moult

strikingA balance Moulting flight feathers

Text & PHOTOGRAPHS PETER RYAN

In the March/April and May/June 2014 issues of African Bird- life, Peter Ryan described how moult plays a key role in the annual cycles of birds. In the final article in this series, he ex- plores how birds replace their flight feathers to minimise the impact on flight, and how large birds cope with the challenges of moulting, given limits on the rate at which feathers grow.

Flight feathers are the largest The gaps in the wings and tail during feathers on most birds and, because of moult do affect flight performance, but their importance for flight, much attention birds have evolved strategies to minimise is focused on their replacement. The first these costs. One of the simplest of these is to article in this series (March/April 2014) limit the number of feathers moulted at the discussed the extreme case of simultane- same time, depending on the importance of ous wing moult, when so many feathers flight to the bird in question. For example, are grown at once that the bird endures flycatchers generally moult fewer primar­ Southern albatrosses tend to replace their a period of enforced flightlessness. Most ies at once than warblers, because they are primaries every two years. Immature birds birds adopt a more gradual approach, re- more reliant on aerial agility. The number moult P8–10 (and sometimes a few inner placing a few feathers at a time, which al- of feathers grown at the same time also de- primaries) sequentially in one year, and lows them to go about their daily activities pends on their position on the bird’s body P1–7 the following year, gradually replacing more or less as normal. and perhaps the size of the feathers. Many more feathers each year as they get older. Flight feather growth is often the most petrels moult up to five of their short, inner Once they start to breed, the number of visible form of moult, because it results in primaries together, whereas they seldom feathers replaced each year depends on the gaps in the wings or tail that are obvious grow more than three outer primaries at the time available between breeding attempts. Fin flight, especially in large, soaring birds. same time. However, smaller species, such as this Indian It can also change a bird’s jizz, causing you The pattern of moult is best known in Yellow-nosed Albatross, can have more to question its identity. For example, Fork- the primaries, the main flight feathers at- eccentric moults. This individual appears to tailed Drongos and Yellow-billed Kites lose tached to a bird’s ‘hand’. The sequence in have three generations of primaries: from the their characteristic forked tails when moult- which the primaries are replaced depends outside of the wing, P10–8 are old, P7 is new, ing their outer-tail feathers, and birds with on the type of bird. In most species, moult P6–2 very old, and P1 new. By comparison, the pointed wings can have rounded wing tips starts with the innermost primary (P1) and remainder of the wing is mostly new. when growing their outer primaries. progresses sequentially to the outermost >

