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August 2014

This month, we take a break from our series on alien invasive plants since most are dormant at this time of year and therefore more difficult to eradicate whilst not actively growing.

Prof. Peter Ryan, well known ornithologist and recently appointed director of the Percy Fitzpatrick Institute at the University of Cape Town, recently published a series of articles on bird moult. These articles appeared in the excellent African Birdlife magazine, which members of Birdlife SA receive bi-monthly.

Moult is a necessary evil that all birds have to face as feathers are dead structures that have to be replaced regularly. Feathers are essential for flight, body insulation and protection as well as for bird appearance. Much time and effort is expended by birds in preening and bathing to ensure that feathers perform their essential functions for as long as possible.

Feathers make up approximately 10% of a bird’s mass. Wing feathers consist of primaries of which there are usually 10. They are the long flight feathers on the bird’s “hand”. The number of secondaries (feathers attached to a bird’s “forearm”) is more variable. The number of contour feathers covering the bird’s body depends on a bird’s size and structure. Most passerines (birds that habitually sing or call and have “normal” feet with 3 toes facing forward and 1 facing backward) have between 1 000 and 3 000 feathers.

Feather wear is highest in flight feathers, especially in long-distance migratory birds.

However even the feathers of sedentary forest birds have to be replaced due to degradation by a combination of physical abrasion, sun damage and attack from bacteria, fungi and feather lice.

Birds moult largely in two ways. Most have protracted moults over a long period while others moult over a much shorter period and are unable to fly, thus requiring safe moulting areas, where predation risk is low.This explains the dominance of water birds taking the latter option. Adult female hornbills are one of the few terrestrial species that undertake a flightless moult replacing all their flight feathers while sealed into their nesting cavities. They are thus entirely reliant on the males until their feathers have grown!

Most birds undertake one complete moult each year, typically after breeding, when they replace all their feathers. Some birds have additional moults, usually confined to head and body feathers during the acquisition of breeding plumage.


References: African Birdlife Volume 2, Numbers 3 & 4


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