Search

Feather moulting is a crucial process in the avian life cycle, which evolved to maintain plumage functionality. However, moulting involves both energetic and functional costs. During moulting, plumage function temporarily decreases between the shedding of old feathers and the full growth of new ones. In flying taxa, a gradual and sequential replacement of flight feathers evolved to maintain aerodynamic capabilities during the moulting period. Little is known about the moult strategies of non-avian pennaraptoran dinosaurs and stem birds, before the emergence of crown lineage. Here, we report on two Early Cretaceous pygostylian birds from the Yixian Formation (125 mya), probably referable to Confuciusornithiformes, exhibiting morphological characteristics that suggest a gradual and sequential moult of wing flight feathers. Short primary feathers interpreted as immature are symmetrically present on both wings, as is typical among extant flying birds. Our survey of the enormous collection of the Tianyu Museum confirms previous findings that evidence of active moult in non-neornithine pennaraptorans is rare and likely indicates a moult cycle greater than one year. Documenting moult in Mesozoic feathered dinosaurs is critical for understanding their ecology, locomotor ability and the evolution of this important life-history process in birds.

The evolutionary history of many organisms is characterized by major changes in morphology and distribution. Specifically, alterations of body mass and geographic distribution may profoundly influence organismal life-history traits. Here, we reconstructed the evolutionary history of flight-feather molt strategy using data from 1,808 Neornithes species. Our analysis suggests that the ancestral molt strategy of first-year birds was partial or entirely absent, and that complete wing flight-feather molt in first-year birds first evolved in the late Eocene and Oligocene (25-40 Ma), at least 30 Myr after birds first evolved. Complete flight-feather molt occurred mainly at equatorial latitudes and in relatively low body mass species, following a diversification of body mass within the lineage. We conclude that both body mass and geographic distribution shaped the evolution of molt strategies and propose that the evolutionary transition towards complete juvenile molt in the Neornithes is a novel, relatively late adaptation., (© 2021. The Author(s).)

Modern flying birds molt to replace old and worn feathers that inhibit flight performance, but its origins are unclear. We address this by presenting and evaluating a ~150 million year old record of molting in a feathered dinosaur from the early bird Archaeopteryx. Laser-Stimulated Fluorescence revealed feather sheaths that are otherwise invisible under white light. These are separated by one feather and are not in numerical sequential order and are mirrored in both wings. This indicates that a sequential center-out molting strategy was already present at the origins of flight, which is used in living falcons to preserve maximum flight performance. This strategy would have been a welcome advantage for early theropod flyers that had poor flight capabilities. This discovery provides important insights into how birds refined their early flight capabilities before the appearance of the keeled sternum, pygostyle and triosseal canal.

AbstractComplex features, such as vision, limbs, and flight, have been lost by many groups of animals. Some groups of birds are more prone to loss of flight than others, but few studies have investigated possible reasons for this variation. I tested the hypothesis that a rare strategy of flight feather replacement is involved in rate variation in the evolution of flightlessness in birds. This strategy involves a simultaneous molt of the flight feathers of the wing, resulting in a temporary flightless condition during molt. I hypothesized that adaptations for this flightless period may serve as preadaptations for permanent flightlessness under conditions that favor permanent loss of flight. I found an elevated rate of loss of flight in lineages with simultaneous wing molt compared with loss of flight in lineages without simultaneous wing molt. This may indicate that birds with simultaneous molt are more prepared to adjust quickly to open niches that do not require flight, such as terrestrial niches on island habitats. These results illustrate how molt strategies can influence the long-term evolutionary trajectories of birds and provide insight into how phenotypic precursors may act as a mechanism of rate variation in the loss of complex traits.

Fault bars are narrow malformations in feathers oriented almost perpendicular to the rachis where the feather vein and even the rachis may break. Breaks in the barbs and barbules result in small pieces of the feather vein being lost, while breaks in the rachis result in loss of the distal portion of the feather. Here, we provide a comprehensive review of 74 papers on fault bar formation in hopes of providing a clearer approach to their study. First, we review the evidence that the propensity to develop fault bars is modified by natural selection. Given that fault bars persist in the face of survival costs, we conclude that they must be an unfortunate consequence of some alternative adaptation that outweighs the fitness costs of fault bars. Second, we summarize evidence that the development of fault bars is triggered by psychological stress such as that of handling or predation attempts, and that they persist because the sudden contractions of the muscles in the feather follicle that control fright moults also causes the development of fault bars in growing feathers. Third, we review external and physiological (e.g. corticosterone) agents that may affect the likelihood that an acute stress will result in a growing feather exhibiting a fault bar. These modifying factors have often been treated as fundamental causes in the earlier literature on fault bars. Fourth, we then use this classification to propose a tentative model where fault bars of different severity (from light to severe) are the outcome of the interaction between the propensity to produce fault bars (which differs between species, individuals and feather follicles within individuals) and the intensity of the perturbation. This model helps to explain contradictory results in the literature, to identify gaps in our knowledge, and to suggest further studies. Lastly, we discuss ways in which better understanding of fault bars can inform us about other aspects of avian evolutionary ecology, such as the physiology of moult, the integration of moult into avian life cycles, and the strategies used to minimize stress during moult. Moreover, the study of fault bars may be relevant to understanding the aerodynamics of flight and the early evolution of flight., (© 2016 Cambridge Philosophical Society.)

