Moult Strategies Research Articles

Moult strategies in the Common Whitethroat Sylvia c. communis in northern Nigeria

Moult strategies in the Common Whitethroat Sylvia c. communis trapped in northern Nigeria in April 1999 are analysed. Differences in the extent of moult between age and sex classes are presented. The secondary moult of adult birds is considered to belong to a new pre‐breeding moult or an arrested moult, beginning at the normal starting positions on the wing, and not as a continuation of a suspended post‐breeding moult. Eccentric moult of primaries was found in nearly 19% of the yearling birds, a pattern that is unusual for Western Palearctic passerines. Some individuals showed resemblance to the split moult pattern described for the Barred Warbler S. nisoria.

Molt of the Ivory Gull

-Molting is a fundamental biological process common to all birds, and understanding molt strategies is an integral part of studying any species' life history. Molt of the monotypic Ivory Gull (Pagophila eburnea) has received variable and often contradictory treatment in the literature. Based upon examination of 152 museum specimens, the unique molt strategy of this species is clarified. Juveniles molt directly into definitive (= adult) basic plumage via a complete molt during April to August of their second calendar year. Adults have only a single molt per cycle, a strategy unique among gulls: their prebasic molt starts in March and April, progresses quickly, and is mostly completed before the pre-laying to hatching period (mid June to early August); adults typically suspend primary molt at this time and conclude molt of outermost primaries in late August or September. Thus molt and breeding overlap broadly in the Ivory Gull, presumably an adaptation to the constraints of its high-latitude winter range and the brief Arctic summer. Received iJuly 2001, accepted 12 August 2001.

Ecological correlates in the evolution of moult strategies in Western Palearctic passerines

Moult is an important process in the life cycle of birds. Passerines differ widely in the number, seasonality and extension of moult episodes, but the incidence of birds ecology on this variation remains largely uninvestigated. We analysed the patterns of moult in European passerines in relation to their distribution, migration and sexual dichromatism. Longer migrations and southern wintering quarters were characteristic of species with complete moults in summer and an additional moult in winter. The main moult in species with larger seasonal changes in sexual dimorphism tended to be scheduled just before the start of the breeding season, suggesting a link between sexual selection and the timing of moult. These patterns strongly support the importance of migration and dichromatism on the evolution of moult strategies.

Escape flight in moulting Tree Sparrows (Passer montanus)

Summary Impaired predator evasion in birds as a cost in different life‐history periods has received increasing attention in the last decade. Evasive abilities in birds have been found to be detrimentally affected by migratory fuel load, reproduction and moult. These results suggest that during these periods of their lives birds suffer from increased predation risk due to impaired evasive abilities. Theoretically, moult should have a detrimental effect on flight, and empirical work on starlings has shown impaired escape ability due to moult. However, a recent theoretical investigation found a surprisingly small effect of moult on flight in birds. In this study, 31 Tree Sparrows, a sedentary species with a slow moult, were used to investigate the effect of natural and manipulated moult on escape ability. No effect was found due to natural moult, however, when experimentally increasing moult gap size a strong negative effect was found. With support from empirical and theoretical work, this is the first study to suggest that slow moult may not increase predation risk due to impaired evasive abilities. Compensatory physiological adaptations probably cause this result and may be very important during moult. Predation risk is probably an important factor in the evolution of moult patterns and moult strategies.

MOLT DURING SPRING MIGRATION: A COMPARISON OF FOUR SPECIES OF RAPTORS

We quantified the extent of molt in Steppe Buzzards (Buteo buteo vulpinus), Marsh Harriers (Circus aeruginosus), Levant Sparrowhawks (Accipiter brevipes), and Eurasian Sparrowhawks (A. nisus) caught during spring migration 1985–1988 in Elat, southern Israel. Ten percent of yearling Steppe Buzzards (58 of 550) and four percent of yearling Marsh Harriers (3 of 77) were actively molting their remiges while on migration. These findings are contrary to suggestions that birds do not molt their flight feathers during migration when they should avoid extra energy expenditure and maintain flight performance. Active molt of primaries and secondaries, however, was not found among adult buzzards and harriers, or in any individual of the two Accipiter species. Molt strategies may be related to differences in flight mode during migration between species that primarily soar (buzzards and harriers) and species that flap more (sparrowhawks). Body condition was not related to the state of molt. This, and the fact that only yearling birds were in active molt of remiges, suggest that a delay in arrival on the breeding grounds may be a significant cost of molt during spring migration.

