Old World swallowtail

Swallowtail caterpillars eat plant leaves and flowers.

The grown-up butterflies sip sweet nectar from flowers.

Swallowtail caterpillars eat only certain plants. Tiny caterpillars munch the leaves of their food plant, and bigger ones prefer to eat its flowers. In Europe they eat plants of the parsley family, such as milk-parsley, wild carrot and wild angelica. In North America, they mostly eat sagebrush plants instead.

Adult swallowtails drink sweet nectar from flowers. As they visit flowers, they can help to pollinate them.

Larval host plants differ across the range. Butterfly Conservation reports that the British race feeds solely on Milk-parsley (Peucedanum palustre), while continental gorganus uses a variety of umbellifers such as Wild Carrot and Wild Angelica. In North America, BAMONA lists sagebrushes (Artemisia), including Arctic wormwood and wild tarragon, and only rarely plants of the parsley family.

Newly hatched caterpillars eat leaves; older caterpillars on parsley-family plants prefer the flowers. Adults feed on flower nectar, and Animal Diversity Web notes that, like other butterflies, they can be effective pollinators.

Swallowtails are strong fliers.

If a swallowtail caterpillar is bothered, it pops out a bright orange fork. The fork has a strong smell that tells others to stay away.

Adult swallowtails fly strongly over meadows and fens. When a female is ready to lay eggs, she flies low over the plants, looking for the right food plant for her caterpillars.

The caterpillars have a clever trick. Behind the head is a hidden orange organ called an osmeterium, shaped like a letter V. When the caterpillar is touched or bothered, it pushes the orange fork out and gives off a strong smell, which tells hungry animals to stay away.

Adults are powerful fliers; UK Butterflies notes that egg-laying females fly low over vegetation searching for suitable plants and usually leave only one egg per plant at a time. In North America, BAMONA lists open hilltops among its habitats.

The larva carries an osmeterium, a V-shaped, orange eversible gland on the first thoracic segment. UK Butterflies describes it being protruded only when the larva is threatened, releasing a pungent, acrid scent. A larva that falls from its plant can hang by a silk thread, and when resting it raises the front half of its body.

In most places there are plenty of swallowtails.

In Britain they are rare. People look after the wet fens where they live.

Across the world the Old World swallowtail is not in danger. NatureServe calls it secure, although it can be rare in some places.

In Britain, the swallowtail is a special case. Its own British kind lives only in the fens of Norfolk, and its caterpillars eat only one plant, milk-parsley. It is listed as Vulnerable in Britain and is fully protected by law. Its future depends on people caring for the fens so that milk-parsley keeps growing.

The species has no global IUCN Red List assessment; NatureServe ranks it G5, demonstrably secure globally though rare in parts of its range (BAMONA), and Butterfly Conservation gives its European status as not threatened.

In Great Britain the native race britannicus is listed as Vulnerable on the GB Red List (2022) and is fully protected under the 1981 Wildlife and Countryside Act. Butterfly Conservation reports an abundance trend of +51 percent (1976 to 2019) but a distribution trend of −27 percent (1996 to 2019). UK Butterflies calls the population stable but highly dependent on the management of its fenland habitat, and Animal Diversity Web notes efforts to reintroduce it to parts of England where it has been lost.