Measuring Marbled Murrelet activity in valley bottom habitat: Bias due to station placement

Survey stations to measure Marbled Murrelet (Brachyramphus marmoratus) acti vity in low-elevation forest are often preferentially placed on stream chan nels because they provide wide visibility and the greatest chances of visua lly detecting birds and thus behaviors possibly associated with nesting. De tections of birds flying along stream channels to access nesting habitat fa rther inland will inflate estimates of activity associated with the adjacen t forest. We compared numbers and types of murrelet detections between 12 p aired stations within 100 m of each other on streambeds and in similar habi tat in adjacent forest during 8 Jun-10 Jul. 1997 in Clayoquot Sound, Britis h Columbia. Circling and below-canopy flight, thought to be indicative of n esting, were observed at all streambed survey stations and at less than hal f of paired forest stations. Numbers of such "occupied" detections were six times greater at streambed than forest sites. Size of opening at survey st ations accounted for much of the difference in detection rates between fore st and streambed stations, but numbers of total, visual, and occupied detec tions, specifically those of circling birds, were lower at forest than stre ambed stations even after the effect of opening size had been considered. C orrelations between opening size and numbers of detections at streambed but not at forest locations, also indicated that differences between streambed and forest stations were not solely a function of opening size. Observatio ns at one pair of stations where murrelets were using a flight corridor ove r the forest station indicated chat a corridor effect may not be confined t o streambed locations. Results indicated that placement of survey stations on stream channels is appropriate if the goal is to determine murrelet pres ence or the occurrence of occupied detections in an area. However, if a com parison of murrelet activity between habitat types is the objective, then f orest stations with comparable opening sizes may be needed to provide unbia sed results.