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above Lesser Jacanas are primary. There are exceptions to outermost primaries first seems Quite simply, large so they are not able to compensate is to replace the outer three prima- one of only a handful of this rule, however. Falcons typi- to make a lot of sense. If you only birds don’t have by increasing the number of feath- ries (P8–10) in their second year, birds reported to replace cally start with P4, with waves of have limited time and energy, why ers grown at once. then the inner seven primaries in their primaries ‘back- replacement moving inwards and not first replace the feathers that enough time to Quite simply, large birds don’t their third year, etc. Together with wards’, starting with the outwards, finishing with the inner experience the most wear, which in replace all their have enough time to replace all changes in bill and plumage col- outermost feather (P10) and outer primaries at roughly the general are the outer primaries? primaries, let alone their primaries, let alone all their our, this allows us to age immature and working inwards same time. And although they tend Two mechanisms have been pro- flight feathers, in one year, without mollymawks up to at least five years towards the body. It is to start with P5 or P6, parrots share posed to regulate moult sequence: all their flight seriously compromising their abil- old. As they get older, progressively not known why this a similar pattern, providing yet the stimulation of adjacent follicles feathers, in one ity to fly. If they can’t resort to a more feathers are replaced, and the pattern is so rare; it has another character in common be- in a sort of chain reaction, or differ- year, without synchronous, flightless moult, their pattern starts to break down. the advantage of replac- tween these two groups which are ential sensitivity to hormonal stim- only recourse is to replace a subset Once albatrosses start to breed, ing the most important now thought to be closely related. uli among feather follicles. Neither seriously compro- of their flight feathers each year. the number of feathers moulted flight feathers first. (But some owls and kingfishers also mechanism has been demonstrated, mising their ability They also don’t want to have several each year depends to a large extent share this unusual moult pattern, so but the notion that feathers trigger to fly feathers growing next to each other, on whether or not the breeding at- opposite, left Many one has to be careful not to read too moult in their neighbours is refut- because this increases the aerody- tempt is successful, and indeed migrant terns replace all much into these similarities.) ed by the eccentric primary moult grow does not keep pace with the namic costs, so most species engage whether they attempt to breed at their inner primaries and Not all primary moult follows the shown by cuckoos. They replace increase in feather length necessitat- in stepwise moult or staffelmauser all. Failed breeders have more time outer secondaries twice same sequence within bird fami- feathers in a seemingly haphazard ed by increasing body size. Feather (wave moult). In young birds this to moult than those that are tied to feathers in a single cycle. Many terns above Woodpeckers a year. When the two lies. Indeed, even within a species sequence, but when studied careful- growth rates range from around two usually starts with a standard moult the colony (and not moulting) until moult some inner primaries twice have stiffened tail feath- moults are well segre- the pattern can vary depending ly there appears to be a systematic to 11 millimetres per day, whereas sequence, starting from P1 and their chick fledges. There is some or even three times a year. This is ers that help to support gated there is a marked on age, sex and location. However, pattern. For example, the Common primaries range from 30 to 700 mil- working slowly outwards. Before evidence that inability to moult best developed in smaller, migrant them while feeding contrast between the the jacanas exhibit an unusually Cuckoo typically replaces its prima- limetres. A male Kori Bustard has to this cycle has reached the outer pri- sufficient feathers ultimately forces species, and has been attributed on vertical trunks. The pale inner primaries and diverse set of moult patterns for a ries in the order P7–9–4–1–2–5– grow almost six metres of primaries maries, however, a second wave of some birds to skip a year’s breed- to their post-breeding (pre-basic) constant abrasion results blackish outer primaries, small, structurally uniform group 10–8–3–6. Such a jumbled sequence on each wing, and when you add in moult starts, and then a third, and ing attempt, and that in at least one moult being so protracted (lasting in heavy wear, especially as is the case with this of birds. Most species show the has the advantage of spreading the the secondaries, the total balloons so on. In some raptors you can see species we know that birds which up to seven months) that the first of the longest, central Sandwich Tern. However, typical in-to-out gradual sequence aerodynamic cost of growing feath- to more than 16 metres of flight up to five moult centres each slowly attempt to breed while retaining a feathers replaced (often grown on pair of feathers, clearly when the two moults of primary replacement, but Afri- ers across the wing, reducing the feathers on each wing! Even if it working their way across the prima- large number of old feathers in their the breeding grounds before the visible in this Bennett’s overlap, the contrast is can Jacanas usually have a flightless overall impact. grew these feathers at the maximum ries alone. wings are more likely to fail their bird migrates) need renewing be- Woodpecker. They have more subtle. moult and the Lesser Jacana starts rate and replaced three feathers at Albatrosses use a slightly different breeding attempt than are birds fore the return migration. However, an unusual pattern of with P10 and works inwards to P1. The problem of the same time, it would take more strategy, moulting different sets of with newer wing feathers. recent evidence from Common tail moult, replacing the It shares this peculiar reverse moult being large than a year to replace all its flight primaries each year. The details vary Before leaving the subject of Terns suggests that the number of central pair of feathers with only a handful of other bird Large birds face the greatest chal- feathers. To make matters worse, depending on the genus, but among large birds, it’s worth noting that feathers replaced a second time also last, once the adjacent species, including the Limpkin and lenge when it comes to scheduling the gaps in the wing and tail formed the Thalassarche albatrosses with some birds exhibit multiple waves serves as a signal of mate quality, feathers have grown. trumpeters. Quite why this might the moult of their flight feathers, during moult have a greater impact which southern African birders of primary moult for reasons other because it is greater in males and be is unknown, but replacing the because the rate at which feathers on flight performance in large birds, are most familiar, the usual pattern than the inability to replace all their increases with age. >

