Density influences accuracy of model-based estimates for a forest songbird

Abstract

Golden‐cheeked Warblers (Setophaga chrysoparia) are endangered songbirds that breed exclusively in the Ashe juniper (Juniperus ashei) and oak (Quercus spp.) woodlands of central Texas. Despite being the focus of numerous studies, we still know little about the size of the range‐wide breeding population and how density varies across the spectrum of juniper co‐dominated woodlands. Models that have been tested and shown to be accurate are needed to help develop management and conservation guidelines. We evaluated the accuracy and bias of density estimates from binomial mixture models, the dependent double‐observer method, and distance sampling by comparing them to actual densities determined by intensive territory monitoring on plots in the Balcones Canyonlands Preserve, Austin, Texas. We found that the binomial mixture models consistently overestimated density by 1.1–3.2 times (actual density = 0.07–0.46 males/ha), and the other two models overestimated by 1.1–29.8 times at low density and underestimated by 0.5–0.9 times at high density plots (actual density = 0.01–0.46 males/ha). The magnitude of error for all models was greatest at sites with few or no birds (<0.15 males/ha), with model performance improving as actual density increased. These non‐linear relationships indicate a lack of sensitivity with respect to true changes in density. Until systematic evaluation demonstrates that models such as those we tested provide accurate and unbiased density estimates for a given species over space and time, we recommend additional field tests to validate model‐based estimates. Continued model validation and refinement of point‐count methods are needed until accurate estimates are obtained across the density spectrum for Golden‐cheeked Warblers and other songbird species.

Comment on O'Donnell et al.'s “Density influences accuracy of model-based estimates for a forest songbird”

“…We first became aware of the O'Donnell et al. () paper when it appeared in the Journal of Field Ornithology's (JFO) Early View website (https://onlinelibrary.wiley.com/toc/15579263/0/0) in September 2015. Upon reading O'Donnell et al., we were surprised to discover that intellectual property we developed as part of several separate works had been used by the authors without our knowledge, engagement, or permission.…”

“…We have no idea why this opportunity was not provided to us, with apparently no recourse, although our inclusion could have potentially helped O'Donnell et al. () avoid scale and method‐specific analysis failures (Laake et al. , Barker et al.…”

Commentary: “Raw data” and intellectual property: a legal issue vs. an ethical issue

“…Although still of the opinion that he was not obligated to do so based on their agreement, Dr. Morrison did eventually provide the City of Austin and the authors of O'Donnell et al. () with the unpublished estimates, but, in doing so, noted that the “Predicted density estimates are based on a regression model that was developed at a much larger scale than the BCP. As such, predictions from the model at a local scale will result in some bias…,” and advised them that “By providing these density and abundance estimates for the BCP, we make no assertions that predicted occupancy is the best model to describe the species‐habitat relationship at this local scale.” Nonetheless, O'Donnell et al., believing that the estimates provide by Dr. Morrison were co‐owned by the City of Austin based on their interpretation of the agreement, included the estimates for BCP in their paper (O'Donnell et al.…”

“…As such, predictions from the model at a local scale will result in some bias…,” and advised them that “By providing these density and abundance estimates for the BCP, we make no assertions that predicted occupancy is the best model to describe the species‐habitat relationship at this local scale.” Nonetheless, O'Donnell et al., believing that the estimates provide by Dr. Morrison were co‐owned by the City of Austin based on their interpretation of the agreement, included the estimates for BCP in their paper (O'Donnell et al. ).…”

“…paper will not be retracted, the Editor and the AFO Council believe that, based on the wording of their agreement, Dr. Morrison was not required to provide the City of Austin with estimates derived using their model, and the use of those estimates by O'Donnell et al. () violated the spirit of appropriate scientific communication and what we view as accepted norms of collaboration and authorship. As such, from an ethical standpoint, if not a legal standpoint, O'Donnell et al.…”