Response of floodplain bird communities to the 2011 Missouri River flood

Background/Question/Methods

Cottonwood (Populus deltoides) forests along the Missouri River provide important foraging and nesting habitat for migrant and resident landbirds. Periodic flooding creates the necessary conditions for cottonwood recruitment and assures continuous succession of forest. Although thought to be beneficial in the long term, the short-term effects of Missouri River flooding on floodplain bird communities is unknown. The construction of 6 large dams has effectively eliminated flood pulses on the Missouri River, thereby decreasing cottonwood recruitment. Despite flood control infrastructure, the Missouri River experienced a high amplitude and long duration flooding event in May–November 2011. To determine the effects of the flooding on breeding birds, we compared densities before the flood (2009-2010) to densities after the flood (2012). During each breeding season, we conducted point count surveys across 77 forest patches stratified by 7 combinations of age and dominant tree species. Using program DISTANCE, we estimated densities for 33 bird species. We hypothesized that bird densities would decline in the first year following the flood and that these declines would occur mostly among species that are associated with early forest successional stages.

Results/Conclusions

In forest patches < 55 years old (established after dam construction), 8 bird species increased in density while 21 species decreased; 4 showed no significant changes. In forest patches > 55 years old, 10 species increased in density, 15 decreased, and 8 showed no significant change. Declines were greater among habitat specialists such as the bell’s vireo, wood thrush, and ovenbird. Generalists such as black-capped chickadee and downy woodpecker were less affected, generally increasing in density or showing no change. Species density estimates across 7 stand ages and types revealed that many species increased their numbers in habitats that are uncharacteristic for the species, though declining in density overall. This may be due to structural alteration of preferred habitat. Late successional stands were largely spared the effects of flooding. Thus, the declines in densities for several bird species associated with this habitat type were not expected.  This may implicate causes other than or in addition to flood-induced structural habitat alteration on bird densities, such as weather effects on nesting and foraging success. Analysis of vegetation data and an additional year of data collection may clarify the potential causes of changes in bird densities after the flood.