Dead Birds

I think I have seen a lot of dead birds lately. Has anyone else noticed this? In the last two weeks I have spotted one dead bird in someone's yard and four dead birds on various sidewalks. It all seems weird to me because I cannot remember the last time I just saw a small dead bird on the ground. They all look like they died of natural causes. Is this a strange phenomenon or something that happens every year? Maybe mating season was especially fierce this year. Maybe it is global warming.