A phylogenetic analysis of the evolution of moult strategies in Western Palearctic warblers (Aves: Sylviidae)

Adult birds replace their flight feathers (moult) at least once per year, either in summer after termination of breeding or (in the case of some long-distance migratory species) in the winter quarters. We reconstructed the evolutionary pathways leading to summer and winter moult using recently published molecular phylogenetic information on the relationships of the Western Palearctic warblers (Aves: Sylviidae). Our phylogenetic analysis indicates that summer moult is the ancestral pattern and that winter moult has evolved 7–10 times in this clade. As taxa increased their migratory distance and colonized northern breeding areas, summer moult disappeared and winter moult evolved. Our data also allows us to trace the historical origins of unusual moult patterns such as the split-moult and biannual moult strategies: the most parsimonous explanations for their origins is that they evolved from ancestral states of summer moult. We briefly discuss our results in the light of recent criticisms against phylogenetic comparative methods and the utility of historical versus functional definitions of adapation.

A phylogenetic analysis of the evolution of moult strategies in Western Palearctic warblers (Aves: Sylviidae)

Adult birds replace their flight feathers (moult) at least once per year, either in summer after termination of breeding or (in the case of some long-distance migratory species) in the winter quarters. We reconstructed the evolutionary pathways leading to summer and winter moult using recently published molecular phylogenetic information on the relationships of the Western Palearctic warblers (Aves: Sylviidae). Our phylogenetic analysis indicates that summer moult is the ancestral pattern and that winter moult has evolved 7–10 times in this clade. As taxa increased their migratory distance and colonized northern breeding areas, summer moult disappeared and winter moult evolved. Our data also allows us to trace the historical origins of unusual moult patterns such as the split-moult and biannual moult strategies: the most parsimonous explanations for their origins is that they evolved from ancestral states of summer moult. We briefly discuss our results in the light of recent criticisms against phylogenetic comparative methods and the utility of historical versus functional definitions of adapation.

Migration and moult of Dunlin Calidris alpina wintering in the central Mediterranean

Dunlin migration in northeast Italy is described. An attempt to identify the main routes and staging areas used by birds wintering in the central Mediterranean is presented. The results of monthly counts from 1990–1995 revealed that the bulk of the population occupied the wintering area in October and left for the breeding grounds in April and May. The analysis of 342 Italian recoveries of foreign ringed birds showed that 65% were ringed during post-breeding migration through the Baltic Sea, whereas just a few birds had been ringed in western Europe. First-year birds arrived in autumn with a single migratory wave, peaking in October. Two categories of adults were identified during post-breeding migration: birds which directly reached Italian wintering sites and birds which arrived after they had suspended their migration for moulting: the Azov/Black Sea wetlands are suggested as possible moulting areas. Out of 2444 adults and 1627 first-years ringed between 1989 and 1996 at our study area, we obtained a total of 42 recoveries abroad and evidence of direct links between Azov/Black Sea and N Adriatic wetlands, both during autumn and spring migrations. Primary moult was observed only in adults arriving early, the second migratory wave being composed of moulted birds. Locally moulting adults adopted a moult strategy characterized by high raggedness scores, typical of resident moulters. Body mass was not affected by primary moult stage or intensity, winter mass values being reached two weeks after the average date of primary moult completion.

The effects of molt on the flight performance, body mass, and behavior of European starlings (Sturnus vulgaris): an experimental approach

The physiological and energy costs of avian molt are well documented, but indirect consequences such as changes in flight performance have received less attention. Here, we report two experiments that investigated flight performance, body mass regulation, and behavior in captive starlings (Sturnus vulgaris). In the first experiment, we found a U-shaped change in take-off escape performance during natural molt: birds ascended at the shallowest trajectories during midmolt. Birds' body mass was also reduced during molt. In the second experiment, we manipulated the plumage of starlings to simulate different stages of flight-feather molt. This allowed us to separate the aerodynamic costs of feather loss from the physiological costs of feather synthesis normally associated with plumage growth. Through observations of flight (take-off, aerial maneuverability, and level flapping-flight speed) and behavioral parameters, we demonstrated that birds in simulated molt have reduced flight performance and reduced body mass. These birds also decrease the time spent performing energetically costly activities and seek areas of relative protection. In the longer term, some aspects of performance return to pretreatment levels, implying compensation for the plumage manipulations. Our results demonstrate that molt incurs significant functional costs that may play an important role in the adaptive radiation of molt strategies and molt patterns observed in avian species.

