Estimating age-dependent survival when juveniles resemble females: Invasive ring-necked parakeets as an example

Many species only show sexual dimorphism at the age of maturity, such that juveniles typically resemble females. Under these circumstances, estimating accurate age-specific demographic parameters is challenging. Here, we propose a multievent model parameterization able to estimate age-dependent survival using capture-recapture data with uncertainty in age and sex assignment of individuals. We illustrate this modeling approach with capture-recapture data from the ring-necked parakeet Psittacula krameri. We analyzed capture, recapture, and resighting data (439 recaptures/resightings) of 156 ring-necked parakeets tagged with neck collars in Barcelona city from 2003 to 2016 to estimate the juvenile and adult survival rate. Our models successfully estimated the survival probabilities of the different age classes considered. Survival probability was similar between adults (0.83, 95% CI = 0.77-0.87) and juveniles during their second (0.79, 95% CI = 0.58-0.87) and third winter (0.83, 95% CI = 0.65-0.88). The youngest juveniles (1st winter) showed a slightly lower survival (0.57, 95% CI = 0.37-0.79). Among adults, females showed a slightly higher survival than males (0.87, 95% CI = 0.78-0.93; and 0.80, 95% CI = 0.73-0.86, respectively). These high survival figures predict high population persistence in this species and urge management policies. The analysis also stresses the usefulness of multievent models to estimate juvenile survival when age cannot be fully ascertained.