Assessing the Effects of Scale and Habitat Management on the Residency and Movement Rates of Migratory Shorebirds at the Tom Yawkey Wildlife Center, South Carolina.

Abstract

I evaluated the influence of inter-wetland distance and the timing of drawdowns on local daily survival (residency) and movement probability of Semipalmated Sandpipers (Calidris pusilla) during the springs of 2006 and 2007 at the Tom Yawkey Wildlife Center (YWC), South Carolina. There is growing interest in determining the scale at which coastal wetlands are functionally connected and how management influences their use by migrant shorebirds. Parameters of interest were estimated using multi-state models and encounter histories obtained from resight and telemetry data. Data were collected in the spatial context of three clusters of multiple wetland units each separated by distances of 2.6 to 4.1 km and two hydrologic treatments--a slow and fast draw down. Mean length of stay was 2.99 d (95% CIs = 2.45 - 3.52) in 2006 and 4.57 d (95% CIs = 2.59 — 8.92) in 2007. Residency probability was influenced by a negative and significant interaction between estimated percent fat at capture and southerly wind speed. This meant that differences in departure rates by birds with varying body conditions were minimized. Sixty-five percent of all marked birds stayed within 2 km from their banding location. Movement probabilities were negatively related to inter-cluster distance and bird density. In the spatial context of YWC there was functional connectivity among the clusters that were ˜2.5 km apart; movement was negligible between units at nearly twice that distance (4.1 km). Contrary to expectations, the average probability of surviving and not moving for birds in slow-managed units was higher than birds in fast-managed units (PhiSS 2006 = 0.488, PhiSS 2007 = 0.654). On average, birds marked in fast-managed units moved out at high rates (Psi 2006 = 0.399, Psi 2007 = 0.467). Higher prey biomass and bird density in slow-managed units influenced observed residency and movement rates. My findings advocate for conservation strategies that identify functionally connected wetland units and suggested that habitat supplementation for shorebirds during peak migration can be met by carefully planned staggered, slow drawdowns. The effectiveness of fast drawdowns is vulnerable to differential prey base quality, presence of birds on previously exposed habitat, failure of water control structures to operate properly, and the possibility of mismatching peak migration and rapid drawdown implementation.