Investigating the population dynamics of California spotted owls without marked individuals

Abstract

Understanding population dynamics is of great interest in many different contexts. Traditionally, population dynamics have often been considered in terms of individual-based demographic parameters (e.g., abundance, survival, and reproductive rates), estimation of which generally requires information from marked individuals. Alternatively, in some situations, it may be appropriate to consider population dynamics at a landscape level where the focus is shifted from numbers of individuals to the status of the population at places on the landscape. One consequence of doing so is that information from marked individuals is no longer required. Recently developed methods allow the estimation of landscape-level population vital rates in the realistic situation where the current status of the population might be misclassified via field methods (e.g., because of imperfect detection). Here, we consider the case of the California spotted owl (Strix occidentalis occidentalis) at the Eldorado study area in central Sierra Nevada, California, USA, where interest is in the occupancy rate of potential nesting territories, and in whether owls in an occupied territory successfully reproduced each year during 1997–2004. We analyzed the data using multistate occupancy models and found no evidence of annual variation in dynamic occupancy probabilities. There was strong evidence of annual variation in successful reproduction, with the pattern of variation being different depending on whether there was successful reproduction in the territory in the previous year. Of the three environmental variables considered, the Southern Oscillation Index appeared to be most important and explained some of the annual variation in reproduction probabilities.