Linked paper: No evidence of a demographic response to experimental herbicide treatments by the White-crowned Sparrow, an early successional forest songbird by J.W. Rivers, J. Verschuyl, C.J. Schwarz, A.J. Kroll, and M.G. Betts, The Condor: Ornithological Applications.
A number of birds that use forests disturbed by timber harvest have been declining for decades in North America’s Pacific Northwest. In this region, timber management often requires spraying competing vegetation with herbicides so that crop trees can grow, but the consequences of this herbicide treatment on bird nesting are poorly understood. We designed an experiment to find out how herbicide application was affecting nesting in the White-crowned Sparrow, a songbird that’s declining in the Pacific Northwest. We treated study sites with different levels of herbicides, as well as establishing control sites that experienced no herbicide application. Then, we located and monitored sparrow nests at each site to evaluate how herbicide intensity influenced the outcomes of nesting attempts and the survival of fledgling birds immediately after leaving the nest.
Although herbicide treatments had a major effect on the vegetation in our study plots, as we had expected, we found no effect of herbicide treatment on nest survival. That is, nests had similar rates of survival regardless of where they were located across the continuum of herbicide intensity. Similarly, the survival of fledgling sparrows did not differ between control sites that received no herbicide and sites where herbicide was applied in a way that simulated current management of industrial forest plantations. These results were surprising, because we know from previous work that herbicides have a strong influence on the extent of broadleaf vegetation, a key habitat component for foraging and nesting for many declining songbirds including the White-crowned Sparrow.
Although our study demonstrated that herbicides reduced plant cover in the general area around sparrow nests, we did find that the amount of concealment provided by nest site vegetation was similar across treatments. This suggests that despite the reduction in vegetation cover from herbicides, sparrows in even the most intensively-treated stands were still able to find suitable hiding places for their nests. Because most nest failures were due to predators, it’s possible that nest predators were impacted more by vegetation cover right around the nests than by a reduction in vegetation cover at bigger scales. We were unable to measure predators in our study, but hopefully future work in this system can improve our understanding of the ecology of songbird nest predators and how they are—or are not—affected by herbicide treatments.
Many declining bird species in the Pacific Northwest are united by their need for recently-disturbed forests that contain broadleaf vegetation. Our work shows that one declining species, the White-crowned Sparrow, is not influenced by herbicide intensity, but it remains unknown whether these results also apply to other species. Therefore, studies that expand beyond sparrows would be especially useful for understanding the effects of herbicides on other species that have similar habitat requirements yet differ in their foraging behaviors.