Large, infrequent disturbance on a regulated river: response of floodplain forest birds to the 2011 Missouri River flood

Abstract

Floodplain forests are dynamic habitats that support a high diversity and abundance of birds. Periodic flood disturbance is important in the establishment and maintenance of the heterogeneous mosaic of vegetation communities across the riverine landscape. Human suppression of disturbance regimes has been implicated in the decline of bird species in these systems. Because few large rivers are not subject to flood control by dams and levees, opportunities to study avian responses to flood disturbance are limited. A large magnitude, long‐duration flood event on the Missouri River, USA, during the summer of 2011 provided an opportunity to quantify post‐flood changes in forest bird densities and species richness relative to pre‐flood conditions on a riverine floodplain impacted by decades of flow regulation. We surveyed 75 forest sites on two segments of remnant floodplain forest along the Missouri National Recreational River (MNRR) in southeastern South Dakota and northeastern Nebraska and examined changes in density for 35 breeding landbird species from pre‐flood (2009–2010) to post‐flood (2012–2014) periods. We used a repeated measures ANOVA design to test the effects of year on average densities of birds and nesting guilds and confidence intervals to determine changes in densities of individual species and species richness. 19 of 35 focal species declined significantly one year after the flood (2012), but abundances of ten species recovered to pre‐flood densities or higher within two years. In 2012, density declines of six species and density increases of two species were significantly correlated with a decrease in woody vegetation density and percent shrub cover. Average bird densities and the density of shrub nesters rebounded in 2013 to pre‐flood levels and continued to increase through 2014. There were no significant changes in species richness at the level of forest habitat types between sampling years. Our results demonstrate short‐term resilience of floodplain bird species to a major disturbance despite declines in early successional habitat and minimal recovery of woody vegetation.