A topic I know a surprising amount about is birds, specifically the birds that live around rivers and wetlands in Bangladesh. I'm a thirty-five-year-old civil engineer in Sylhet, so it's got nothing to do with my job, but it's been my passion for years.
I learned mostly by going out and looking. My uncle Habib used to take me to the haor, the big seasonal wetlands near our home, when I was about ten. He had an old pair of binoculars and a battered bird guide, and he'd point things out and make me write the names in a notebook. Later I joined an online group of birdwatchers in Bangladesh, and now I read field guides and listen to recordings of bird calls.
I've been interested for around twenty-five years, although it became more serious in my late twenties, when I bought a decent camera. Now I go out almost every weekend in winter, which is the best season because migratory birds arrive from places as far away as Siberia.
The reason it interests me so much is that birds tell you how healthy a place is. If certain ducks stop coming to a lake, it often means the water is polluted or the fish have gone. So birdwatching has made me more aware of environmental problems in a very direct way. I also find it calming. Sitting quietly beside the water at sunrise, waiting, is the opposite of my busy working week. And there's the thrill of spotting something rare. Last January I saw a Baer's pochard, a duck that's become very uncommon, and I was shaking with excitement.