I imagine that most birds, especially those in the city, don't live long enough for cancer to be a significant cause of death.
I imagine if your diet consists of things like "french fries sitting in a puddle of antifreeze and oil leakage from the side of the road" you could get cancer quite quickly :-)
I'm not really convinced that's the cause of cancer...
My aunt used to eat antifreeze and motor oil with her french fries and she lived to 103.
I don't know about cancer, but definitely some chemical poisoning -> thus proving pavel_lishin's point :).