I am working on bird counts from winter  season and correct bird abundances for imperfect detection. I would like to use Kendall estimator ( Kendall WL, Hines JE, Nichols JD, Campbell Grant EH (2013) Relaxing the closure assumption in occupancy models: staggered arrival and departure times. Ecology 94:610–617), however I have data from two surveys only (the method requires three surveys). The estimator is being calculated, however confidence intervals cannot. In addition, I used Royle estimator ( Royle JA (2004) N-mixture models for estimating population size from spatially replicated counts. Biometrics 60:108-115). This method has some assumptions about the populatoin closure, though. The estimates  from both methods are strongly correlated thus it suggests both methods do a job. I wonder if it is reasonable to present both estimates in a manuscript or just to stay with the Royle estimator (with confidence intervals but possibly with violated assumptions...). Below I attach a graph.

Thank you