Power to Detect Trends in Marbled Murrelet Breeding Populations Using Audiovisual and Radar Surveys

We used pilot data collected in 2001–2004 to compare the power of radar and audiovisual survey approaches to detect trends in breeding population size and differences in trends between populations of marbled murrelets (Brachyramphus marmoratus) in northwestern California. Radar counts of murrelets were almost triple in inland Reserves than in Conservation Areas, and audiovisual counts were 7 times greater. Variation in counts was statistically significant among survey sites but not among years. Although annual variation in radar counts was not statistically significant (P = 0.13), mean radar counts more than tripled from 2001 to 2002, a difference that we considered to be biologically significant, and the radar counts reflected a large increase in the proportion of breeders between these 2 years as determined by radiotelemetry in another study (Acord et al. 2004). Audiovisual counts were much more variable than radar counts overall (CV = 1.10 versus 0.41) and within survey sites (CV = 0.94 versus 0.23). As a result, approximately twice the audiovisual survey effort was needed to detect trends with reasonable power (80%). Power to detect trends in murrelet breeding populations was most sensitive to the duration of the monitoring program and the magnitude of the trend; only relatively modest gains in power were realized by increasing the number of surveys or sites. A monitoring program designed to detect differences in trends between breeding murrelet populations required greater survey effort than a program designed to detect overall trends. Despite the fact that gains in power to detect trends from using radar were offset by the cost of purchasing radar equipment, we advocate the use of radar over audiovisual surveys for monitoring murrelet breeding populations because radar reflected changes in breeding effort whereas audiovisual surveys did not. We also advocate the use of radar because it may provide an estimate of the number of breeding individuals in certain situations, it can be used under poor viewing conditions, it samples larger areas, and it detects a higher proportion of inland-flying murrelets.