Factors influencing the distribution of Clapper Rails in Mississippi’s tidal marshes
Rush, Scott Anderson
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For many of the marsh birds that inhabit the tidal ecosystems of the northern Gulf of Mexico, such as the Clapper Rail (Rallus longirostris), there exists little information on habitat use, reproduction and response to prey availability. Further, natural and human-induced processes continue to act upon these estuarine ecosystems. During the four year period 2005 to 2008, I studied the spatial and reproductive ecology of Clapper Rails in tidal marshes of the northern Gulf of Mexico. Through application of occupancy models, reproductive metrics, radio-telemetry and chemical tracers I provide an evaluation of factors that influence the spatial, reproductive and trophic ecology of Clapper Rails within these tidal systems. Projections from occupancy models indicate that although Clapper Rails occupied much of the area surveyed (71%). However, small-scale habitat alteration such as the loss of emergent marsh and increased cover of the halophyte Juncus roemerianus may influence future distributions of these birds. Tidal marsh habitat is characterized by periodic tidal flooding, a condition that has shaped many aspects of the Clapper Rail’s reproductive ecology. The results of this study indicate that Clapper Rails may limit nest loss to tidal flooding by placing nests in structurally complex habitat. Further, evidence indicates that as tidal height increases within a breeding season the size (volume) of Clapper Rail eggs increased and clutch size decreased, relative to the date of nest initation. Thus, evidence suggests that Clapper Rails persue an adaptive trade-off that mitigates the loss of productivity due to tidal flooding and the mortality of recently hatched young, tied to a seasonal increase in predation pressure. Moreover, physiochemical conditions differ between the marine and freshwater estuaries studied. Radio-telemetry and chemical tracers (carbon and nitrogen stable isotopes and egg yolk lipid concentrations) provided evidence that prey availability influences the movement and distribution of Clapper Rails as well as their subsequent allocation of resources to reproduction. Collectively, these findings indicate that Clapper Rail populations are spatially and temporally dynamic. Conservation initiatives developed for this species, and similar tidal marsh-dependent organisms should account for variation in population drivers that can act at multiple spatio-temporal scales.