Our squadron suffered many birdstrikes in W.Germany in the early 1980's. Mainly swifts, swallows and pigeons usually resulting in minor damage to paint and a few dents. We used to fly with landing lights on to minimise the risk as most birds would avoid us if they could see us. We always used to land on asap to check for damage and then be on our way. Once took a pigeon straight down the no. 2 engine but the Turmo engine didn't even cough - one thing about a strong but basic engine. Lots of feathers and partly cooked meat in the jet pipe but nil damage.
However, the boss had a wood pigeon come through the lower transparency at 140 kts and hit the cyclic square on. He suffered severe bruising of the inner thighs. Would have been much worse if the cyclic hadn't taken the impact.
A colleague hit a seagull in a Gazelle and ended up with a cut head despite helmet and visor.
I once hit a black-eared kite (4 1/2 foot wingspan, weight about 3kgs!) at 150 kts at midnight in the far east. Fortunately it hit the swept-back upper structure just above the windscreen but the impact was severe and I am certain it would have penetrated the glass windscreen.
I was always very glad of my helmet and clear visor. I am now required to fly using just a headset and quite frankly I feel very vulnerable. I would suggest anyone who has the choice opts for the helmet and visor.