Feather Growth and Replacement Strategies Changes the Way Birds Look
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MOULT In the March/April 2014 issue of African Birdlife Peter Ryan described how feathers grow, and how moult plays a key role in the annual cycle of birds. In the second article on this fasci- nating topic, he expands on how birds have evolved different strategies to cope with the numerous constraints they face shedload when replacing their feathers, and explores how moult FEATHER GROWTH AND REPLACEMENT STRATEGIES changes the way birds look. FEATHERS have allowed birds to flourish across the globe, but they need to be replaced regularly if they are to function effectively. Feathers grow from follicles in a bird’s skin and, with the excep- tion of the powder down, feather growth is episodic, resulting in dis- crete generations of feathers. The type of feather produced changes during the life of a bird. Precocial chicks hatch with a layer of down to help them keep warm. Flamingo and penguin chicks moult into a second downy plum- age before the first contour feathers Fdevelop. However, passerine chicks have only a few wisps of down be- cause they rely on their parents to keep them warm when they first hatch, and chicks of some hole- nesting species such as kingfishers Many birds use a second body and woodpeckers lack any down – moult to vary their appearance their juvenile contour feathers are for breeding. This male Southern the first feathers they grow. Red Bishop is completing moult Contour feathers provide water- into its distinctive, brightly proofing and protection, and im- – at the expense of feather quality. Chicks of precocial birds, coloured breeding (alternate) prove insulation by shielding the In general, juvenile feather quality such as this Black- plumage, but many old, drab, down layer. In many passerines increases with the duration of the winged Stilt, have a non-breeding (basic) feathers and at least some other orders of nestling period. first plumage of down still have to be shed. Male birds, the juvenile contour feath- The trade-off between feather feathers. Such chicks bishops have to replace all their ers are poor facsimiles of adult growth rate and quality persists only start to grow con- body feathers in order to acquire feathers, weighing less and hav- throughout a bird’s life. For exam- tour feathers midway a full set of breeding finery. ing fewer barbs. Juveniles of these ple, in Grey Plovers feather wear through the nestling Female bishops may also replace species look ‘soft’ as a result. Their rate increases in relation to the period. some body feathers at this time, first set of body feathers is replaced speed of moult, probably because but lose fewer than adult males. within a few months of fledging, slower growth allows more mela- The flight feathers are not which might seem to be a waste nin (which enables a feather to bet- moulted until after breeding, of resources. However, these birds ter withstand wear) to be deposited but male widows with extravag- typically face a high risk of preda- in each feather. However, the dura- ant tail streamers replace their tion as nestlings, so evolution has tion of moult is determined more tail feathers in both the pre- and favoured strategies that allow them by moult intensity (the number of TEXT & PHOTOGRAPHS PETER RYAN post-breeding moults. to leave the nest as soon as possible feathers growing at once) than > RICHARD DU TOIT 36 AFRICAN BIRDLIFE MAY/JUNE 2014 MOULT 37 continent in late summer. Many and within species. Some birds The simplest strategy involves of the western species that mi- moult once a year, usually after only one moult per year, which grate before moulting only travel breeding, and so the only change usually takes place after breeding. as far as the Mexican monsoon to their feathers’ appearance dur- This post-breeding moult gives belt, moult there, then continue ing the year is due to wear. Other rise to what the Humphrey–Parkes further south to their main winter- species moult twice or even more system calls the ‘basic’ plumage ing areas. frequently, allowing them to re- and so is known as the pre-basic By comparison, in Eurasia there fresh their plumage and potentially is no obvious effect of climate on change their appearance seasonally. when and where passerines mi- These changes are often linked to grating into Africa moult; more breeding activity, with the complete than 40 per cent of species delay post-breeding moult generating the the onset of moult until they reach non-breeding or winter plumage, Africa, irrespective of the location and the pre-breeding moult (usu- of their breeding grounds. Migrant ally incomplete because it doesn’t passerines also tend to moult later involve the flight feathers) generat- in Africa than their New World ing the breeding plumage. counterparts. This is thought to The