When you come to think of it, suburban gardens and town parks are like pockets of the countryside - bees, shrubs, flowers, artificial lakes, waterways, plenty of grass - numerous mini-habitats far closer together than you would find in open country. The fact that everything is cluttered together means that the birds get used to a lot going on at very close quarters: they become far less shy.
Features of the town can also, unintentionally, provide an ideal habitat for birds: high buildings with ledges and cornices, girders and cranes, arches and "cave-like" structures, chimneys, street-lamps, overhead wires, fountains - and so on. For birds it's food, however, which is likely to be the main attraction to sharing living quarters with predatory Man.