Bitcoin (and any other crypto) is taxable whenever you exchange it no matter for what (fiat, other cryptos, products, services, etc). Donations can be claimed if requisites are met but AFAIK you are not required to report them.
In your particular case you'll have to report if you exchange BTC for any other coin, sell it for fiat or use it to buy something directly. For Uncle Sam cryptos are not money but property. That means that even if you pay for a pizza with BTC, for the IRS you first sold your BTC for USD (at a gain or loss) and used that USD to buy the pizza.
See the IRS notice shedding some light on this (it's short and readable by humans):
https://www.irs.gov/pub/irs-drop/n-14-21.pdfI'm not a tax expert. Taxes and crypto is still a gray mix for everyone. If you really want to be safe check with an accountant.