U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version



Margenau, E.L., N.W. Cooper, D.J. Brown, D.M. Donner, P.P. Marra, and P. Ryan. 2022. Spatially structured brown-headed cowbird control measures and their effects on Kirtland’s warbler long-term population sustainability. Wildlife Research 50(10):771-781. doi: 10.1071/WR22037


U.S. government work


Context: Brown-headed cowbirds (Molothrus ater), through brood parasitism, can exert extrinsic population growth pressures on North American songbirds. Cowbird removal programs may reduce parasitism rates on host species but can be expensive and difficult to implement throughout a host species’ breeding range.

Aim: We estimated cowbird abundance and nest parasitism rates within Kirtland’s warbler (Setophaga kirtlandii) primary breeding range in Michigan, USA, and determined the maximum sustainable parasitism rate for Kirtland’s warblers under several spatially structured cowbird removal designs.

Methods: We conducted point counts to estimate cowbird abundance and monitored nests to quantify nest parasitism rates during 2019–2021. We used the modelling software STELLA to determine the maximum sustainable parasitism rate for Kirtland’s warblers under different spatially structured cowbird removal scenarios (complete, core-only, and no removal).

Key results: Cowbird abundance and parasitism rates remained low following cowbird trap closures in 2018. In the simulation study, complete removal was the most robust scenario with no replications having <1000 Kirtland’s warbler males. The core-only removal scenario had a substantially higher sustainable parasitism rate in the peripheral breeding area than the no removal scenario. Assumed hatch-year dispersal distance had the greatest impact on the maximum sustainable parasitism rate in the core-only scenario.

Conclusions: Low cowbird abundance and nest parasitism following suspension of cowbird removal efforts showed resuming the removal program may not be required in the short-term. If cowbird abundance increases, however, adaptive cowbird removal programs can be used to sustain Kirtland’s warbler populations long-term.

Implications: Our results indicate that incorporating spatial structure of host species’ habitat into designing cowbird removal programs may minimise costs of cowbird management while sustaining populations of Kirtland’s warbler and possibly other host species that are affected by brood parasitism.