Avian botulism is a primary, year-round threat to adult survival in the endangered Hawaiian Duck on Kaua‘i, Hawai‘i, USA

ABSTRACT Adult survival is the most important demographic parameter influencing population dynamics for many bird taxa. Thus, understanding how survival probabilities and causes of mortality vary throughout the annual cycle is critical for developing informed and effective management strategies. In this study, we used radio-telemetry data to evaluate the effects of biotic (e.g., sex, peak [September–April] vs. off-peak [May–August] nesting seasons) and abiotic factors (e.g., rainfall, year, bi-monthly interval) on adult survival, estimate annual survival probabilities, and identify primary sources of mortality for Hawaiian Ducks (Anas wyvilliana), an endangered, non-migratory dabbling duck, on the island of Kaua‘i, Hawai‘i, USA over 2013 and 2014. Additionally, we used contemporaneous Hawaiian Duck carcass recovery and surveillance data to examine temporal and climatic associations with avian botulism outbreaks. Our results suggested bi-monthly survival decreased with total rainfall during the preceding 2-month interval. Survival did not vary with sex, between peak and off-peak nesting seasons, or between the two years of this study. Annual survival probabilities (62–80%) were relatively low compared to the closely related Laysan Duck (Anas laysanensis) on Laysan Island. Primary causes of mortality included avian botulism and presumed predation by cats (Felis catus). The botulism surveillance dataset revealed support for the effect of rainfall on the number of sick and dead birds recovered (n = 216), with generally a greater number of recoveries during months with middle-range total rainfall during the concurrent and preceding months. Our study provides critical baseline demographic data for population monitoring and highlights the importance of managing botulism risk and non-native mammalian predators for the recovery of the endangered Hawaiian Duck. LAY SUMMARY Understanding how adult survival probabilities and causes of mortality vary throughout the annual cycle is instrumental for develo in informed and effective conservation strate ies We used radio-telemetry data to evaluate factors influencing adult survival and identify sources of mortality for the endangered Hawaiian Duck on Kaua‘i, and we used botulism surveillance data to assess factors associated with avian botulism outbreaks. Avian botulism was the primary cause of adult mortality and posed a risk year-round. Increased rainfall was associated with lower survival and higher numbers of sick and dead birds attributed to botulism except during wettest periods when botulism recoveries decreased Survival did not differ between sexes or peak and off-peak nesting seasons, but the rate of predation was higher for females, and all female predation events (n = 4) occurred during the peak-nesting season. Our study provides critical demographic data for population monitoring and highlights the importance of managing botulism risk and non-native mammalian predators (e.g., feral cats) for Hawaiian Duck recovery.