Using time-of-detection to evaluate detectability assumptions in temporally replicated aural count indices: an example with Ring-necked Pheasants

Abstract

The validity of treating counts as indices to abundance is based on the assumption that the expected detection probability, E(p), is constant over time or comparison groups or, more realistically, that variation in p is small relative to variation in population size that investigators seek to detect. Unfortunately, reliable estimates of E(p) and var(p) are lacking for most index methods. As a case study, we applied the time-of-detection method to temporally replicated (within season) aural counts of crowing male Ring-necked Pheasants (Phasianus colchicus) at 18 sites in southern Minnesota in 2007 to evaluate the detectability assumptions. More specifically, we used the time-of-detection method to estimate E(p) and var(p), and then used these estimates in a Monte Carlo simulation to evaluate bias-variance tradeoffs associated with adjusting count indices for imperfect detection. The estimated mean detection probability in our case study was 0.533 (SE = 0.030) and estimated spatial variation in E(p) was 0.081 (95% CI: 0.057-0.126). On average, both adjusted (forp ) and unadjusted counts of crowing males qualitatively described the simulated relationship between pheasant abundance and grassland abundance, but the bias-variance tradeoff was smaller for adjusted counts (MSE = 0.003 vs. 0.045, respectively). Our case study supports the general recommendation to use, whenever feasible, formal population-estimation procedures (e.g., mark-recapture, distance sampling, double sampling) to account for imperfect detection. However, we caution that interpreting estimates of absolute abundance can be complicated, even if formal estimation methods are used. For example, the time-of-detection method was useful for evaluating detectability assumptions in our case study and the method could be used to adjust aural count indices for imperfect detection. Conversely, using the time-of-detection method to estimate absolute abundances in our case study was problematic because the biological populations and sampling coverage could not be clearly delineated. These estimation and inference challenges may also be important in other avian surveys that involve mobile species (whose home ranges may overlap several sampling sites), temporally replicated counts, and inexact sampling coverage.RESUMEN. El uso de tiempo de detección para evaluar la suposición de detectabilidad eń ındices de conteo auditivos temporalmente replicados: un ejemplo con Phasianus colchicus La validez de tratar los conteos comoíndices de abundancia se basa en la suposición de que la probabilidad de detección esperado, E(p), es constante a través del tiempo o entre grupos de comparación, o más realísticamente, que la variación en p es pequeño en relación a la variación en el tamaño de la población que los investigadores buscan detectar. Desafortunadamente, las estimaciones confiables de E(p) y var(p) son insuficientes para la mayoría de los métodos deíndice. Aplicamos el método de tiempo de detección a conteos auditivos temporalmente replicados (dentro de la temporada) de...