How do they keep birds from eating the fruit? In my personal but limited experience, birds tend to eat certain kinds of fruit long before it ripens enough for human consumption.

The birds do eat some, but that's fine. The birds in this city don't appear to have any shortage of food. Some plums or berries will be ruined or taken, but mostly the garden manages itself just fine. Same as you'll find wild berries in huge numbers – many have already been eaten by wild animals, but there's still plenty left too.

One perk of this style of gardening is that, for various reasons, pest moderation seems to happen automatically. No pesticides are required, the garden is watered by humans only a handful of times in summer, and no fertilizers besides city compost are used. Everything is lush and happy. Insects are everywhere, and perhaps birds are eating those instead of berries. It's hard to say exactly how each biome functions, but this one is very productive and animals aren't a problem for it.