Bumping along a dusty ranch road at the base of the Glass Mountains in west Texas, a fellow grad student and I were intent on finding grasshoppers for a research project. When we stopped, I noticed a large group of cave of swallows swirling around an opening on the side of a mountain. We climbed up the slope and found a sinkhole about 180 feet deep. Cave Swallows flew all around in what we soon realized was an important nesting site, a colony of 500 birds.
Swallows are small songbirds known for their fast, smooth flight. Of the eight species that breed in North America, the Cave Swallow has the most limited distribution. One population breeds in the Caribbean and south Florida, and the other occurs in Texas, north-central Mexico, and southern New Mexico. It’s not threatened or endangered, but until the last few decades, we didn’t know much about it. My efforts to learn more started at that Texas sinkhole and continue to this day.
In the fall of 1978, I moved back to my home state of New Mexico and spent the next couple of years, off and on, observing Cave Swallows at the mouth of the large entrance to world-famous Carlsbad Cavern in the national park of the same name. I had seen the birds in New Mexico before, but for a long time, the only way to find them was to make a long, hot hike into the backcountry of the park to a few obscure caves.
Fortunately that changed in 1966, when three pairs showed up at the cavern. The cave, which is visited by hundreds of thousands of people each year, suddenly became one of the easiest places to see the bird. (It’s also known for the nightly spectacle of Mexican free-tailed bats exiting the cave in mind-boggling numbers.) For a while, going to the entrance between March and October was one of the only sure ways to add the Cave Swallow to your life list. The population soon grew noticeably.
In 1980, I requested permission from the National Park Service to band Cave Swallows at the cave. Part of my justification was that it was one of only three bird species in North America for which the winter range was not known. (The others were Ross’s Gull and Antillean Nighthawk; today we know where the gull winters, but the nighthawk’s winter range remains a mystery.)
Monitoring a species is difficult at best if you don’t know where it breeds, its migration path, or where it winters. So the quest began, but the start was rocky.
The administration, for reasons that were never clear, said I could band birds only on the top of the ridge, near the cave entrance, at dawn. The nets had to be down half an hour before the cave opened for visitors. The restrictions made my work extremely difficult. Once the birds exited the cave, they scattered, and my three-meter-high nets, placed amid cactus, sotol, and ocotillo, were never in a good spot. The birds simply flew over them.
In the first two years and after many trips, only 30 birds had been banded. I revisited the administration and told them that the project was done unless we could band inside the cave entrance. We simply had not been able to get the numbers we needed to make it worthwhile. The authorities finally gave permission to band in the evenings after the cave entrance was closed to visitors and we were deep enough in the cave not to be seen by others. The project quickly got off the ground.
On our first evening in the cave, August 5, 1982, we banded 30 swallows — as many in one night as we had banded in the last two years. A few days later, we got 68 more birds and had almost 500 by the end of the year. By 1984, we were banding 1,000 birds a year at the site and recapturing several hundred more. While the original project had been to band as many swallows as we could and hope for a recovery on the unknown wintering grounds, it quickly evolved into gathering as much information about the species as possible. We measured the wings and tails, weighed the birds, and noted their ages. Later we looked for brood patches so we could tell which birds were male and which were female. We also noted what insects the swallows were carrying, whether they were holding mud for a nest, and which ones had ectoparasites (organisms that live on the bodies of birds and other animals).
In other words, we tallied anything and everything we could on each individual bird. On retraps (that is, swallows we captured, banded, released, and then captured again), we weighed them and compared the data with the weight on the last encounter.
So how do we capture birds inside an iconic natural landmark? At the cavern’s amphitheater, where people sit to watch the thousands or hundreds of thousands of bats, we move down into the cave just out of sight of the seating area. We stretch a net that is tied to 10-foot-long aluminum poles across a wide spot in the cave, and volunteers hold it in place. After birds strike the net and become entangled, we lower it, quickly remove the birds, and collect the data. Birds are released immediately after being processed.
We’ve had our share of unusual birds. In 1986, we banded a pure white young Cave Swallow. Its eyes, feet, and bill were colored normally, so it wasn’t an albino.
We’ve also banded a few hybrids — birds with intermediate characteristics between Barn and Cave Swallows. Oddly, the two dozen or so hybrids that we’ve found have come in the last week of August to the second week of September, and then they were never seen again. The only exception was an adult hybrid we found one spring. It was banded and released. Then it disappeared.
