Grassland songbird use of waterfowl nesting cover in southwestern Manitoba

I compared the abundance and reproductive success of grassland songbirds in managed waterfowl nesting cover (native and introduced dense nesting cover [DNC]) and unmanaged (hay, crop) habitats, and associated these differences with vegetative characteristics. My study was conducted from May-July of 1994-1996 in the rural municipalities of Odanah and Minto, southwestern Manitoba. Study plots were 400 x 200 m (8 ha) in size and grid-marked with colored flags at 50-m intervals to facilitate spot mapping of bird detections. In order to assess reproductive success, I derived a behavioral index of productivity (BIP) for each recognized songbird territory and I monitored grassland songbirds nests in each study plot. I also measured vegetation characteristics within each study plot. Total bird abundance did not differ among native DNC, introduced DNC, and hay fields; However, crop fields had less than 25% as many birds as the other 3 habitats (F₃, ₆₅ = 27.2, P = 0.001). Horned larks (Eremophila alpestris) were significantly more abundant in crop fields, but all other birds except sharp-tailed sparrows (Ammodramus caudacutus), eastern kingbirds (Tyrannus tyrannus), and western meadowlarks (Sturnella neglecta) were significantly more abundant in hay, native DNC, or introduced DNC. Native DNC, introduced DNC, and hay were extremely similar, with only a few exceptions. Mayfield nest success for all species combined ranged from 18-26% among habitat types (extremes represented by crop and native DNC, respectively), but this small amount of variation was not significant. These values are low compared to other estimates for grassland songbirds. Structural diversity, residual cover, and vegetation height were important vegetation characteristics for predicting abundance of grassland songbirds. Incorporating additional native grass, forb, and shrub species into seed mixtures would add more structural diversity to DNC for a variety of species. Although DNC was attractive to grassland songbirds, BIP and nest success estimates were low in this habitat, raising the possibility that DNC may function as an ecological trap for grassland songbirds. Additional research should focus on monitoring reproductive success of grassland songbirds, including a critical evaluation of the BIP method.