Movements and temporary emigration of moulting and wintering Harlequin Ducks

Robertson, Gregory J.
Cooke, Fred
Goudie, R. I.
Boyd, W.S.
Canadian Wildlife Service

Over 2500 Harlequin Ducks (Histrionicus histrionicus) have been marked with individually identifiable tarsal leg bands in the Pacific Northwest of North America. Two of these populations have been intensively studied near Vancouver, British Columbia over three winters (1994-1996). Intensive marking and surveys were done at these two sites (White Rock and Point Roberts, WA) to assess movement patterns and emigration of Harlequin Ducks. Because the population size and the number of birds assessed for bands was known on all surveys, independent estimates of p* could be obtained, allowing temporary emigration to be estimated. Some proportion of the marked population of both sexes temporally emigrated from the White Rock study site in the mid-winter and in the spring. Independent assessments of p* also provided the ability for us to estimate the probability that an individual bird was not present at the study sites for a given period of time. Some birds were not sighted for long periods of time (i.e. months) while other birds were sighted as often as expected based on our estimates of p*. The estimated number of banded and unbanded birds also varied significantly over the year at White Rock. Some unmarked birds moved into the White Rock study area after the moult while some marked birds departed. Only one male changed moulting grounds (1 of 50, WR to PR), no females exchanged moulting sites (0 of 71). After the wing moult some WR females (9 of 47 (19.1%) in 1995) were sighted at PR , however a reciprocal movement of PR females to WR was not detected (0 of 24 birds). Similar movements of males were not seen. On the scale of months our p value approaches 1.0, therefore individual which stay longer than a month after moving should be detected. Smaller intensive studies (either through intense re-sightings effort or radio tracking a small portion of the population) can provide valuable insights when designing and analyzing a large scale CMR data set to eliminate potential sources of bias due to temporary emigration. For example, we found low levels of exchange during the moult, yet afterwards significant numbers of females moved to other sites. Therefore, a re- sighting effort done after the moult would result in fewer local females being resighted, thereby underestimating female survival as compared to the males.