Responses of Red-winged Blackbirds, Yellow-headed Blackbirds and Marsh Wrens to glyphosate-induced alterations in cattail density

The effects of herbicide-induced changes in wetland emergent vegetation on densities of territorial male Red-winged Blackbirds (Agelaius phoeniceus), Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), and Marsh Wrens ( Cistothorus palustris) were assessed in northeastern North Dakota. In 1990 and 1991, 23 cattail-dominated wetlands were randomly assigned to 0% (reference wetlands), 50%, 70%, or 90% areal spray coverages with glyphosate herbicide. Two pears post-treatment, densities of redwings were higher in the reference wetlands ((x) over bar = 1.59 +/- 0.24 [SE]/ha) than in the 90% treated wetlands ((x) over bar = 0.55 +/- 0.14/ha, P = 0.063). Yellowheads were more abundant in reference wetlands ((x) over bar = 3.80 +/- 0.83/ha) than in treated wetlands ((x) over bar = 2.05 +/- 0.40/ha, P = 0.061). Likewise, wrens were more abundant in reference wetlands ((x) over bar = 2.21 +/- 0.27/ha) than in treated wetlands ((x) over bar = 0.66 +/- 0.13/ha, P = 0.001). Percent coverage of live emergent vegetation (largely cattails, Typha spp.) was positively correlated with blackbird and wren numbers (P less than or equal to 0.1). Results of this study suggest that numbers of these three wetland-dwelling species were limited by altering cattail density. Staggering vegetation management treatments on large wetland complexes may help diversify the stages of cattail regeneration and provide heterogenous nesting and foraging habitat for these birds.