The bird spikes are not harmful to birds unless they actually come into contact with the bird spikes.
The bird spikes that are installed on buildings and other places basically just act as a visual and physical barrier to prevent birds and pigeons from landing on the building and other places that bird spikes are.
If the bird were to actually sit on the bird spike then the bird spike could harm the bird or pigeon but since most birds and pigeons avoid those bird spikes the birds don't get hurt.
Since the birds avoid coming into contact with the bird spikes the bird spikes are a good humane way of deterring birds and pigeons from buildings and any other area that the spikes are on.
If the bird does try to land on the bird spike they may receive a slight prick as a result but other than that they will not be harmed.