nature of the feathers grown be related to the greater rate of is determined by the bird’s hor- feather wear in Africa, where most monal condition, however, not migrants winter in savanna habi- the timing of moult. By manipu- tats, compared to the extensive lating hormone levels, researchers use of forest and woodland in the can induce juveniles to develop Neotropics. It seems that birds de- adult plumage and adults to retain lay their moult in Africa because their breeding plumage during the they need fresh feathers for the post-breeding moult. Photoperiod northward migration and subse- also can affect the type of plum- quent breeding season. Further age grown. Southern Red Bishop support for this hypothesis comes males kept under artificially long from Phylloscopus warblers; those day lengths retain their breed- migrating to the forests of South- ing plumage in the post-breeding Juvenile passerines such by the rate of individual feather duced smaller clutches. Similarly, THE RATE OF MOULT East Asia moult earlier than their moult, even though their testes as this Cape Robin-chat growth. Interestingly, there is little when moult by Common Starlings DEPENDS IN PART ON African counterparts. regress. And even in nature one typically fledge with evidence that moult intensity af- was delayed experimentally, the THE TIME CONSTRAINTS Less is known about patterns sees variations in the intensity of moult. This so-called ‘basic strate- Large gulls show a dizzy- soft, lax contour feath- fects feather growth rates. However, birds increased moult intensity, of moult duration among tropical expression of breeding plumage, gy’ is found in albatrosses, petrels, ing array of ‘immature’ ers, which are replaced a bird’s nutritional and health sta- resulting in reduced feather mass, IMPOSED BY OTHER and southern temperate birds. In linked to individual differences in raptors, swifts and a few gulls. plumages because they within a few months of tus does affect the duration and strength and durability. ACTIVITIES IN A Bird’S general, moult is more protracted hormone levels and the degree to Some birds have an additional undergo two moults the birds leaving the nest. extent of moult. Albatrosses with The rate of moult depends in ANNUAL CYCLE. IN in the south, and in some cases can which feather growth coincides post-juvenile moult in their first per year, and the colour large numbers of stomach nema- part on the time constraints im- GENERAL, BIRDS DON’T occur at a low level year round. In with periods of peak hormone year, replacing the hastily-grown of their back feathers todes moult fewer feathers per year posed by other activities in a bird’s one interesting study, Red-billed levels. The bewildering diversity feathers with a ‘formative’ plumage depends on the timing MOULT WHILE BREED- than birds with few parasites. annual cycle. In general, birds Quelea primary moult duration was of immature gull plumages, which (grown during the pre-formative of moult as well as on In one of the most compelling don’t moult while breeding or ING OR MIGRatING estimated to increase from 75 days are a pastiche of juvenile and adult moult). This ‘complex basic strate- the bird’s age. This Kelp demonstrations of the costs im- migrating. As a result, immature in Namibia through 83 days in Bo- body feathers, results from indi- gy’ is exhibited by gamebirds, ducks, Gull, about 16 months posed by rushing moult, Jan-Åke birds can afford to take longer to after arriving at their non-breed- tswana to 101 days in Gauteng and vidual differences in hormonal rails, cranes, several seabird groups, old, mostly exhibits a mix Nilsson and Erik Svensson de- moult than adults, and migrant ing grounds. 124 days in the Eastern Cape. This status and the timing of moult. and many terrestrial birds. It is of older feathers from its layed the onset of moult in adult species tend to moult faster than There are interesting geograph- discrepancy apparently resulted Confusion between moult se- widespread especially in temperate first alternate plumage Blue Tits by prolonging the birds’ resident species because they have ic differences in the prevalence from differences in feather growth quence and function led Phil systems where breeding is strongly and fresh feathers from breeding season. Although the the added constraint of a migra- of these three strategies. Among rates rather than in the intensity of Humphrey and Ken Parkes to pro- seasonal, because juvenile plumage its second basic plumage, tits completed their post-breeding tion. Some birds complete their North American migrant passer- moult, and reflected regional dif- pose a new terminology for moults is grown before the annual pre-basic but still retains a few moult on time, the quality of their moult before leaving their breed- ines, barely 10 per cent of species ferences in when moult started, as and the resultant plumages.