The reproductive success of ducks nesting in association with gulls on islands in Miquelon Lake, Alberta, was studied over a period of 3 years. Gulls protected nesting ducks by preventing other egg-eating birds from foraging on the islands. Pilfering of duck eggs by gulls was correlated with the point in incubation when ducks were trapped and handled. Without trapping, we estimated hatching success would exceed 90%. However, duckling mortality, concentrated in the 1st week posthatching, was positively correlated with the numbers of associated gulls. The survival of ducklings declined to zero when the number of nesting gulls approached 500 pairs. The origin and maintenance of this ecologically unstable relationship is discussed.

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BOAG, D. A. 1986. Dispersal in pond snails: potential role of waterfowl. Can. J. Zool. 64: 904-909.Young pond snails of three species (Lymnaea stagnalis, Stagnicola elodes, and Helisoma trivolvis) were allowed free access to waterfowl feathers floating on the surface of shallow containers. Snails adhering to the feathers were subjected to simulated flight conditions by placing them in an air stream. The size of these snails, the length of time they adhered to the feathers while in the air stream, and their survivability for various lengths of time under these conditions were recorded. Those snails that adhered to the feathers were mainly small individuals (<2.5 mm long in all three species). The proportion that remained clinging to the feathers declined sharply with time; after 15 min of exposure only 6% of L. stagnalis, 18% of S. elodes, and 15% of H. trivolvis remained. Survivability also declined with time exposed to the simulated flight conditions; after 15 min only 50% of L. stagnalis, 23% of S. elodes, and 15% of H. trivolvis were still alive. Larger individuals tended to survive better than smaller ones for any given exposure time, but they also tended to fall off the feathers sooner than smaller ones; no snail more than 3 rnrn long was recorded adhering to the feathers for more than 4 min under the test conditions of simulated flight (air speed of 41 k d h ) . These results suggest that despite a low proportion of the snail population adhering to the feathers (< 1 % of those available at any given time) and the relatively high rates of loss, either by falling off the feathers or dying in situ after adhering, the probability of successful dispersal for distances up to 10 km remains high (a mallard flying for 15 min at 4 1 km/h was deemed capable of carrying at least three surviving individuals of any of the species tested over a distance of about 10 krn). Of the three species tested, S. elodes had the greatest probability of successful dispersal, which may explain its almost ubiquitous distribution in the water bodies of central Alberta. BOAG, D. A. 1986. Dispersal in pond snails: potential role of waterfowl. Can. J. Zool. 64: 904-909.De jeunes gastropodes d'Ctang appartenant a trois especes (Lymnaea stagnalis, Stagnicola elodes et Helisoma trivolvis) ont Ct C mis en prCsence de plumes d'oiseaux aquatiques a la surface de rkcipients peu profonds. Les gastropodes accrochds aux plumes ont ensuite Ct C placCs dans un courant d'air de fason a simuler les conditions de vol. La taille de ces gastropodes, la durCe de leur maintien sur les plumes dans le courant d'air et leur capacitC de survivre pour diverses pCriodes de temps dans ces conditions ont Ct C dCterminCes. Les gastropodes qui restaient accrochCs aux plumes Ctaient surtout des petits individus (<2,5 mm de longueur chez les trois espkces). Le nombre d'individus qui restaient accrochCs aux plumes diminuait rapidement en fonction du temps; aprks 15 minutes d'exposition, seulement 6% des L. stagnalis, 18% des S. elodes et 15% des H. trivolvis Ctaient encor...

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The characteristics of a large population of Columbian ground squirrels (Spermophilus columbianus), the numbers of which varied between ca. 700 and 1000 animals emerging in the spring, were studied over a 7-year period in southwestern Alberta. Over time, the density of nonjuvenile squirrels varied between 11.6 and 16.1/ha, with a sex ratio that consistently favoured females; the density of juveniles varied between 4.6 and 20.7/ha, but with a sex ratio that was usually balanced. Spatially, the density of squirrels also varied with a certain part of the study area (17%) annually supporting proportionately more animals of each sex than the remaining 83%. Dispersal was recorded primarily among yearlings, with more males than females involved in this movement. Most dispersal occurred in late June and early July, centred on the time juveniles emerged from natal burrows. We recorded squirrels dispersing up to 6.7 km. Survival of squirrels in their 1st year of life was lower (females 40%; males 35%) than in any subsequent year (females 50–64%; males 37–54%). These attributes of Columbian ground squirrels are compared with those of other members of the genus, and the evidence they provide for population regulation is discussed.