Photoperiod is the major cue used by birds to time breeding seasons and molt. However, the annual cycle in photoperiod changes with latitude. Within species, for temperate and high latitude species, gonadal maturation and breeding start earlier at lower latitudes but regression and molt both occur at similar times at different latitudes. Earlier gonadal maturation can be explained simply by the fact that considerable maturation occurs before the equinox when photoperiod is longer at lower latitudes - genetic differences between populations are not necessary to explain earlier breeding at lower latitudes. Gonadal regression is caused either by absolute photorefractoriness or, in some species with long breeding seasons, relative photorefractoriness. In either case, the timing of regression and molt cannot be explained by absolute prevailing photoperiod or rate of change in photoperiod - birds appear to be using more subtle cues from the pattern of change in photoperiod. However, there may be no difference between absolute and relative photorefractory species in how they utilise the annual cycle in photoperiod to time regression., (Copyright © 2013 Elsevier Inc. All rights reserved.)

In avian species that have evolved life-history strategies wherein molt and breeding overlap, there are potential conflicts between the regulatory roles of baseline prolactin and corticosterone in parental care (positive) and moult (negative). We describe seasonal patterns of hormonal secretion, moult, and parental behaviour in sibling species of giant petrels (Macronectes spp.) which begin moult during the incubation/early chick-rearing stage of reproduction. With the exception of male Southern giant petrels (Macronectes giganteus), prolactin secretion and moult in Northern (Macronectes halli) and female Southern giant petrels conformed to those observed in all other avian species, with the initiation of moult coincident with decreases from peak prolactin levels. However, male Southern giant petrels began moulting early in incubation when prolactin was increasing and had not yet peaked, which suggests a requirement of prolactin for incubation behaviour and a dissociation of prolactin from moult. Corticosterone showed little seasonal variation and no relationship with moult. When comparing prolactin, corticosterone, and moult in failed vs. active breeders, we found that failed breeding enabled a more rapid down-regulation of prolactin, thus facilitating a more rapid moult. We present specific examples of the behavioural ecology of giant petrels which we conclude help mediate any potential hormonal conflicts between parental care and moult., (Crown Copyright © 2011. Published by Elsevier Inc. All rights reserved.)

Food limitation greatly affects bird breeding performance, but the effect of nutritive stress on molt has barely been investigated outside of laboratory settings. Here we show changes in molting patterns for an entire native Hawaiian bird community at 1650-1900 m elevation on the Island of Hawaii between 1989-1999 and 2000-2006, associated with severe food limitation throughout the year beginning in 2000. Young birds and adults of all species took longer to complete their molt, including months never or rarely used during the 1989-1999 decade. These included the cold winter months and even the early months of the following breeding season. In addition, more adults of most species initiated their molt one to two months earlier, during the breeding season. Suspended molt, indicated by birds temporarily not molting primary flight feathers during the months of peak primary molt, increased in prevalence. Food limitation reached the point where individuals of all species had asymmetric molt, with different primary flight feathers molted on each wing. These multiple changes in molt, unprecedented in birds, had survival consequences. Adult birds captured during January to March, 2000-2004, had lower survival in four of five species with little effect of extended molt. Extended molt may be adaptive for a nutrient stressed bird to survive warm temperatures but not cool winter temperatures that may obliterate the energy savings. The changing molt of Hawaiian birds has many implications for conservation and for understanding life history aspects of molt of tropical birds.

Blood and cloacal swabs were collected from 100 (66 female, 34 male) wild Tundra Swans (Cygnus columbianus) molting in northwestern Alaska, USA, 25-28 July 2008, to establish hematologic and serum chemistry reference values and to isolate enteric Salmonella spp. and Escherichia coli O157:H7. Plasma biochemistry and hematology values did not vary significantly by sex or age. Tundra swans had high levels of creatine kinase, lactate dehydrogenase, amylase, and alkaline phosphatase compared with some other avian species (values were up to 7 times greater), possibly indicating capture myopathy. However, concentrations were much lower (up to 8 times lower) than in other waterfowl exposed to similar or more intensive capture methods. White blood cell count and hematocrit values were similar to other waterfowl species, and enteric Salmonella spp. and E. coli O157:H7 were not present among birds sampled. Our data provide the first biochemical, hematologic, and bacteriologic reference values for wild Tundra Swans.

