the purple martin migration boat cruise on a night when we could make the trip.

Photo credit: Roberts

The cruise was aboard Catch the Spirit, a dinner yacht that is also available for public booking; see the Spirit of Lake Murray website. We pushed off from the dock located on an arm of the lake near Oswald Park, and soon were within sight of Dreher Island State Park and the main body of Lake Murray. Our destination: Bomb Island.

The 12-acre island is formally known as Doolittle Island, named for the commander of the 1^st US air raid on the Japanese Home Islands during WWII. The raid was planned in Columbia, and B-25 pilots often practiced on the island, hence the nickname. Records indicate that at least five B-25s crashed into Lake Murray during this period; three were immediately salvaged, and at least one remained abandoned in 150 feet of water until September 2005 when it was once again brought to the surface.  (Today the aircraft can be viewed at the Southern Museum of Flight in Birmingham, Alabama.)

More recently Bomb Island is famous for a different form of aviation - the purple martins. Today Bomb Island (also labeled Lunch Island on some maps) is the largest purple martin sanctuary in North America, with hundreds of thousands of birds flying up to 160 miles every evening to roost there. From the first week of July through the end of August, the purple martins begin congregating over the island at dusk, putting on quite a show with their murmuration.

Purple martins are the largest type of swallow in North America, on average 7.5 inches long and weighing 1.9 ounces. The males are solid black, appearing a glossy purple in the sun. As we cruised through the waters of Lake Murray to a sun slowly slinking in the sky, we started seeing the birds swooping overhead, passing us on their way to the island. Two, then four, then ten at a time. Before we even saw the dozens of boats gathered around Bomb Island we could see the cloud of martins circling above.

The closer we approached, the thicker the sky was with martins. According to the TNC naturalist, the swooping and swirling is a social behavior, not a feeding or mating ritual. The masses appear to descend into the trees to roost, only to take up into the sky again and again for an encore, all the while hundreds more birds are flying in from around the lake.

In late summer after leaving the nesting colony, the purple martins gather in large flocks to socialize, rest, and feed on the insect populations in the area, building and storing fat that will fuel their trip to their wintering grounds in Brazil. The dry island with low thick brush provides a sanctuary from predators and a micro-climate warmer and less windy than land. An individual martin may use a roost for several weeks before moving on, but the migratory roost lasts for about 8 to 12 weeks or more until all the birds are gone.

Every morning the purple martins exit the roost all at once, just before sunrise. The air is so thick with the birds that they show up on the National Weather Service radar; to see it for yourself visit the NOAA radar website for the region between 6-7am, and check “loop” & “auto update.” On the one morning I was able to get online in time there was some cloud cover over Columbia, but the bulls-eye was still visible.

The Lowcountry and GA populations are better visible on this radar shot, source here

Although the Bomb Island roost is well established and has been used for many consecutive years, the number of birds can range widely. A wildlife research biologist at Clemson University studied the roost in 1995, making visual surveys to determine size of the roost, and then comparing it to radar images from NOAA. His estimate was that the total roost population numbered at least 700,000 birds, possibly the largest purple martin roost in the world. However in 2014 the martins didn’t show. Using doplar radar researchers found a roost on Lake Monticello, with a portion of the birds possibly moving southeast to join a roost on Lake Moultrie. They were back again a few years later... My best guess as to the number of birds this year – maybe somewhere around 200,000-300,000? Regardless, it was simply breathtaking.

Out west purple martins usually nest in abandoned woodpecker nest cavities, but here in the southeast the martins depend on humans to build them bird condos, made from wood or gourds. There are two practical reasons for attracting a martin colony to your homestead; first is that they are voracious insect eaters, and the second is to scare away certain less desirable birds. The birds prefer roosting in homes placed away from tall trees – another reason why they might return to Bomb Island year after year, as the bombing runs destroyed most of the trees years ago.

There are still a few weeks left to catch the purple martins before they leave on their epic journey to South America. Grab your binoculars, load up the boat, and head for Lake Murray; a cruise to Bomb Island to enjoy the aerial acrobatics show will become a summer tradition.