Timing and Extent of the Molts of Adult Red-Eyed Vireos on Their Breeding and Wintering Grounds

The molt of adult Red-eyed Vireos (Vireo olivaceus) on their breeding grounds was studied at three widely separated sites in northeastern North America: North Point, James Bay (NPJB), Ontario; Vermont Institute of Natural Science (VINS), Woodstock, Vermont; and Powdermill Nature Reserve (PNR), Rector, Pennsylvania. Unlike other North American vireos and most other North American passerines, adult Red-eyed Vireos undergo an incomplete molt on the breeding grounds. This molt typically replaces most or all of the body plumage, about half of the wing coverts, the tertials, and the central rectrices. Extent of molt did not differ among localities; however, timing of the molt was 1-2 weeks earlier and onset of molt occurred over a longer period at PNR than at VINS or NPJB. Based on examination of museum specimens, flight feathers not molted on the breeding grounds are replaced on the wintering grounds beginning in early January ; average duration of primary molt on the wintering grounds is 78 days. It is unclear whether or not feathers molted on the breeding grounds are molted again at this time. This issue needs to be resolved before Humphrey-Parkes terminology can be applied to breeding and wintering ground molt episodes in this species. Selective advantages associated with a protracted nesting season and with early onset of migration both may have had a role in shaping the molt strategy of adult Red-eyed Vireos. Rapid onset of fall migration following nesting in this seasonally frugivorous species may have been influenced by the phenology of fruit availability and abundance en route to and especially on its Amazonian wintering grounds.

Dyar's Rule and the Investment Principle: optimal moulting strategies if feeding rate is size–dependent and growth is discontinuous

We consider animals whose feeding rate depends on the size of structures that grow only by moulting (e.g. spiders' legs). Our Investment Principle predicts optimum size increases at each moult; under simplifying assumptions these are a function of the scaling of feeding rate with size, the efficiency of moulting and the optimum size increase at the preceding moult. We show how to test this quantitatively, and make the qualitative prediction that size increases and instar durations change monotonically through development. Thus, this version of the model does not predict that proportional size increases necessarily remain constant, which is the pattern described by Dyar's Rule. A literature survey shows that in nature size increases tend to decline and instar durations to increase, but exceptions to monotonicity occur frequently: we consider how relaxing certain assumptions of the model could explain this. Having specified various functions relating fitness to adult size and time of emergence, we calculate (using dynamic programming) the effect of manipulating food availability, time of hatching and size of the initial (or some intermediate) instar. The associated norms of reaction depend on the fitness function and differ from those when growth follows Dyar's Rule or is continuous. We go on to consider optimization of the number of instars. The Investment Principle then predicts upper and lower limits to observed size increases and explains why increases usually change little or decline through development. This is thus a new adaptive explanation for Dyar's Rule and for the most common deviation from the Rule.

Plumage Condition Affects Flight Performance in Common Starlings: Implications for Developmental Homeostasis, Abrasion and Moult

Variation in length and asymmetry of wing primary feathers can arise from a breakdown of developmental homeostasis, feather abrasion and incomplete growth during moult. Indirect predictions have been made concerning the impact of primary length and asymmetry on the flight ability of birds, but they have not been explicitly tested. Here we provide evidence from both natural variation in primary feather condition and experimental manipulations of primary feather length and asymmetry to indicate that these factors influence aspects of flight performance in the Common Starling Sturnus vulgaris. Damaged and incompletely grown primary feathers reduce escape flight performance. Experimentally reduced primary lengths reduce take-off speed; increased primary asymmetry decreases aerial manoeuvrability. A comparison of the experimental and natural plumage data indicates that birds may be able to adapt to a change in wing morphology, perhaps reducing the effects of feather loss or damage on flight. The results from this study indicate that primary feathers are under strong stabilising selection to maximise developmental homeostasis and reduce feather asymmetry. These findings are also of ecological importance to the damageavoidance and moult strategies of these birds. This is the first experimental evidence to indicate a quantitative reduction in flight performance with feather lengths and asymmetries typical of those observed during flight feather moult and feather damage in any species.

