Declines of Migratory Songbirds in a Deteriorating Swamp Forest, a Link to Baldcypress Leafroller Outbreaks

Forested wetlands exist in a balance of hydrologic conditions that facilitate flooding and periodic drying, making their long-term sustainability vulnerable to hydrological disruptions. We demonstrate here a link between swamp forest deterioration, decreases in caterpillar outbreak frequency, and corresponding decreases in specialist swamp forest songbirds. Coastal Louisiana baldcypress-tupelo swamp forests have experienced a variety of threats over the last 150 years, first by nearly complete clear-cutting, and then through damming and leveeing rivers and deltaic distributaries. Maurepas Swamp is the second largest swamp forest in the United States, but historical hydrological disruptions combined with subsidence has left it persistently flooded, thereby preventing tree regeneration. We examined long-term changes in bird and caterpillar populations, predicting that swamp specialists would be disproportionately affected by deteriorating conditions compared to forest generalists. We conducted standardized avian point counts in closed-canopy relic (20 points) and open-canopy degraded (40 points) swamp forest in 2003-2005 and 2019-2020. We estimated bird density between forest types and times periods using a distance-based hierarchical modeling approach to adjust for imperfect detection. We used the U.S. Forest Service National Insect Disease Survey database to assess lepidopteran outbreak frequency and extent and used a longitudinal mixed effects logistic regression model to measure changes through time. As predicted, three swamp specialist bird species (Prothonotary Warbler [Protonotaria citrea], Northern Parula [Setophaga americana], and Yellow-throated Warbler [Setophaga dominica]) decreased in density by >50% over the 16-year period, corresponding to a near complete absence of baldcypress leafroller caterpillar (Archips goyerana) outbreaks after 2006. Generalist Red-bellied Woodpeckers (Melanerpes carolinus) also decreased through time, although less dramatically than specialists, perhaps explained by regional rather than localized contexts. In contrast, 12 of 16 generalist bird species densities remained stable or increased over time, as did forest tent caterpillar (Malacosoma disstria) outbreaks. We suggest that changes in baldcypress leafroller outbreak frequency in part be driving changes in specialist bird densities, which to our knowledge would represent the first described trophic linkages between these taxa. We summarize four hypotheses that may explain changes in baldcypress leafroller outbreak frequency, with the link between nutrient inputs from hydrological connectivity perhaps having the best support to date.