The blackcap is a partial migrant; birds from the colder areas of its range winter in scrub or trees in northwestern Europe, around the Mediterranean and in tropical Africa.

The blackcap and its nearest relative, the garden warbler, are an ancient species pair which diverged very early from the rest of the genus at between 12 and 16 million years ago.

[3] The nearest relatives of the garden warbler outside the sister group are believed to be the African hill babbler and Dohrn's thrush-babbler, both of which should probably be placed in Sylvia rather than their current genera, Pseudoalcippe and Horizorhinus respectively.

[8] Fossils and subfossils of the blackcap have been found in a number of European countries; the oldest, dated to 1.2–1.0 million years ago, are from the Early Pleistocene of Bulgaria.

[11] The differences between subspecies are small, making subspecific boundaries hard to define, and the exact distribution of S. a. heineken is unclear, since birds from northwest Africa may be of this form.

Blackcaps breeding in the north of the range have an earlier and shorter post-juvenile moult than those further south, and cross-breeding of captive birds shows that the timing is genetically controlled.

The main call is a hard tac-tac, like stones knocking together,[17] and other vocalisations include a squeaking sweet alarm, and a low-pitched trill similar to that of a garden warbler.

[22][23] The continental breeding range of the blackcap lies between the 14–30° July isotherms, and is occupied by the nominate subspecies, the other forms being restricted to islands or fringe areas in the Caucasus and eastern Iberia.

[16] Birds on the Mediterranean and Atlantic islands and in the milder west and south of the main Eurasian distribution often winter within the nesting range, but populations elsewhere are migratory.

The blackcap is a leap-frog migrant; birds from the north of the breeding range travel furthest south, whereas Mediterranean breeders move much shorter distances.

[25] Similar experiments using birds from southern Germany and eastern Austria, on opposite sides of the migratory divide, demonstrated that the direction of migration is also genetically determined.

[27] In recent decades, substantial numbers of continental European birds have taken to wintering in gardens in Great Britain, and, to a lesser extent, Ireland,[29][30] where the blackcap was formerly just a summer visitor.

Although the British climate is sub-optimal, compensatory factors include the ready availability of food, (particularly from bird tables), a shorter migration distance, and the avoidance of the Alps and the Sahara Desert.

This suggests a trade-off between the cost of travelling long distances of migrants, and the flexibility required by sedentary individuals to tolerate a wide variety of environmental conditions.

Other habitats, such as parks, large gardens and overgrown hedges, are used as long as they meet the essential requirements of tall trees for songposts and an established understory.

[12] In Africa, habitats include cultivated land, acacia scrub, mangroves and forest, and these warblers are found at altitudes up to 3,600 m (11,800 ft) in the east of the continent.

Territorial boundaries are established initially by loud singing, performed while the male displays with his crown raised, tail fanned and slow wingbeats.

[37][38] Similar songs are a feature of the Sylvia warblers as a group, and it has been suggested that this promotes interspecific competition and helps to segregate territories between related species.

[19][39] It appears more likely from later studies that segregation of sympatric species, other than the blackcap and garden warbler, is due to subtle habitat preferences rather than interspecies aggression.

A male attracts a female to his territory through song[41] and a display involving raising the black crown feathers, fluffing the tail, slow wingbeats, and a short flapping flight.

[49] The blackcap feeds mainly on insects during the breeding season, then switches to fruit in late summer, the change being triggered by an internal biological rhythm.

When migrants arrive on their territories they initially take berries, pollen and nectar if there are insufficient insects available, then soon switch to their preferred diet.

Blackcaps eat a wide range of invertebrate prey, although aphids are particularly important early in the season, and flies, beetles and caterpillars are also taken in large numbers.

Although any suitable fruit may be eaten, some have seasonal or local importance; elder makes up a large proportion of the diet of northern birds preparing for migration, and energy-rich olives and lentisc are favoured by blackcaps wintering in the Mediterranean.

[51] Blackcaps defend good winter food sources in the wild,[36] and at garden feeding stations they repel competitors as large as starlings and blackbirds.

Blackcaps have evolved adaptations which make it difficult for the parasitic species to succeed, despite the cuckoo's tendency to lay eggs which resemble those of their host.

The open habitat and cup nest of the warbler make it a potential target for the cuckoo; it may have experienced much higher levels of parasitism in the past, and countermeasures would have spread rapidly once they evolved.

The study concluded that 45.5% of the males and 22.7% of the females were affected, but the number of parasites was small, and the ability to store fat for the migration flight was unimpaired.

[1] Blackcaps and other small birds are illegally trapped and hunted in large numbers in Mediterranean countries, particularly in Lebanon, Syria, Palestine, Malta, Libya, Egypt and Cyprus, where they are considered as a delicacy.

There are occasional nesting records from outside the main range, such as in northern Israel and the Faroes, and wandering birds may appear further afield in Iceland or on the islands of Arctic Russia.