They’re a lot alike. Both birdwatching and gardening can be done at home, cost very little and take a minimum of effort. They sort of feed off each other too. Birdwatching, like gardening, can be done quite well at almost no cost. "Unless you want to" is the key phrase; there are no books you absolutely need to have, no clubs to join, no trips to take, no special clothes etc. Unless you want to do these things.