Reducing conservation-reliance through adaptive management

When Kirtland’s Warblers were declared endangered in 1966, researchers identified two limiting factors: habitat limitation and parasitism by Brown-headed Cowbirds. 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, resulting in a population average of less than one warbler young produced per breeding pair. 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 more than three young per breeding pair, 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. Beginning in 1991, a combination of natural forest regeneration after a large fire and active creation of new breeding habitat allowed Kirtland’s Warbler populations to dramatically increase over the next two decades, resulting in a population of over 5000 individuals today.

The cowbird-trapping program was both necessary and effective, but was designed for a critically endangered and spatially isolated population, not the larger and more robust population spread across 1.5 million ha that exists today. The current robustness of the Kirtland’s Warbler population, combined with funding uncertainty has led program participants and agency partners to challenge the necessity of the cowbird-trapping program. The scientific community has also challenged the necessity of cowbird control programs in general, arguing that they are often unnecessary and that they shift funds away from more important management actions such as habitat protection.

With that in mind, I designed a reduced-trapping experiment to reduce both the costs of the program and the conservation-reliance of this endangered species. In 2015 and 2016, we closed 11 cowbird traps and found that less than 1% of nests were parasitized. Using an adaptive management approach, we increased the scale of the experiment in 2017, closing seven more traps. However, the nest parasitism rate was still leas than 1% ! Finally, in 2018, we closed all cowbird traps across the Michigan breeding range for the first time since 1972 and again found less than 1% parasitism. We are currently working on a manuscript outlining these exciting results. However, we have received two more years of funding from USFWS to continue our experiment and to work on developing a long-term cowbird monitoring protocol.

In the spring of 2018, the USFWS proposed to remove 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 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 comes directily from the Endangered Species Act, and will disappear if Kirtland’s Warblers are delisted. We are confident that by assessing the need for continued cowbird trapping, we can reduce the costs of cowbird management, and in turn reduce the consservation-reliance of this endangered species. Furthermore, by developing a long-term cowbird monitoring protocol, we can ensure that if cowbirds become a threat to Kirtland’s Warblers in the future, managers will know and have time to act.