EVALUATION OF RADIO-TRACKING AND STRIP TRANSECT METHODS FOR DETERMINING FORAGING RANGES OF BLACK-LEGGED KITTIWAKES

We compared strip transect and radio-tracking methods of determining f oraging range of Black-legged Kittiwakes (Rissa tridactyla). The mean distance birds were observed from their colony determined by radio-tra cking was significantly greater than the mean value calculated from st rip transects. We determined that this difference was due to two sourc es of bias: (1) as distance from the colony increased, the area of ava ilable habitat also increased resulting in decreasing bird densities ( bird spreading). Consequently, the probability of detecting birds duri ng transect surveys also would decrease as distance from the colony in creased, and (2) the maximum distance birds were observed from the col ony during radio-tracking exceeded the extent of the strip transect su rvey. We compared the observed number of birds seen on the strip trans ect survey to the predictions of a model of the decreasing probability of detection due to bird spreading. Strip transect data were signific antly different from modeled data; however, the field data were consis tently equal to or below the model predictions, indicating a general. conformity to the concept of declining detection at increasing distanc e. We conclude that radio-tracking data gave a more representative ind ication of foraging distances than did strip transect sampling. Previo us studies of seabirds that have used strip transect sampling without accounting far bird spreading or the effects of study-area limitations probably underestimated foraging range.