Initially plane parts (engines, wings, cockpits) were tested against dead birds but nowadays we have so many live strikes every year that in practice we know wh

Initially plane parts (engines, wings, cockpits) were tested against dead birds but nowadays we have so many live strikes every year that in practice we know what the proxy based on gelatin is for a variety of birds. Gelatin is harder than flesh but softer than bone and the overall effect is that on average it has the same effect as birds because it is more like an average representation. While parts of birds are relatively tough they aren't in sufficient volume or weight to penetrate any more. That isn't to say a bird strike isn't pretty dangerous because they can be. Jet cockpits get severely damaged by them as do engines and you can see in the video the effect on a wing.