I have to point you to an article on the CBC News site, I hope you read it.
I know those birds have really good eyesight but a scarecrow on the shore of a 3 km wide lake, gimme a break. This without the fact that migrating birds usually land after sunset.
A better idea would be to have the CEO of Syncrude and the ministers from both the Alberta and federal governments who are supposed to be responsible for the environment dress up as scarecrows and spend one full migratory cycle (N to S and S to N) on an island of floating crude in the middle of the lake making noise and looking scary.
If no birds died in the hellish "Pit of Doom" during this period, let The Three Stooges come ashore. It just might work, they sure scare the hell out of me.