HawkTalk Issue 90 Spring 2022

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HAWKTALK SPRING 2022 ISSUE 90

The Marvels of Moulting

Photo: Charles James Sharp

In the last edition of HawkTalk , we included an insight into feathers and promised to include a follow-up article to explore what happens when a bird moults; a truly fascinating process. Most birds moult their feathers annually because over time feathers wear out or may become damaged. Moulting is when a bird sheds those old feathers to make way for new ones. Damage to bird feathers occurs almost continuously, through exposure to UV light, abrasion against vegetation and occasional breakages that might happen when escaping a predator. The damage caused by these factors reduces feather efficiency, which affects not only how well the bird can fly, but also the protection feathers provide (insulation, waterproofing and camouflage) or how effective they are during displays. To keep all of these functions working correctly birds must replace their feathers. Moulting feathers and growing new ones requires a lot of energy though, because the bird has to change its general physiology and behaviour so it can produce the different proteins (amino acids) that help with feather production – all whilst having its flight performance reduced. Compared with a non-moulting bird, a Robin with a moult lasting around 54 days, needs to find an extra 3 percent of energy each day during the early and late parts of its moult, and up to 20 percent more during mid-moult. This is why moult usually occurs during the summer when food is readily available. Timing of moult is critical. A moulting bird will not fly as well or be able to keep as warm as a bird with new feathers, so it is important that moult is completed quickly before the onset of colder weather or migration. In most birds in the UK, moult of all the flight and body feathers usually occurs just after breeding, during late summer. They will also often have another, more limited, moult of body feathers in the spring, just before breeding.

Juvenile Blackcap moulting its head feathers into adult plumage

Non-moulting male Blackcap

Newly-hatched birds grow their first full set of feathers in the nest. These are kept through the winter before some of the body feathers are replaced during the spring to give a similar plumage to adults – just in time for breeding. After nesting, they will then undergo a complete moult, in which all flight feathers are replaced. Feathers are very worn by this time, so this full set of new feathers are essential to help them survive the coming winter weather. In subsequent years, the moult will follow the same basic pattern; occurring after nesting each year. This timing is the reason many birds seem to disappear and the countryside becomes devoid of birds in late summer; many are replacing feathers and staying in cover and inconspicuous to reduce the chance of being predated. In UK, songbirds’, moult is a sequential process starting with the replacement of the innermost primary. The new feather grows at the base of the old feather, forcing it out and growing in its place. Once around 30-40 percent of its full-grown length, the next primary starts to grow and so on until all primaries are replaced. Replacement of the tail feathers and the inner wing feathers (secondaries and tertias) usually starts when moult in the primaries reaches the fourth, fifth or sixth primary. Moult then continues away from the body in the primaries and inwards towards the body in the secondaries. This sequence helps to minimise disruption to flight performance. For many larger species, especially larger raptors, replacing all flight feathers in one go would cost too much energy and take too long. In these species, moult is usually partial and involves the replacement of perhaps just one or two primaries, five or six secondaries and three or four tail feathers each year. Moult then stops over the winter before resuming at the points they left off in the following summer. Examination of the different ages of feathers in these species’ wings can be used to work out how old the bird is. So, next time you see a Buzzard circling overhead, take a closer look and see if you can detect those feathers which have been replaced.

Buzzard wing – darker feathers are new replacements of those recently moulted