When I take photographs on a trip to another country or another city, I often give myself a theme to form a sequence of images. In New England I took lots of photographs of fire hydrants. In Venice more recently, it was shrines. I always take photographs of skulls, whenever I see them.
In Rome I looked for old Fiat Cinquecentos, but I thought of this half way through, and besides - they are becoming hard to find. I never set myself the task of finding images of birds, but looking through my photographs, I discovered rather a lot of them.