Wp. Smith et Vl. Harke, Marbled murrelet surveys: site and annual variation, sampling effort, and statistical power, WILDL SOC B, 29(2), 2001, pp. 568-577
Populations of the marbled murrelet (Brachyramphus marmoratus) have decline
d and become fragmented throughout the Pacific Northwest, apparently becaus
e of the loss of older forests from logging. Because clearcut logging conti
nues throughout a large portion of the bird's range, there is a pressing ne
ed to develop rigorous monitoring protocols. We conducted 10-minute dawn su
rveys at fixed point-count stations along 2 roadway segments during mid-Jul
y 1991-1996 to quantify annual and with in-year spatial variation in detect
ing marbled murrelets. Mean detections/station (pooled across stations) var
ied among years and ranged from 11.3 to 33.8. The greatest within-year diff
erence among stations was a range of 0-74 detections, which occurred in the
same vicinity in 1994; within-year variation between roadway segments was
not significant (F-1,F-21 = 0.46, P = 0.51). For a specified effect size of
0.40, we determined that the sample size needed to detect significant diff
erences among years, for alpha = 0.05 and power (1-beta) = 0.95, was 22 poi
nt-count stations. Number of years required to detect an annual decrease in
detection of 10% with one visit to a station/year varied according to the
precision of estimates, which varied annually and ranged between 28-46 year
s and 12-19 years for alpha = beta = 0.05 and 0.10, respectively. By visiti
ng each station 20 or 12 times annually, it may be possible to detect a 10%
annual decrease in detection in 6 years (i.e., 50% cumulative decline) wit
hout compromising power or error rate. Assuming that dawn survey detections
reasonably index habitat distribution or relative abundance, these results
provide preliminary estimates of sampling effort necessary to detect local
population decline or within-year differences in murrelet habitat distribu
tion in the Yakutat Foreland of southeastern Alaska.