Bike box or bag?

Do you need to alert them, or just book a ticket as normal with hold luggage?

Depends on the airline and the size of your bag. The Aerus bike bags I've got have always travelled as normal hold luggage, they're well under the maximum suitcase size, and fully packed are well under 20kgs. It was about 13kgs with just the Colnago. Usually, I have to take the case to oversized baggage and in smaller airports they'll often ask you to open the case.

Always check to see if the airline has an additional charge for bikes. If your bike bag is under the maximum suitcase size and there is an additional charge for bikes, whatever you do, never say there's a bike inside. They'll always try to charge you the higher price, and at the airport that can be expensive.

Strangely enough, I'm flying with Norwegian Air to the Canaries in May and they charge less for bikes than standard hold luggage!