The breeding season lasts from early March to late July, and chicks are often found in a nest well into August. During this period, blackbirds rear 2-3 broods. In a good year, fourth broods may be attempted. Weather determines the timing of the breeding season.
Warm or cold spells in spring can bring the breeding season forward or delay it by several days. Dry weather in June can shorten the season and even cause starvation of late broods. The nesting season starts up to two weeks earlier in gardens than in woodland.
The nest, built by the female, is low down in any suitable cover. Trees, shrubs and climbers are preferred, but nests can be found inside buildings, occasionally even on the ground.
The nest is a substantial cup of grass, straw, small twigs and other plant material. It is plastered inside with mud and lined with fine grass. It can take two weeks to complete, and sometimes the same nest is used for successive broods.
The normal clutch size is 3-5. Larger clutches are laid in woodland than in gardens. The female incubates alone, and the chicks hatch 13-14 days later. Only the female broods the chicks, but both parents feed them. Chicks in gardens are fed on earthworms when they are available; woodland chicks are fed mainly on caterpillars.
The chicks are ready to fledge at 13-14 days, but if the nest is disturbed, they can leave and survive as early as nine days old. This ability to fledge early is an important anti-predator adaptation. The young birds creep and flutter from the nest, and remain in nearby cover for the following few days.
They are flightless at first, but within a week will have learned to fly. By this time, they begin to experiment with foods, learning by trial and error what is edible. As their skills and confidence grow, they begin to explore their parents' territory and range more widely. The young become independent three weeks after leaving the nest, and leave the natal area shortly after. They are not driven away by the male.
Fledged young are often left in the care of the male, while the female prepares for the next nesting attempt. The last brood of the season is usually divided between the parents, with each adult taking sole care of some of the young.
There are considerable losses at the egg and chick stage, with at best 30-40% of nests producing fledged young. Despite smaller clutch sizes, birds in towns fledge more chicks per nest than birds in the countryside.