In a mild winter, robins start courtship in January, but the breeding season normally begins in March. The birds pair only for the duration of the breeding season.
Most nests are located on or near the ground in hollows, nooks and crannies, climbing plants, hedgebanks, tree roots, piles of logs and any other situations that provide a fully concealed cavity.
If a nestbox is provided to attract robins, it needs to be open-fronted, and placed in a hidden location in a climber or other such vegetation. Robins are famous for nesting in all kinds of unlikely locations, including sheds, kettles, boots, hanging baskets, coat pockets, under car bonnets, in farm machinery, even on boats in daily use.
The cup nest, built by the female alone, is made of dead leaves and moss, lined with hair. Courtship feeding is a very prominent activity, and the male can supply over a third of his mate's food intake during nest building and egg laying.
This extra food is important and can make a difference to the clutch size, particularly since a complete clutch represents about 90% of the females total body weight. Normal clutch size is 4-6, with one egg laid each day, usually early in the morning.
The birds are very sensitive to any disturbance during the nest building and egg laying, and will easily desert the nest if they think that the nest has been discovered. Unless the birds are used to people, it is best to stay clear of the immediate vicinity of the nest until the incubation starts.
Once the clutch is complete, incubation is by the female alone for 13 days. The shells of the hatched eggs are removed immediately from the nest by the female, who sometimes eats part of them for extra calcium.
The chicks hatch naked, and are totally dependent on their parents for food and warmth. Both parents look after the nestlings. Feather growth will become evident with the appearance of quills at three days of age. By five days the eyes start to open and they are completely open by eight days.
By this time, rows of feathers will start to appear on backs and flanks. The body is more or less feathered by 10 days. Flight feathers are the last to grow, and as the chicks fledge at 14 days, they will not be able to fly for another couple of days.
The young are tended by their parents for up to three weeks after fledging. Frequently the care of the fledged young is left to the male, while the female prepares herself for the next nesting effort. Robins have two broods a year. Three successful broods a year is not uncommon, and in a good year even four are known.
These multiple broods result in a long breeding season, and nestlings can be found until late July. There are considerable losses during the egg and chick stage only about 57% of eggs from completed clutches result in fledged youngsters.
Despite being fiercely territorial against their own kind, robins do not normally bother about birds of other species near their nests. The parental instincts of robins are highly developed, which has led to the frequent reports of robins feeding the chicks or fledglings of other species. The most regular recipients of this unexpected care are song thrush, blackbird, spotted flycatcher and willow warbler.