This is a large and handsome butterfly, with a wingspan of around 90 millimetres. It is mostly seen gliding gracefully within a few metres of the ground, or sipping nectar from flowers.

It seems to be self-introduced in Australia, being first recorded in 1871. By that time, its favourite foodplants were well-established. Cotton-bushes and milkweeds, including one known as the swan plant, are the usual host plants for wanderer caterpillars in Australia. These plants are from southern Africa, not from the butterfly's American homeland.

Somehow, two names have arisen for this butterfly. In North America, it is known as the monarch, while in Australia, it is the wanderer.

Although it is uncommon here, it could not be classed as rare. It is more common in milder parts of Victoria, both north and south of the Divide.

Victorian populations are temporary, with migrating adults colonising Victoria each spring. The adults can live up to four of five weeks.

Some people grow the swan plant (Gomphocarpus (Ascelepias) fruticosus) to attract the butterflies. This often results in butterflies appearing and laying their eggs. The plant is a weed in some places. The striped caterpillars grow to 40mm or more, and the green chrysalis (pupa) is daintily ornamented with gold.

Unlike many introduced plants and animals, wanderer butterflies do no harm to native plants or garden plants because of their special preference for the introduced swan plant and other milkweeds.

LAKE BIRDS COUNTED

Although many of Lake Wendouree's waterbirds remain at similar numbers to December, the coot population has risen. The current total is now 55, whereas just eight were counted in early December.

With numerous new families seen, wood ducks have almost doubled, at 101, as have black ducks at 117. Numbers of both purple swamphen and dusky moorhen have risen. The swan population - at 114 - has dropped slightly in the six weeks since the previous count.

Compared with a count 50 years ago, the swan numbers are less than a quarter, but they were more numerous in the 1970s. There were fewer swamphens and moorhens in 1972, but more great crested grebes and musk ducks. There were no white ibises, little pied cormorants and little black cormorants, and only 26 silver gulls. The cormorants and ibises did not breed here 50 years ago, but they do so now. Thirty species were recorded on the 1972 count, compared with 46 in January this year.

What is the name of this plant? The flowers were more vibrant pink than the photo suggests. C. V., Buninyong.

Your flowers belong to the common centaury, a common summer-flowering introduced plant. Another name is pink stars.

It grows in grassy places and on roadsides. Although originally from Europe, and sometimes regarded as a weed, it grows mostly in disturbed areas with other exotics, rather than in places where native vegetation dominates.

Botanically it is Centaurium erythraea. There is another very similar plant with slightly richer pink flowers and narrower petals. It is exotic too, and it tends to flower slightly later.

Common centaury has a long European history as a medicinal plant, and is still collected today by some Ballarat people of European background.

The plant was once regarded as a native Australian, but is now recognised as an exotic. The flowers open only in sunny weather.