| Summary: | A population of northern shovelers, Anas clypeata, was studied at Delta, Manitoba, to determine if behavioural mechanisms contributed to the spacing of breeding pairs. Further evidence supporting the contention that the Shoveler is a territorial species was obtained. Aggression of territorial drakes was localized about a loafing bar and defended boundaries existed between adjacent territories. The aerial pursuit flight was also shown to deter other shoveler pairs from establishing in the pursuer's territory. In 94.1 per cent of pursuit flights, the pursued birds(s) left the chaser's territory. Pursuit flight frequency reflected the density of pairs in the area studied. Flight frequency was the highest during pre-laying then decreased when incubation began. A subsequent increase in frequency coincided with an influx of presumably re-nesting pairs into the study area from elsewhere in the marsh. Flights were associated with aggression, rarely with rape, suggesting that aggression, rather than sex, was the primary motivation.
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