dartford warblers first breed when they are one year old. they are usually monogamous and the pair bond can persist from one year to the next. the male sometimes builds several simple nests (cock nests) of which one is chosen by the female, but it is more usual for both birds to participate in constructing the breeding nest. in southern england the birds breed on heathlands, sometimes near the coast, and nest in either common gorse ("ulex europaeus") or common heather ("calluna vulgaris"). the compact cup-shaped nest is located in dense bushes, usually within of the ground. it is formed mainly of grasses and is lined with a layer of finer material that can include thin roots and feathers. the eggs are laid from early april in southern france and spain, and from mid-april in southern england. the clutch is typically 3–5 eggs which are smooth and glossy, with a white or occasionally pale green ground and marked with brown speckles which are sometimes concentrated at the larger end. the average size of an egg is with a weight of of which 6 percent is shell. the eggs are incubated for 12–14 days mainly by the female. the chicks are fed arthropods by both parents. the nestlings fledge 10–14 days after hatching and are then fed by their parents for a further two weeks. usually two and occasionally three broods are raised in a year.