Trade-offs between self-maintenance processes can affect life-history evolution. Integument replacement and the stress response both promote self-maintenance and affect survival in vertebrates. Relationships between the two processes have been studied most extensively in birds, where hormonal stress suppression is down regulated during molt in seasonal species, suggesting a resource-based trade-off between the two processes. The only species found to differ are the rock dove and Eurasian tree sparrow, at least one of which performs a very slow molt that may reduce resource demands during feather growth, permitting investment in the stress response. To test for the presence of a molt-stress response trade-off, we measured hormonal stress responsiveness during and outside molt in two additional species with extended molts, red crossbills (Loxia curvirostra) and zebra finches (Taeniopygia guttata). We found that both species maintain hormonal stress responsiveness during molt. Further, a comparative analysis of all available species revealed a strong relationship between molt duration and degree of hormonal suppression. Though our results support trade-off hypotheses, these data can also be explained by alternative hypotheses that have not been formally addressed in the literature. We found a strong relationship between stress suppression and seasonality of breeding and evidence suggesting that the degree of suppression may be either locally adaptable or plastic and responsive to local environmental conditions. We hypothesize that environmental unpredictability favors extended molt duration, which in turn allows for maintenance of the hormonal stress response, and discuss implications of a possible trade-off for the evolution of molt schedules.

The majority of bird species studied to date have molt schedules that are not concurrent with other energy demanding life history stages, an outcome assumed to arise from energetic trade-offs. Empirical studies reveal that molt is one of the most energetically demanding and perplexingly inefficient growth processes measured. Furthermore, small birds, which have the highest mass-specific basal metabolic rates (BMR(m)), have the highest costs of molt per gram of feathers produced. However, many small passerines, including white-plumed honeyeaters (WPHE; Lichenostomus penicillatus), breed in response to resource availability at any time of year, and do so without interrupting their annual molt. We examined the energetic cost of molt in WPHE by quantifying weekly changes in minimum resting metabolic rate (RMR(min)) during a natural-molt period in 7 wild-caught birds. We also measured the energetic cost of feather replacement in a second group of WPHEs that we forced to replace an additional 25% of their plumage at the start of their natural molt period. Energy expenditure during natural molt revealed an energy conversion efficiency of just 6.9% (±0.57) close to values reported for similar-sized birds from more predictable north-temperate environments. Maximum increases in RMR(min) during the molt of WPHE, at 82% (±5.59) above individual pre-molt levels, were some of the highest yet reported. Yet RMR(min) maxima during molt were not coincident with the peak period of feather replacement in naturally molting or plucked birds. Given the tight relationship between molt efficiency and mass-specific metabolic rate in all species studied to date, regardless of life-history pattern (Efficiency (%)  = 35.720 x 10(-0.494BMRm); r² = 0.944; p  =  or < 0.0001), there appears to be concomitant physiological costs entrained in the molt period that is not directly due to feather replacement. Despite these high total expenditures, the protracted molt period of WPHE significantly reduces these added costs on a daily basis.

The cuttlefish Sepia apama Gray (Mollusca: Cephalopoda) is a seasonally abundant food resource exploited annually by moulting albatrosses throughout winter and early spring in the coastal waters of New South Wales, Australia. To assess its nutritional value as albatross forage, we analysed S. apama for water, lipid protein, ash contents, energy density and amino acid composition. Because albatrosses consistently consume S. apama parts preferentially in the order of head, viscera and mantle, we analysed these sections separately, but did not identify any nutritional basis for this selective feeding behaviour. The gross energy value of S. apama bodies was 20.9 kJ/g dry mass, but their high water content (>83%; cf <70% for fish) results in a relatively low energy density of 3.53 kJ/g. This may contribute to a need to take large meals, which subsequently degrade flight performance. Protein content was typically >75% dry mass, whereas fat content was only about 1%. Albatrosses feed on many species of cephalopods and teleost fish, and we found the amino acid composition of S. apama to be comparable to a range of species within these taxa. We used S. apama exclusively in feeding trials to estimate the energy assimilation efficiency for Diomedea albatrosses. We estimated their nitrogen-corrected apparent energy assimilation efficiency for consuming this prey to be 81.82 ± 0.72% and nitrogen retention as 2.90 ± 0.11 g N kg(-1) d(-1). Although S. apama has a high water content and relatively low energy density, its protein composition is otherwise comparable to other albatross prey species. Consequently, the large size and seasonal abundance of this prey should ensure that albatrosses remain replete and adequately nourished on this forage while undergoing moult.

