So far this spring it has been found at Lake Goldsmith and Lake Wendouree. At both places there was just a single bird. The Lake Wendouree bird was sighted on the mud islands in the lake's centre.

The marsh sandpiper is one of a dozen migratory wading birds visiting us from the northern hemisphere. Some of these are confusingly similar in appearance, especially the smaller species.

The marsh sandpiper is a slim, elegant and active bird, greyer and plainer than most, and it is larger than the common sharp-tailed sandpiper. Its long bill is finer than that of the similar but larger greenshank, and its legs have a yellowish tone.

With most of the similar species being birds of marshes, its name is no help in its identification. It is found mostly on inland waters - especially brackish ones - rather than on the coast.

The mud islands in the centre of Lake Wendouree provide suitable sheltered resting and feeding places for many birds that seldom visit the busy shoreline. Sharp-tailed sandpiper, red-kneed dotterel and pied stilt are often found there in small numbers, with red-capped plover and black-fronted dotterel sometimes seen also.

A few weeks ago there were 900 sharp-tailed sandpipers at Lake Goldsmith.

A few local bird species can have dark or light plumage. Examples are little eagle and white-bellied cuckoo-shrike.

In both cases, the "light" plumage is the usual colour, while the dark plumage is scarcer.

Birds can have all sorts of colour aberrations or mutations (such as albinism and melanism), but the two species mentioned have these dark forms frequently and consistently enough for them to be illustrated in bird books.

In both cases, the dark birds share the same distribution as the normal, lighter, ones. It can make identification difficult.

This phenomenon seems to occurs most often in raptors, with grey goshawk and brown falcon being further local examples.

Similar examples from other parts of Australia include a shearwater, the reef heron, a skua, an owl and a butcherbird. The Gouldian finch has three different plumages, but these relate to head colour, rather than light or dark general plumage.

Common heath - which can have white, pink or red flowers - is a well-known similar example in the plant world.

What is the difference between a beak and a bill?

L.J., Nerrina

In general usage, the words beak and bill are interchangeable.

The word beak is often used when pecking, tearing and nipping are the main purposes. Parrots, hawks, magpies and finches are often regarded as having "beaks", while a duck is often said to have a "bill".

Beaks are often strong and bills often weaker. Thus, a spinebill and a spoonbill have bills, as also do most birds with slender, flattened or round-tipped bills.

While many waterbirds - such as ducks, stilts and sandpipers - are often regarded as having bills, "beak" seems the more appropriate term for the stronger-billed waterbirds such as herons, cormorants and swamphen.

A vet or an ornithologist may refer to a cockatoo's or a hawk's bill, but stronger bills such as these are usually referred to as beaks by most people.