Reassessing the threat of brown-headed cowbird parisitism to Fort Hood's population of the endangered black-capped vireo using experimental and modeling approaches

Background/Question/Methods

Prior to management (trapping and shooting), cowbirds (Molothrus ater) parasitized >90% of the nests of the federally endangered black-capped vireo (Vireo atricapilla) at Fort Hood Military Reservation, Texas, USA.  Vireo nest survival was <5%.  After implementation of cowbird management in 1988, parasitism frequency eventually fell below 10%, nest success increased to 30-50% annually, and the vireo population steadily increased.  Arguably, Fort Hood’s vireo population has increased to the point that per-capita risk of parasitism has decreased to a sustainable level and cowbird management may no longer be needed.  To assess this assumption, we implemented an experimental and modeling approach.  In 2006, we ceased cowbird management on the west side of Fort Hood while continuing cowbird management on the east side.  We are also using population projection models to determine sustainability of Fort Hood’s vireo population under different parasitism frequency and nest success scenarios.  The results of our experiment and modeling efforts could influence cowbird management decisions at Fort Hood in the future.  Diminished threat of cowbird parasitism has been cited as a justification for down-listing the vireo.  Thus, our results will also help assess whether the threat of cowbird parasitism has actually been diminished.

Results/Conclusions

Prior to the cessation experiment (1997–2005), parasitism frequency was similar between the east (3.8%) and west (5.4%) sides of Fort Hood.  After initiation of the experiment (2006–2008), parasitism frequency has steadily increased on the unmanaged west side (7.9% to 23.1%) while remaining at a low level on the managed east side (1.4–4.4%).  Prior to the cessation experiment, nest survival was also similar between the east (0.371) and west (0.351) sides. After initiation of the experiment, nest survival was lower on the unmanaged west side than on the managed east side.  Preliminary modeling efforts suggest that, three years post-cessation of cowbird management, nest survival might already be insufficient to maintain a stable or increasing vireo population on the west side of Fort Hood.  These results seem to suggest that the threat of cowbird parasitism has not been diminished.  However, the effect of cowbird parasitism on nest survival is confounded by higher frequencies of nest depredation on the west side.  Data from additional years will be needed to determine whether Fort Hood’s vireo population will be able it sustain itself in the absence of cowbird management.  Currently, we plan to continue the experimental cessation through 2011.