Abstract
A rapidly increasing number of species are at risk of extinction and recovery often requires expensive, time-consuming, and challenging management actions. Captive breeding and reintroduction programs are increasingly used to bolster population sizes and genetic variability of rare species but these efforts rarely monitor animals after release and hence prevent assessments of their effectiveness. Habitat quality is often a primary predictor of success for reintroduction and translocation programs. Post-release monitoring is essential for understanding the ways captive-released animals interact with their new environments as well as how these animals survive and contribute to the population once they are released. We conducted post-release monitoring of an endangered marshbird, the light-footed Ridgway’s rail (Rallus obsoletus levipes). Light-footed Ridgway’s rails are found in coastal wetlands of southern California and northern Mexico, where many wetlands have been lost or significantly degraded by coastal development. Light-footed Ridgway’s rails face continued threats from low genetic variability, continued development, and sea level rise. A captive breeding and reintroduction program was created to help bolster population sizes and increase genetic variability. Over the past 20 years, over 700 captive-raised juvenile rails have been released into marshes in their US range. Post-release monitoring of captive rails has been limited and, hence, we know little about the survival and habitat selection of captive-released rails from this program. We attached solar powered GPS transmitters to captive-released and wild-caught juvenile rails to compare survival and habitat selection.
Chapter 1 compares survival of captive-released and wild-caught light-footed Ridgway’s rails based on a suite of environmental covariates that may affect survival in the wild (e.g., raptor abundance, tidal water levels, time since release/capture) and analyzes survival of captive-released rails based on factors encountered prior to release and the date that they were released into the wild. Captive-released rails had low initial daily survival probability when compared with wild-caught rails, but their survival increased as time since release increased and matched wild-caught rail survival after 100 days in the wild. Captive rails released earlier in the year (June- August) had higher average daily survival than rails released in September- December. Our results suggest that earlier release dates can help managers increase post-release survival of captive rails in the wild.
Chapter 2 considers the effect of changing water levels on habitat selection of captive-released and wild-caught rails. Light-footed Ridgway’s rails are found in tidal wetlands where daily tides vary from 1-2-meters every 6-7 hours, drastically changing the landscape available. We analyzed habitat selection of captive-released and wild-caught rails at 3 tidal stages: 1) all rail locations (regardless of tidal height), 2) all rail locations when water levels were below 0.82 meters (i.e., low tide), and 3) all rail locations when water levels were above 1.21 meters (i.e., high tide). Differences between wild-caught and captive-released rails were the most pronounced at high tide and captive-released rails selected microhabitats within the marsh at high tide that may leave them exposed to rising water and increased predation risk. Our results suggest that efforts to expose captive rails to tidal fluctuations, changes in water level, or changes in habitat availability prior to release may help improve post-release survival. Our study highlights the importance of monitoring animals after release and refining habitat selection analyses based on the unique environmental conditions of the target species.