Nexen says it believes the duck died before the company put fences, wildlife cannons and other deterrents in place.
About five million litres of bitumen, sand and produced water leaked from the pipeline near Fort McMurray last Wednesday.
Nexen says it regrets the spill and adds it remains committed to minimizing its impact on the environment and wildlife.
More than 1,600 ducks died after they landed on a toxic Syncrude tailings pond in northern Alberta in 2008, and the company was fined $3 million.
Two years later, more than 550 birds had to be destroyed when an early winter storm forced them to land on waste ponds belonging to Syncrude and Suncor.
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