Critical Demographic Parameters for Declining Songbirds Breeding in Restored Grasslands

Land area occupied by tallgrass prairie has declined throughout the midcontinental United States during the past 2 centuries, and migratory birds breeding in these habitats have also experienced precipitous population declines. State and federal agencies have responded by restoring and reconstructing grassland habitats. To understand consequences of restoration for grassland bird populations, we combined demographic data collected over 4 breeding seasons (1999–2002) in northern Iowa, USA, with population projection models to estimate population growth rates of 2 declining migratory songbirds, dickcissels (Spiza americana) and bobolinks (Dolichonyx oryzivorus). To determine what parameters were critical for conservation of these species, we estimated relative contributions of nest predation, brood parasitism by brown-headed cowbirds (Molothrus ater), annual survival, and renesting to population growth using elasticity analysis. Based on model simulations, the population growth rate for dickcissels was not high enough to be stable without immigration into the area (λ < 1). For bobolinks, populations could only be stable (λ = 1) if annual survival was relatively high (adult survival >0.7, with juvenile survival between 0.2 and 0.5). Population growth rates were most sensitive to adult survival across a wide range of parameter estimates, whereas sensitivity to brood parasitism and renesting were consistently low. Elasticities associated with nest predation were highly variable and dependent on survival estimates. In the absence of changes in other demographic parameters, eliminating brood parasitism would not be enough to ensure stable populations of either species. Only management focused on increasing adult survival or decreasing nest predation could produce stable populations. Our results underscore the need for reliable adult survival estimates and conservation strategies focused throughout all phases of the annual cycle. In addition, our modeling approach provides an effective framework for investigating the importance of demographic parameters to population growth rates of birds that are influenced by nest predation, brood parasitism, and renesting. Although habitat restoration is one of the few alternatives for conserving communities in threatened landscapes, restoration strategies also need to have positive effects on population dynamics for species of concern, which has not been demonstrated in this grassland system.