Answer:
As days grow longer in spring, robins suddenly develop an urge to move northward, to separate from other robins and live on a territory, and to build a nest and raise babies. But they can’t separate from every single robin! In order to raise babies, they need to choose one particular robin who they’ll allow on their territory to be their mate.
The first thing robins do in spring is to arrive on and claim a territory. We can easily observe males doing this; they sing as well as chase other males off the territory. Females are much quieter, but equally determined to defend their territory against other females.
Females arrive on territory a few days to a week behind the males. Most of the males’ territorial battles are over by then. Females probably select a territory by how good the habitat looks, but often notice the territory in the first place because of the male’s singing. When a female enters a male’s territory, he doesn’t chase her away. He does notice her, often singing even more. If she likes what she sees and hears, she starts looking around for a good place to build a nest. It turns out that even though robins are devoted mates, they first fell in love—not with each other but with the territory!
The mates stay closely bonded during the summer season. Each is responsible for certain jobs as they raise their babies. The female builds the nest, though the male often brings her some of the materials. He continues to sing as she lays the eggs and incubates them. When the babies hatch, both the parents get busy feeding them. When the babies fledge (leave the nest), both parents continue to follow them and feed them for a few days. But then the female gets busy building a new nest and laying new eggs. While she incubates the new brood, the male continues taking care of the older babies. He leads them to a stand of trees in the evening where they will roost with other robins. By the time the new eggs hatch, the older babies are ready to be on their own, and the male is able to help feed the new babies.