During my 30 + years around the air yard, I’ve seen plenty of nests in other pilots’ airplanes. In the intervening days between my last flight 10 days ago and yesterday a local mama bird built a nest and laid four eggs on top of my engine’s #1 and #3 cylinders. I’m grateful that both sides of the cowling of my Comanche open wide, permitting me unlimited access to my engine. I set the nest aside and used compressed air to blow the remaining twigs and sticks off the engine.
If your cowling doesn’t permit a good engine inspection prior to flight, get some cowl plugs. These plugs are a lot less expensive than changing a cylinder due to a nest that’s blocked the cooling air.