Do birds have nests all year round, or just when they have eggs/chicks?

The short answer is that birds that build nests leave them until they are destroyed. But some do not build nests. Most birds have nests all year round but only use them when rearing young. Others (like the cowbird) lay eggs in other birds nests (i.e.- they never build a nest) and even allow the other birds to feed the cowbird's young! Birds will nest when there is a combination of factors that is favorable for them. This includes temperature, food, and time of year. Some birds lay eggs on the ground (especially birds that nest on islands) and build no nests at all.