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 vegetatio n on densities of territorial male Red-winged Blackbirds (Agelaius pho eniceus), Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), an d Marsh Wrens ( Cistothorus palustris) were assessed in northeastern N orth Dakota. In 1990 and 1991, 23 cattail-dominated wetlands were rand omly assigned to 0% (reference wetlands), 50%, 70%, or 90% areal spray coverages with glyphosate herbicide. Two pears post-treatment, densit ies 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 refe rence wetlands ((x) over bar = 3.80 +/- 0.83/ha) than in treated wetla nds ((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, Typh a spp.) was positively correlated with blackbird and wren numbers (P l ess than or equal to 0.1). Results of this study suggest that numbers of these three wetland-dwelling species were limited by altering catta il density. Staggering vegetation management treatments on large wetla nd complexes may help diversify the stages of cattail regeneration and provide heterogenous nesting and foraging habitat for these birds.