Birdwatch

How birds use preen oil to keep their feathers in tip top condition.

-

KEEPING warm and dry is essential to birds’ survival and the best natural way to do this is their plumage. Both waterproof and insulating, feathers have to be in top condition and to keep them fully functionin­g birds have to regularly bathe and preen. Primary and secondary feathers, as well as tail feathers, must have a continuous surface to be effective as they flap and move against air resistance.

Preening ensures that feathers are fit both for flight and for adequate insulation, and most of it is done using the beak. Wing, tail and contour feathers all have a central shaft with vanes on either side. The vanes are made from barbs which radiate out and are interlinke­d by barbules.

When a bird preens it usually passes the feather between its mandibles and gently nibbles to relink any barbules which have become separated to form an unbroken surface which is waterand windproof. Sometimes a closed or open beak is stroked across the feather surface. Most preening takes place when a bird is perched, but in-flight preening has been observed in ibises, herons, albatrosse­s, gulls, terns, swifts, martins and swallows.

Birds have an extra additive to provide waterproof­ing in the form of preen oil which is applied over the feather surface. This waxy oil comes from a preen, or uropygial, gland, situated at the base of the tail in most species (in kiwis it is adjacent to the cloaca).

Most birds have this gland, although a few either lack one or have one which has become vestigial. Those without one include emu, ostrich, rheas and cassowarie­s, as well as frogmouths, potoos, some pigeons, Amazon parrots, some macaws and some woodpecker­s.

The gland looks like a small nipple and may have a feather by the opening to act as a wick. It is most developed in more aquatic species such as pelicans and seabirds like tubenoses.

The oil is used by rubbing the beak at the gland opening and then transferri­ng it to the feathers. The feet can be used to reach areas the beak can’t, such as the head; oil is transferre­d to the feet with the beak.

Studies have shown that birds spend between 5% and 30% of their time preening, averaging at 8-9%. Waterbirds preen the most, with gulls spending 15% and divers as much as 25%.

Some birds with long necks preen using toes which have a special pectinate claw, which acts like a comb. A study of pectinate claws showed that 17 out of 118 bird families reviewed possessed them, but not all species within the family did. These included frigatebir­ds, cormorants, herons, storks, grebes, terns, owls and nightjars; dippers are the only passerines with them. Birds which have no preen gland, or an inactive one, may have ‘powder down’, special feathers which never moult and have tips that break to form a slightly oily powder that can be spread over the plumage.

Preening also helps to keep both the feathers and the beak supple, as the oils stops them from becoming dry and brittle. Preen oil has been shown to contain bacteria which help to keep feathers free of parasites such as feather lice.

The oil may also have a distinct aroma which can be detected by birds. It consists of a mixture of various waxes and oils and its compositio­n has been shown to vary depending on the sex and age of individual birds, as well as the time of year. Common Starlings have been shown to be able to distinguis­h the sex of another by smell alone and this has been attributed to the preen oil on the feathers.

Hoopoe appears to have a particular­ly active preen gland. Renowned for is rather smelly nests, it actually transfers the preen oil to its eggs, turning them from blue-grey to greenbrown. The colour is due to bacteria which help to keep the embryos free from infection as well as helping to prevent feather degradatio­n. It seems the only down side is the aroma from the oil which really stinks! Chris Harbard

 ??  ?? Hoopoe has a particular­ly active preen gland, producing a very smelly oil.
Hoopoe has a particular­ly active preen gland, producing a very smelly oil.

Newspapers in English

Newspapers from United Kingdom