Comparison in survival of Laysan ducks (Anas laysanensis) between the endemic population on Laysan Island and the reintroduced population on Midway Atoll, Hawai`i

Background/Question/Methods

Population variation in life history can be important for predicting establishment and persistence of reintroduced populations of endangered species. The Laysan duck (Anas laysanensis) is an endangered bird native to the Hawaiian Archipelago that was extirpated from most islands after introduction of mammalian predators. Laysan ducks were restricted to a single remote island, Laysan Island (4.1 sq. km), for nearly 150 years. Since the species is not known to disperse between Hawaiian Islands today, 42 wild birds from Laysan Island were translocated to another mammalian-predator-free atoll (Midway Atoll; 6.0 sq. km) during 2004-2005 to reduce extinction risk. Mark-resight data were collected on both islands to estimate survival through 30 June 2012.  Birds marked in each year, 2004-2009, were considered as separate cohorts in the analysis, using log-rank tests in a multifactor design (cohort, age, sex, and island) to compare main effects and their interactions. Kaplan-Meier survival estimation was used to calculate survival curves and display differences among survival rates for 2004-2012.

Results/Conclusions

Comparison between islands indicated survival was fairly similar, and generally high, in the two populations during 2004-2008, although differences between islands appeared after that time frame. We will discuss hypotheses concerning differences in survival between populations.  The log-rank analysis of survival indicated very significant differences (P < 0.0001) among cohorts in survival, with the later cohorts (2007-2009) having lower survival. There was also a very significant interaction (P = 0.0003) between cohort and sex, with females having lower survival in some cohorts. A population decline of 42% was observed on Laysan Island between 2010 and 2012 after consecutive storms and Japan’s Tōhoku earthquake-generated tsunami in 2011 hit both islands, and this catastrophe is evident in the survival curves. To further reduce extinction risk for the species, 28 wild birds were moved from Midway Atoll to Kure Atoll in August 2014. Survival on Kure Atoll has been very high since translocation, which bodes well for the establishment of a third, small population for this species.