Moult in birds

The woods which were so alive with bird song and nesting activity only a few weeks ago now seem quiet and deserted. Only occasionally is a bird to be seen flitting through the forest canopy, or heard chirping from some scrub. Where have they gone?

The answer is that they are still mostly there, but undergoing their moult. Without this annual replacement, feathers will wear out by abrasion, deterioration by ultra-violet light, or by chewing by ectoparasites. Then the birds are without physical protection, insulation and the ability to fly.

During moult, when some feathers are missing, full flight agility is impaired and birds are vulnerable to predation. Therefore they keep quiet, skulking in the undergrowth, unseen, keeping out of the way of predators as far as possible.

In some birds (especially waterfowl like the swans, geese and ducks) the loss of all the flight feathers ("primaries") occurs simultaneously, and the birds become flightless for a period.

Swan Lake: moulting swans become flightless and often aggregate into flocks during the moulting period

Masses of white feathers washed up on the shoreline is a certain indicator that moult is in progress

This mother swan is in full primary post-nuptial moult

Detail from the picture above showing the new primary feathers just starting to grow through. This bird can not fly during this process

In smaller birds, the moult is not so catastrophic and takes place over an extended period, dropping and replacing a tract of feathers in sequence. Bird ringers (banders) have a unique opportunity to observe moult and to record its progress. This gives an insight into this essential physiologically stressful process and to pin-point what factors (for example, climate change) might effect its timing and duration.

Bird-ringer Thomas rings and examines a swan

In smaller birds Primary moult can be recorded and each feather give a rating score according to whether is is old, growing or new and fully replaced.



Primary moult takes place from the "inside out".

TOP: Moult just started. The outside 7 long primary feathers are old, then two are growing.

MIDDLE, more advanced, only 4 long old feathers remain.

BOTTOM primary moult almost complete, only one unmoulted long primary at the outside remains. The next three are growing.

In real life: you can see that this Chaffinch has two long old feathers remaining on the outside, the third is shorter and is still growing. The ones inside them are new.

This male Bullfinch looks a little scruffy as he moults his plumage

A moulting Great Spotted Woodpecker