What kind of bird seed was it, ie black oil sunflower, mix of various seeds, etc?

Where are you located

You're planning to dump chemicals on your lawn because a few blades of grass are a bit wider than others?

It's probably not the bird seed, but just survival of the fittest. Just mow and don't worry.

Bird seed that looks like grass is most likely corn, millet or annual canary seed. Other seeds most often used in mixes for birds are broadleaf like sunflower or thistle. All of these grass seeds will want to grow taller than lawn turf, so if you just cut your grass regularly these taller growing grasses will wither.