Migrating birds face many hazards, but it is not widely known that the impact skyscrapers have on migrating bird populations, especially in North America. I met a Canadian wildlife officer last year who described sitting in a city office block and hearing bird-strikes on the windows "with the frequency of raindrops". Researchers from the Fatal Light Awareness Project collected the corpses of birds over a two year period, 2008-9, from only two tower blocks in Toronto and totalled more than 900 dead birds, pictured below:
The good news is that these buildings have been refitted with less reflective glazing, which has apparently significantly reduced bird strikes and that future new buildings will have to follow suit. More details here. Researchers estimate that between 1 and 10 birds will hit each building in North America every year. That is a lot of buildings and a lot of dead birds.