Moult in relation to migration in birds—a review

This study reviews different ways of combining the moult with other essential activities like breeding and migration. Most species tend to avoid moult during the breeding. A majority of short-distance migrants change their feathers in summer before autumn migration, while most long-distance migrants perform a winter moult in the tropics after autumn migration. A complete moult both in summer and winter has only been recorded in a few passerines. Birds like albatrosses and larger eagles have a serial moult, changing only a number of their primaries every year. Another strategy common among long-distance migrants is to change some feathers on the breeding grounds, suspend moult, and complete it in the winter quarters. A special adaptation allowing faster moult is the simultaneous shedding of all wing quills. During this time birds are flightless and many species perform spectacular moult migrations to congregate in areas rich in food and free of predators. The moult strategy may vary between closely related species and a comparative study of three different groups (raptors, waders and warblers) is presented. The most clear differences in strategy seem to be related to migration distance.

Moult strategies of White Wagtail Motacilla alba and Yellow Wagtail Motacilla flava in central Sweden

The wing moult of the White Wagtail Motacilla a. alba and the Yellow Wagtail M. f .flava has been recorded since 1973 at Kvismaren, central Sweden. Based on data from 558 White and 78 Yellow Wagtails in active moult the wing moult duration of the former was estimated at 55–65 days and of the latter at 35–40 days. The moult sequence was the same in both species. The only significant difference was the speed of the moult. The reason for this difference in moult strategy is probably the difference in migration destination of the two species. The Yellow Wagtail arrives later than the White Wagtail, stays a shorter time in central Sweden and starts the migration towards tropical Africa just before finishing the moult in the middle of August. Swedish White Wagtails winter in the east Mediterranean, arrive almost a month earlier and leave a month later. It is difficult to compare the present results with data from other parts of Europe as there are plenty of misrepresentations published some decades ago and quoted in later works. Therefore, methods of moult data evaluation are also briefly discussed.

Differential Timing of Autumn Migration between Sex and Age Groups in Raptors at Falsterbo, Sweden

The migration of raptors at Falsterbo, Sweden was studied during five autumns, 1986-1990, to investigate the intraspecific temporal sequence of migration for different age and sex classes. The analysis is based on counts of between 45 and 72 321 individuals of 14 species. The percentage of juveniles varied among species and to some extent between years. This reflects a variable tendency of different migrants to concentrate at Falsterbo and fluctuating breeding success from year to year. Most species showed an even sex ratio. Marked exceptions were Marsh Harrier Circus aeruginosus and Hen Harrier C. cyaneus in which females constituted 60 and 61% respectively of adult migrants. This biassed sex ratio agrees with the tendency towards polygyny in these species. In tropical (long-distance) migrants adults migrated ahead of juveniles (Honey Buzzard Pernis apivorus, Montagu's Harrier Circus pygargus, Osprey Pandion haliaetus, and Hobby Falco subbuteo). It is suggested that it is advantageous for adults to arrive early on the wintering grounds, finish moult and accumulate energy reserves before spring migration and the next breeding season. Those short-distance migrants in which adults migrated before juveniles at Falsterbo are species whose majority leave Scandinavia in the winter (Hen Harrier, Common Buzzard Buteo buteo, Roughlegged Buzzard B. lagopus, Merlin Falco columbarius and Peregrine F peregrinus). Adults leaving first have a greater possibility to secure a good winter territory forcing the juveniles, migrating later, to settle in less suitable habitat or to winter further south. In species where juveniles migrated ahead of adults a large proportion of the adults spend the winter close to the breeding grounds (Red Kite Milvus milvus, Goshawk Accipiter gentilis and Sparrowhawk A. nisus). The presumably subdominant juveniles may be excluded from the breeding territories by the adults and therefore migrate south, followed by a varying proportion of adults. Females generally migrated before males at Falsterbo due to different moult strategies; females starting and finishing moult of the flight feathers before males. This difference in timing of moult and migration is ultimately dependent on a marked role division between the sexes during the breeding cycle. In Honey Buzzard where both sexes share the breeding duties equally no difference in timing of migration was found. Social dominance, importance of early arrival at the winter quarters and moult strategies are the factors best explaining the observed differential timing in migrating raptors at Falsterbo.

Sex-Specific Differences in Molt Strategy in Relation to Breeding in the Wandering Albatross

The extent of molt of primary feathers was studied in relation to the status and to the duration of the interval between breeding attempts in the Wandering Albatross (Diomedea exulans). The replacement of primaries lasts for more than one season, birds showing a typical wave molt with one to three foci occurring on a wing. Molt never occurs on the breeding grounds. In breeding as in nonbreeding individuals, males invariably renew more feathers than females. Breeding males and females renewed each year an average of 8.3 and 7.3 primaries per wing, respectively. The extent of primary molt of breeders was directly related to the duration of the interbreeding period in females but not in males. Males and females breeding for the first time had fewer new feathers than did experienced birds. When they visited the nesting colonies, immature birds had fewer new feathers than did breeding birds of the same sex. In immature birds, the extent of molt was related to the body condition (mass) of males but not of females. The extent of molt was inversely related to mass gain from one season to the next in nonbreeding males. These results suggest that molting in the Wandering Albatross is probably an important constraint that could compete with breeding, particularly in females. Possible reasons for sex-specific differences in molt extent are discussed.

