When Italo Calvino talks about the city of Esmeralda as one of the travels of Marco Polo he describes it as a city containing a network of canals and routes so that the residents of Esmeralda never have to travel through the same routes every day to reach the same location. He also describes the invisible airy paths of the swallows that fly around the whole city.
I think that this text is a metaphor for all the choices and the decisions that we have to make in our life which influence our future. We all have our goals in life and even though our choices and decisions are influenced by a lot of things including the people surrounding us, the decision is finally ours and the responsibility of it too. To shape our life and form the structure of it, we move forward by making a decision from many options which further decides the path of the structure. These decisions can prove to be good or bad but at the time of choosing that option we always choose what is looks like the most convenient for us. There will always be some obstacle in our life but like the swallow, described darting down to gulp the mosquito and flying around as if fearless of the danger of the other predators shows how we should try to solve our problems instead of running away from them and live freely.