Second North American Sea Duck Conference - Patuxent Wildlife ...

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
MEASUREMENT OF PROBABILITY DETECTION RATES
FOR NEST SEARCHES IN DENSELY VEGETATED
COMMON EIDER COLONIES
Scott Gilliland, Peter Hicklin, Katherine Mehl, and C. Davison Ankney.
Canadian Wildlife Service; scott.gilliland@ec,gc.ca
The American subspecies of the common eider (Somateria mollissima) is an important game bird in
eastern North America. Currently, this population is not regularly monitored throughout its range,
and managers are considering development of a monitoring program for this population. Two general
monitoring programs are being considered: spring counts of males and counts of nests at breeding
colonies. Monitoring programs that utilize nest counts continue to be an attractive option because of
the potential to make precise counts of nests at large colonies. But, breeding effort in eiders may vary
annually which may influence effectiveness colony counts, and the precision of nest counts has not
been evaluated. Throughout much of the southern portion of their range large numbers of eiders nest
on islands with dense vegetative cover (up to 12000 pairs per colony) which can be difficult to search
completely. We use a data set collected from a forested eider colony in the Wolves Archipelago, New
Brunswick to evaluate optimal timing of nest surveys and estimate probability detection rates. Over a
three-year period, the colony was searched 7-8 times annually. During each search, the fate/status of
each nest was recorded and each nest was marked with a numbered marker. In 1984, 1985, and 1986,
respectively, we found a total of 500, 1032 and 858 nests. We use mark-recapture analyses to estimate
total breeding effort and to evaluate probability detection rates of nest searches. Initial analyses
indicate that only a portion of nests initiated in the colony may be detected during single surveys of
the colony. Surveys conducted late in incubation (17-23 days after the mean laying date) detected the
largest proportion of the total nest counts which accounted for only 62%, 76%, and 65% of the total
nests found in 1984-86, respectively. Our initial results indicate annual variability in colony counts
may in part be due to variability in detection probabilities among years. We suggest that monitoring
programs based on counts of eider nests should incorporate estimates of detection probabilities to
improve the precision and accuracy of these surveys.
NOV. 7-11, 2005 ANNAPOLIS, MARYLAND, USA
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