Moult Strategies of Four Species of Grassfinches Living in the Same Area

SummarySchoepfer, M. (1988). Moult strategies of four species of grassfinches living in the same area. Emu 89, 102–111.Moult of the remiges was studied over a period of two years in sympatric populations of the Red-browed Firetail Emblema temporalis, the Diamond Firetail E guttata, the Double-barred Finch Poephila bichenovii and the Plum-headed Finch Aidemosyne modesta from the central western slopes of New South Wales. Primary and secondary moult-rates were calculated from individuals captured more than once during the same moult and the mean duration of primary and secondary moult was estimated by multiplying the rate by the score of the completed moult. There were significant differences within and among species, years and sites. P. bichenovii moulted at the slowest primary and secondary rate and continued its moult throughout winter at an even slower rate. This moult-strategy may be non-adaptive for populations in New South Wales. E. temporalis had variable primary moult-rates compared between years and sites, which may reflect food supply and climate; no significant differences could be found between the secondary rates. The rates of A. modesta were characterised by a fast primary and a slow secondary moult which could be adaptive for regular seasonal movements. E. guttata replaced both primaries and secondaries at a fast rate in both years and represented a more conventional moult pattern. Among species there was more variation in the rates of primaries than of secondaries. Moulting seasons were highly synchronised in both years and fairly constant. Moult occurred from November until May, except for P. bichenovii. Moult and breeding overlapped in all four species and a high proportion of birds caring for eggs or nestlings actively replaced remiges.

Delayed moult in the SiskinCarduelis spinus

Moult of body feathers was found to occur in the population of Siskins throughout the period November to April in a wintering area in NE Spain. Body feather moult occurred in both sexes, in adults and first‐year birds, and was particularly apparent in early spring. This probably reflected an adaptive opportunistic moult strategy.

Influences of status and recent breeding experience on the moult strategy of the yellow‐nosed albatross Diomedea chlororhynchos

The primary moult of individually colour‐ringed, adult yellow‐nosed albatrosses at nests on Gough Island was examined in 1983 and related to the status of each bird and its breeding history in the previous year. Adults renew only about half of their primaries each winter and suspend moult while breeding. Birds that bred successfully renewed fewer primaries than did unsuccessful birds or nonbreeders. There were no differences in primary moult between the sexes or in relation to size. Yellow‐nosed albatrosses show complex wave moult as an adaptation to slow renewal of flight feathers. The energy, nutrient or time requirements for feather renewal may conflict with breeding annually so that there is a trade‐off between the extent of moult desirable to maintain flight efficiency and the benefits of breeding in successive seasons.

The moult of the Little Stint Calidris minuta in the Kenyan rift valley

Moult data were collected during 1967–80 from some 6900 Little Stints in the southern Kenyan rift valley.Adults typically moulted from summer to winter body and head plumage during September and early October, soon after arrival. The complete pre‐winter wing and tail moult began in most adults between mid‐September and early October. Some birds finished by December, but others continued until February and March. Individual duration was usually between 100 and 150 days. Adults which completed this moult early often remoulted outer primaries between January and early April.Young birds acquired first‐winter body plumage during October and early November. Some 90% had a complete pre‐winter wing and tail moult. This usually began between December and early February, and finished during March or early April, taking about 70–100 days. In about 10% of young birds, flight feather moult was restricted to the outer primaries and inner secondaries. Birds adopting this strategy typically began moult late, during January or February. Short periods of suspension were common during pre‐winter wing moult, particularly in adults. The difference in moult speed between adult arid first‐winter birds was attributable in the primary, secondary and tail tracts to differences in numbers of growing feathers.Practically all birds completed a pre‐summer moult involving the entire body and head plumage, most of the tertials, some or all of the tail feathers and many wing coverts. Most birds began this moult between early February and late March, and finished between mid‐April and early May. It was typically later and more rapid in first‐year birds than adults. In late birds, the onset of pre‐summer moult was linked to the final stages of pre‐winter moult.The wing moult of the Little Stint in different wintering areas is discussed. First‐winter moult strategy is compared with that in other small Calidris species.