Estimating population abundance is fundamental to species conservation. Conservation of threatened and endangered species, in particular, necessitates evaluation of quantifiable criteria to advance, and ultimately achieve, recovery. For instance, the draft Recovery Plan for the federally endangered least Bell’s vireo (Vireo bellii pusillus) recommends that downlisting to threatened status may be achieved when stable or increasing populations, each consisting of several hundred or more breeding pairs, are both protected and managed during five consecutive years across vireo metapopulation areas. One such area includes the Santa Clara River Valley of southern California. At present, the U.S. Fish and Wildlife Service and its partners are piloting a multi-year, point-count study of least Bell’s vireos across the valley study area. Our population assessment follows a dependent, double-observer sampling approach to enhance observer detection of vireos during repeat survey visits between 80 (spring 2021) and 104 (spring 2022) sites. While controlling for ecological- and observational-process variation, we analyzed preliminary vireo counts using single-season N-mixture models and estimated that hundreds of breeding pairs currently occupy the valley, which begins to evaluate recovery criteria and demonstrates the utility of standardized, stratified-random surveys to evaluate vireo population quantities and recovery outcomes across its range.

Dr. Andrew Dennhardt is a U.S. Fish and Wildlife Service biologist in the Listing & Recovery Division of the Ventura Fish and Wildlife Office. He received his Bachelor's of Science from Southern Illinois University, his Master's from West Virginia University, and his Ph.D. from Michigan State University---studying myriad aspects of population and community ecology at each step along the way. Today, his work focuses on evaluation and recovery of federally threatened and endangered species such as the endangered arroyo southwestern toad and Santa Barbara County population of the California tiger salamander as well as the least Bell's vireo, the focal species of his presentation today.