Kirtland’s Warbler Conservation & Management

Breaking news October 2019: Kirtland’s Warbler’s successful recovery leads to de-listing as an endangered species!

When Kirtland’s Warblers were declared endangered in 1966, researchers thought that parasitism by Brown-headed Cowbirds was the main limiting factor. Brood parasitism is a fascinating evolutionary strategy in which one individual deceives another unrelated individual into hatching and raising its young. In the late 1960’s, nearly 70% of Kirtland’s Warbler nests were parasitized by cowbirds. When cowbird eggs are laid in a Kirtland’s Warbler nest, the cowbird eggs hatch earlier, and the cowbird nestlings outcompete Kirtland’s Warbler nestlings for food resulting in their starvation. To combat this problem, the United States Fish and Wildlife Service (USFWS) began a cowbird-trapping program in 1972, and parasitism rates were reduced to ~6% by the following year. This management action allowed Kirtland’s Warblers to produce three times more young than before trapping, greatly increasing their potential for population growth. However, the population did not actually increase for nearly 20 years, indicating that parasitism was not the sole factor limiting the population.

Kirtland’s Warblers almost exclusively breed in young jack pine forest, which are created naturally only through large, intense wildfires. Large-scale wildfire suppression in the early 20th century resulted in very little remaining Kirtland’s Warbler breeding habitat. Early attempts at creating habitat through small prescribed fires created new habitat but Kirtland’s Warblers would not use the habitat. In 1980, one such prescribed burn escaped control and burned over 100 times the area that had been planned. Ten years later this created vast amounts of new Kirtland’s Warbler habitat. This new habitat in combination with large-scale planting of habitat, allowed the Kirtland’s Warbler population to dramatically increase from a low of 167 males in 1987 to over 2300 males today.

In the fall of 2019, the USFWS formally removed the Kirtland’s Warbler from the Endangered Species List. The amazing recovery of Kirtland’s Warblers is a huge success story for the Endangered Species Act, the management agencies involved (USFWS, USFS, MDNR), and the Kirtland’s Warbler Recovery Team. However, with this change in listing status, comes new challenges. For example, all of the funding for cowbird trapping came directly from the Endangered Species Act and disappeared with the delisting.

To help reduce Kirtland’s Warbler’s reliance on conservation efforts, we recently carried out a 4-year adaptive management experiment. We slowly reduced and then eventually suspended the cowbird trapping program altogether for the first time since the program’s inception in 1972. We found that the abundance of cowbirds and the rate of parasitism did not increase despite these drastic cutbacks in trapping. This represents a large reduction in conservation reliance and a large cost-savings each year. However, because we do not understand why cowbirds have decreased in the region, we are developing a long-term cowbird monitoring protocol, to ensure that if cowbirds become a threat to Kirtland’s Warblers in the future, managers will know and have time to act. For more detail about our cowbird trapping experiment see our recent publication in the Journal of Wildlife Management.

While a focus on breeding grounds conservation has been tremendously successful for Kirtland’s Warblers, we have reason to believe this may change in the future. Global climate change is expected to dramatically alter the wintering habitat Kirtland’s Warblers depend upon. Kirtland’s Warblers winter throughout The Bahamas, and current predictions suggest that 11-80% of the Bahamian land mass may be lost due to sea level rise in the next century. Moreover, a long-term drying trend is predicted for the region. Similar effects, in combination with habitat loss due to human development, may threaten migration and stopover habitats as well. However, until very recently we did not understand exactly where Kirtland’s Warblers wintered or the routes they take on spring and fall migration.

To better understand Kirtland’s Warbler migration routes, timing, migratory connectivity, and their wintering distribution we recently tracked individuals using light-level geolocators. Using these data we were able to describe their migration routes for the first time. Then by combining geolocator data, with 15 years of on-the-ground surveys, and sightings from eBird we refined the Kirtland’s Warbler wintering range, finding that Kirtland’s Warblers winter throughout The Bahamas and Turks and Caicos, but primarily use the central Bahamas. Finally, by combining data from our coded-radio project (see below) and a Bahamas banding and Michigan re-sighting effort, we were able to show that Kirtland’s Warblers have weak migratory connectivity. In other words, individuals wintering on any one Bahamian island tend to migrate to sites across the whole breeding range rather than one particular part of the breeding range.

In 2017, we began a project using Lotek’s new coded-radio tag technology and the Motus Wildlife Tracking System’s network of automated telemetry towers . Over the past three years, we have tagged over 160 Kirtland’s Warblers on Cat Island, The Bahamas and successfully relocated ~60-70% of them in Michigan. This innovative approach allows us the unique opportunity to directly study the same individuals on the wintering grounds, through migration, and on the breeding grounds – something not currently possible in other songbirds. Data analysis is ongoing, but we hope to learn more about how winter conditions carry-over to affect survival and performance during migration and reproductive success during the breeding season. Many of these relationships were first discovered using American Redstarts as a model organism, but using indirect methods like stable isotopes. It is our hope that by more directly studying carry-over effects in Kirtland’s Warblers we can better understand the causal pathways and mechanisms underlying carry-over effects. Scott Weidensaul recently wrote a great article about our research that was published in Audubon Magazine – This Little Warbler Could Lead to Big Discoveries About Migration.

We are also collaborating with Heather Skeen at the University of Chicago and The Field Museum who is working to understand how changing environments influence the composition of gut microbiota. Our Bahamas to Michigan tracking study provides a powerful platform to investigate individual-level changes across the annual cycle. Gut microbiomes have been studied within single seasons, and across different seasons, but never before in the same individuals during both the wintering and breeding periods. Microbiomes are an integral aspect of vertebrate biology, influencing behavior, immune response, reproductive success, and overall host health. Identifying how and why the composition of the microbiome changes throughout the annual cycle may contribute to our understanding of how the host adapts to changing environments.