by Alexandra Forsythe
Want to start a stampede? Turn toward a crowd of birders and yell, “Kirtland’s Warbler!”
The Kirtland’s is one of the most sought-after birds, especially in the springtime when a variety of warblers travel through Indiana, dressed in their most colorful finery, on the way to their breeding grounds. The Kirtland’s is not the most colorful of the warblers, nor is it one of most beautiful singers, but it is quite elusive, making it one of the most prized birds for listers.
The Kirtland’s Warbler’s story is one of conservation success. In 1970, they were heading toward extinction. Today, they are rebounding, but their ability to succeed rests largely in our hands. Fortunately, the Kirtland’s has captured the attention of brilliant scientists in the birding world and many studies are underway to determine the best methods of helping these birds continue to prosper.
Via email, I spoke with Dr. Nathan Cooper who is at the helm of one of the most comprehensive studies on the Kirtland’s. He and Pete Marra, director of the Smithsonian Migratory Bird Center, obtained funding to study the life history of the Kirtland’s Warblers using geolocators and radio-tracking devices. Dr. Cooper, his assistant Tom Ryan and volunteer David Bryden, are committed to spending time in the field, both in the breeding grounds and in the wintering grounds, to learn more about the birds’ movements, survival rates and habitat use.
The first step in the study was to place geolocators on adult birds. The geolocators monitor light levels and give the scientists an indication of the latitude and longitude of the birds. This allows them to track migration patterns and identify wintering locations. In all, 60 adult males were fitted with the devices.
Next, the scientists wanted to place radio-tracking devices on 7-day old nestlings. Finding and watching the nests was challenging. Dr. Cooper explains: “If you get too close [the adults] fly away or become anxious and won’t visit the nest. And if you’re too far, you lose them in the dense vegetation.” The transmitters would help the scientists determine survival rates and the habitats used by the fledglings.
The news is both good and bad for the chicks. In his blog, Dr. Cooper notes: “Most of our nestlings survived long enough to make it out of the nest…Predation rates during the first few days out of the nest have been fairly high…Causes of death are somewhat unclear…It’s quite difficult to tell what type of animal killed a fledgling even with the carcass in hand.” I asked Dr. Cooper whether he had an educated guess regarding the number of Kirtland’s that are killed by cats. He didn’t have a specific estimate, but he sent me a paper that should be required reading for everyone: “The impact of free-ranging domestic cats on wildlife of the United States” by Loss, Will and Marra, 2012. The study found that domestic cats kill an astounding 1.4 – 3.7 billion birds in the United States every year!
The survival rates for Kirtland’s through their second year are more unclear. In response to my question, Dr. Cooper stated: “This is something we are still working on getting a good estimate for. In general it is a very difficult question to answer for most songbirds. More and more studies are investigating the post-fledgling period and finding that anywhere between 22 and 70% of birds survive until they gain independence from their parents. For Kirtland’s it is looking like ~50% of fledglings survive this period, but the analyses aren’t complete yet. Estimating survival after the post-fledgling period until the following breeding season is even more difficult for most species. However, recent estimates (“Using complementary approaches to estimate survival of juvenile and adult Eastern Kingbirds”, Redmond and Murphy 2012) suggest only about 20-30% of birds survive from leaving the nest until the next breeding season.”
Despite the challenges from Cowbirds, habitat loss and other obstacles, Kirtland’s Warblers are making a comeback. When I asked about his prognosis for their long-term survival, Dr. Cooper was cautiously optimistic. “Kirtland’s Warblers are a real success story for the Endangered Species Act. After reaching a low of only 200 males in the world, there are now over 2000 males. With continued management, I think the species has a very strong probability of long-term survival. One of the unknowns however, is how their wintering habitat will change as the Caribbean grows in population size, and as climate changes. This has driven us to want to better understand this period through the use of light-level geolocators to track birds throughout the annual cycle.”
The next stage of the study is underway. Dr. Cooper traveled to the Bahamas in early March to study the Kirtland’s in their wintering grounds. The team will travel to Cuba (another potential wintering grounds for the Kirtland’s) next year. His wish list for future studies includes a reevaluation of the Cowbird control program and the development of smaller GPS tracking devices suitable for Kirtland’s. “I am eager for this day to come because studying the same individuals on both the wintering and breeding grounds would be a dream come true.”
Be sure to follow the progress made by Dr. Cooper and his team here: http://www.nathanwcooper.com/news/. In the meantime, Dr. Cooper suggests that everyone support local and national conservation organizations like the Smithsonian Migratory Bird Center and Nature Conservancy. He also asks that we all help to protect migratory stopover habitats, and that we buy Smithsonian-certified bird-friendly coffee (http://nationalzoo.si.edu/scbi/migratorybirds/coffee/). He’s encouraged by the support he has found at the local level. “I’m always surprised by how many local people in the many areas I’ve worked truly care about birds and the environment. Many important issues often get lost at the national level, but when you talk to people who live in these wild areas, it’s clear how much they care for and respect the land.”