The Cave Swallows of Carlsbad Cavern

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 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.

Grasshoppers quickly took backstage as we watched the swallows spiraling down and feeding nestlings. In a few days, I returned to band some of the birds. Little did I know that more than 30 years after that 1978 encounter, I’d still be studying the species.

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.

Underground bird study
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.

Where they winter
After a decade of work, we had learned a lot about Cave Swallows, but we still were not getting any recoveries from their wintering range. One winter, Jim Walters, a good friend and long-time supporter of the project, and I drove all over the cave-rich areas of northeastern Mexico, thinking they might be wintering there. But after many miles and many caves, we never saw a single Cave Swallow.

I even spent a year teaching on the south edge of the Amazon basin in Bolivia, hoping that perhaps Cave Swallows might winter somewhere in South America. Their closest relatives, Barn and Cliff Swallows do, so it seemed natural Cave Swallows might join them. Although I saw tens of thousands of other swallows, I never got even a hint of a Cave Swallow. In my absence that year, a Park Service employee, Tom Bemis, carried on the banding.

After returning, I didn’t know what to do except just keep banding. We banded in backcountry caves and learned about the birds’ movements from cave to cave. We kept building up our numbers of newly banded birds (8,000 by the end of 1991), and each retrap told us more about longevity, survival rates of different classes of birds, and weight change influenced by rainfall. By the end of 1991, we had more than 7,400 retrapped birds, which included many cases of multiple recaptures of the same individual. So we plodded on, not knowing that a bird we had banded in 1988 would provide an answer to the question that sparked this quest.

In the first week of January 1992, duck hunter H.L. Walther walked onto his front porch the morning after a storm in San Patricio, in the west-central state of Jalisco in Mexico, and found a dead bird. As he prepared to discard it, he noticed a band on its leg and removed it. Walther sent it to the U.S. Bird Banding Laboratory in Maryland, and just like that, we had a clue. But could Jalisco really be where the swallows wintered? No records existed for the west coast of Mexico, so perhaps the bird had just been lost. The picture finally came into focus in 1994, when ornithologist Oliver Komar discovered thousands of Cave Swallows wintering in El Salvador. Komar is now science director for the El Salvadoran conservation group SalvaNatura. Further searches found that the majority of the population wintered from the west coast of Mexico south to El Salvador and casually to northwestern Costa Rica. Walther’s bird hadn’t been lost after all.

Why did it take so long to discover the wintering range? I had looked for the species in Mexico in caves in the winter, but in fact, the birds seem to avoid them. In El Salvador, the birds were roosting in stands of sugar cane, grain fields, and mangroves. They would feed almost until dark, when they would dive into the vegetation. Most birders in Central America focus on rainforest, elfin cloud forest, and other habitats where the tropical gems can be found. So it’s not too surprising that the swallows were overlooked. After living in Latin America for five years, I doubt that I’ve spent more than a few hours looking for birds in sugar cane and certainly never as the sun is going down.

New nest sites
Cave Swallows have shown us that we should never engrave in stone too much of what we know about any species. While I’ve been studying the birds in New Mexico, the swallows have expanded their range significantly in Texas. In the mid-1980s, they started using bridges, culverts, and old buildings for nest sites. Many colonies have sprung up along highways, so today the species is as easy to see in much of the Lone Star State as it is at Carlsbad Cavern.

Thanks to reports from birders, we also know that Cave Swallows have been wintering in Texas since the 1980s. The overwhelming majority heads south, but a few birds spend the cooler months in Texas along streams, rivers, and lakes.

More perplexing have been the now annual pre-migration movements of Cave Swallows into the eastern United States, mostly in October and November. The flights seem to be associated with low-pressure weather systems.

A few old records on the east coast were thought to be associated with hurricanes, but the recent sightings, which can involve hundreds of birds per year, are harder to explain. Swallows have been reported from Wisconsin to Maine and south to the Gulf coast, with the exception of only a couple of states. Even after more than 30 years, we still have much to learn about the Cave Swallow.

I started to study the species by hoping to answer a simple question — where do the birds go in winter? — but over time, my colleagues and I have learned about longevity, sex ratios, production of young, variations in plumage, food habits, local movements, and much more. In the first years of our project, the birds arrived by mid-March and were generally gone by mid-October. Now they usually arrive in early February, and some stay through the middle of November.

We wouldn’t have progressed this far without help from volunteers. Through our 2010 season, more than 5,000 people from 41 states and 20 nations have held net poles, removed birds, written data, or performed other duties. Several thousand college students, as well as young children and a congressman, have made our project successful. I hope they have taken a moment to reflect on the experience and realized there is so much yet to learn and so much to protect.

Steve West is the staff scientist for the New Mexico Wilderness Alliance and a former high-school and college science teacher. He wrote the Cave Swallow account (No. 141) in the Birds of North America reference series, and he is the author of Northern Chihuahuan Desert Wildflowers (Falcon, 2000). West also has worked with conservation groups in Latin America and has conducted grassland-bird surveys in New Mexico for several decades.

Originally Published August 20, 2010