Demographic and behavioral characteristics of wintering sparrow populations in a South Carolina salt marsh
Abstract
Throughout the world, coastal communities both human and natural face the existential threat of rapidly rising seawater levels resulting from global climate change. Since 2012, my collaborators and I have been monitoring a community of sparrows that overwinter within the extensive salt marshes of Kiawah Island in South Carolina, USA. The three species that comprise this community of “marsh sparrows” are Seaside Sparrow (Ammospiza maritima), Saltmarsh Sparrow (Ammospiza caudacuta), and Nelson’s Sparrow (Ammospiza nelsoni), and they can all be readily captured during above-average high-tides during which they are forced to occupy a few salt marsh patches that remain above water. This dissertation documents long-term banding efforts, field experiments, and GIS analyses to assess the current status of the Kiawah Island marsh sparrows, their capacity to cope with environmental changes, and ultimately their prospects for survival over the next 75 years. The three chapters of this dissertation take on each of these subjects in turn.
In Chapter One, I compiled eight years of bird-banding data and used open-population capture-recapture models to estimate annual survival and wintering abundance of the three “marsh sparrow” populations. I employed three different population modeling approaches that included both classical mark-recapture analyses as well as a relatively new spatial modeling approach. To determine which model was best, I simulated bird populations on Kiawah Island as well as the sampling regime (i.e., banding efforts) to see which modeling approach yielded results that best reflected the input parameters for the simulation. Annual survival for all three species generally ranged from 0.5 to 0.8 across all years of the study. Density/abundance estimates indicated that as many as 1,848 marsh sparrows occupy the ~1,650 hectares of salt marsh surrounding Kiawah Island. Although the capture and banding efforts did not fully conform to the expectations of a mark-recapture analysis, my simulation efforts indicated that a classical Cormack-Jolly-Seber model provided the most accurate estimates of annual survival.
In Chapter Two, we used radio surveillance methods and translocation experiments to study the movement abilities and site fidelity of Nelson’s, Saltmarsh, and Seaside Sparrows. The vast majority of translocated birds returned successfully to their initial capture sites (presumed winter home range area), and they generally did so within a few days, regardless of whether they were displaced across fragmented or continuous marsh landscapes. We found no clear evidence that return success differed among species or between landscape treatment (continuous marsh vs. fragmented marsh). Return times did not appear to be significantly influenced by species, landscape treatment, or translocation distance. The high rate and speed of returns following displacement indicates a strong fidelity in marsh sparrows to wintering home ranges and associated high-tide roost sites.
In Chapter Three, we evaluated losses of wintering habitat for marsh sparrows at Kiawah Island under future sea-level rise (SLR) scenarios. We specified four different SLR scenarios in our local models and found that even the most moderate predictions for SLR entailed dramatic losses of salt marsh habitat on Kiawah Island. For the more severe SLR scenarios, our models predicted that Kiawah would not likely sustain overwintering marsh sparrows by the year 2075.
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