58 african birdlife july/august 2014 MOULT 59 to quantify than flight-feather moult. During the complete pre- basic moult, replacement of body contour feathers generally overlaps with wing moult, but contour feath- ers on different parts of the body moult at slightly different times. In general, the showiest feathers are replaced last so that they are in the best possible condition at the start of the breeding season. The im- portance of the timing of moult of different feather tracts has taken on additional importance in the past few decades thanks to the increas- ing use of stable isotopes in feathers Giant petrels are Other feather tracts Tail moult usually is symmetri- as natural tracers (see box). among the largest birds Moult of the secondaries and tail cal, starting with the central pair Much remains to be learned that undergo a com- feathers tends to start once primary of feathers and moving outwards. about the basics of bird moult. plete primary moult moult is well advanced. Secondary However, woodpeckers and tree And the more we learn, the more each year without moult typically commences in the creepers, which have particularly exceptions we find to the patterns losing the ability to fly. innermost feathers (tertials) and stiff tail feathers adapted to support described here. It is clear that the They do this in part by works outwards, while a second them while climbing trees, replace extent of moult varies among in- overlapping moulting wave starts with the outermost sec- the central pair last, once the rest dividuals, depending on the bird’s with breeding, and in ondary and works inwards. How- of the tail is fully grown and can resources and the time available for part by moulting sev- ever, there are numerous exceptions help to brace the bird while the cen- moult. Hopefully this series of arti- eral primaries at once. to this pattern. Many long-winged tral pair grows. The number of tail cles has instilled a new respect for This Southern Giant birds that have lots of secondaries feathers grown at once also varies birds and the pressures they face to Petrel has lost the three replace large numbers of them at greatly, even among closely-related renew their feathers. inner primaries, show- once. They offset the loss of wing species. For example, Cory’s Shear- If you want further information ing the ‘stepped wing’ area this incurs at least in part by first waters replace their tails very slowly, on moult, Steve Howell’s Peterson shape characteristic of replacing their greater coverts before growing only one feather at a time, Reference Guide Molt in North many giant petrels. starting their secondary moult. By whereas many White-chinned Pet- American Birds (2010) provides comparison, primary coverts tend to rels produce all their tail feathers at a very readable account of moult moult in tandem with their associ- once. The reason, if any, for this dif- in general as well as a family-by- ated primary (with the exception of ference is unknown. family description of moult patterns juveniles of several disparate groups, The timing and sequence of body in North American birds. Unfortu- where the primaries are moulted moult tends to be poorly docu- nately there is no equivalent sum- without replacing their coverts). mented – in part because it is harder mary for African birds. 

Feather tracts, moult and stable isotopes lthough birds appear to be covered in feathers, in most birds attention on the need to understand the timing of moult in other the follicles are restricted to specific feather tracts or pterylae, feather tracts. withA bare patches in between (apteria). Exceptions to this pattern Stable isotopes occur in several elements commonly found in living include ostriches, mousebirds and penguins. It is assumed that the organisms, such as hydrogen, carbon, nitrogen, oxygen and sulphur. ancestral condition was to have a uniform covering of feathers, but The rare heavy isotopes of these elements tend to accumulate in at least in the penguins this feature probably evolved more recently biological tissues because they are less easily metabolised. Stable iso- to ensure an even covering of feathers for insulation. The presence of topes thus give an indication of the trophic level of an animal (that is, apteria improves the ability of birds to thermoregulate on hot days: where it sits in a food web) and, given consistent regional differences by exposing these bare patches, they can radiate body heat passively in some elements, can reflect where an animal has been feeding. – provided the air is cooler than their skin. Because feathers are dead structures, they ‘trap’ the stable isotope The timing of moult varies among and within feather tracts. Until signature of the animal at the time they are formed. By sampling recently, almost all attention on moult phenology centred on the a range of feathers grown at different times, we can infer seasonal flight feathers, and especially the primary feathers. However, the changes in diet and location. However, this presupposes we know increasing use of stable isotopes as natural tracers has focused when each feather is grown.

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