look again aircraft have exhaust systems world wide. some exxhaust are dirty cause the engine is not getting maintance issues taken care of. . we hae many contrails here by a major airport 2 miles from my home ..............................................................we have yet to find dead birds on our sidewalks dead beas on our sidewalk or dead ants..contaiminted air will kills birds first ..before humans would die .. keep your eye on the birdie, then you know your air is safe ..