The functional life span of feathers is always much less than the potential life span of birds, so feathers must be renewed regularly. But feather renewal entails important energetic, time and performance costs that must be integrated into the annual cycle. Across species the time required to replace flight feather increases disproportionately with body size, resulting in complex, multiple waves of feather replacement in the primaries of many large birds. We describe the rules of flight feather replacement for Hemiprocne mystacea, a small, 60 g tree swift from the New Guinea region. This species breeds and molts in all months of the year, and flight feather molt occurs during breeding in some individuals. H. mystacea is one to be the smallest species for which stepwise replacement of the primaries and secondaries has been documented; yet, primary replacement is extremely slow in this aerial forager, requiring more than 300 days if molt is not interrupted. We used growth bands to show that primaries grow at an average rate of 2.86 mm/d. The 10 primaries are a single molt series, while the 11 secondaries and five rectrices are each broken into two molt series. In large birds stepwise replacement of the primaries serves to increase the rate of primary replacement while minimizing gaps in the wing. But stepwise replacement of the wing quills in H. mystacea proceeds so slowly that it may be a consequence of the ontogeny of stepwise molting, rather than an adaptation, because the average number of growing primaries is probably lower than 1.14 feathers per wing.

Concentrations of organochlorines (OCs), brominated flame retardants (BFRs) and mercury (Hg) were measured in eggs of six seabird species breeding in the Gulf of St. Lawrence, Canada. Stable nitrogen (delta15N) and carbon (delta13C) isotopes were used as ecological tracers to measure trophic level and connectivity with benthos, respectively. Concentrations, patterns as well as ecological tracers varied significantly between species. The sum of polychlorinated biphenyls (SigmaPCBs) was the most important group measured in all seabird species based on concentration followed generally by the sum of chlorinated pesticides (SigmaCPs), the sum of brominated flame retardants (SigmaBFRs) and finally total Hg (THg). SigmaPCBs, SigmaCPs and SigmaBFRs increased with trophic level, whereas THg did not. Only SigmaBFRs increased with a higher connectivity with the benthos. Seabird species resident to the Great Lakes-St. Lawrence ecosystem showed higher Hg and BFR levels than migratory species. Molt patterns were used to explain variations of contaminant levels., (Copyright 2010 Elsevier Ltd. All rights reserved.)

We used allometric scaling to explain why the regular replacement of the primary flight feathers requires disproportionately more time for large birds. Primary growth rate scales to mass (M) as M(0.171), whereas the summed length of the primaries scales almost twice as fast (M(0.316)). The ratio of length (mm) to rate (mm/day), which would be the time needed to replace all the primaries one by one, increases as the 0.14 power of mass (M(0.316)/M(0.171) = M(0.145)), illustrating why the time required to replace the primaries is so important to life history evolution in large birds. Smaller birds generally replace all their flight feathers annually, but larger birds that fly while renewing their primaries often extend the primary molt over two or more years. Most flying birds exhibit one of three fundamentally different modes of primary replacement, and the size distributions of birds associated with these replacement modes suggest that birds that replace their primaries in a single wave of molt cannot approach the size of the largest flying birds without first transitioning to a more complex mode of primary replacement. Finally, we propose two models that could account for the 1/6 power allometry between feather growth rate and body mass, both based on a length-to-surface relationship that transforms the linear, cylindrical growing region responsible for producing feather tissue into an essentially two-dimensional structure. These allometric relationships offer a general explanation for flight feather replacement requiring disproportionately more time for large birds., Competing Interests: The authors have declared that no competing interests exist.

The value of stable isotope analysis in tracking animal migrations in marine environments is poorly understood, mainly due to insufficient knowledge of isotopic integration into animal tissues within distinct water masses. We investigated isotopic and moult patterns in Cory's shearwaters to assess the integration of different stable isotopes into feathers in relation to the birds' transoceanic movements. Specimens of Mediterranean Cory's shearwater Calonectris diomedea diomedea caught accidentally by Catalan longliners were collected and the signatures of stable isotopes of C (delta(13)C), N (delta(15)N) and S (delta(34)S) were analysed in 11 wing and two tail feathers from 20 birds, and in some breast feathers. Based on isotopic signatures and moult patterns, the feathers segregated into two groups (breeding and wintering), corresponding to those grown in the Mediterranean or Atlantic regions, respectively. In addition, feathers grown during winter, i.e. moulted in Atlantic waters, were grouped into two isotopically distinct profiles, presumably corresponding to the two main wintering areas previously identified for Mediterranean Cory's shearwater in tracking studies. N signatures mainly indicated the Mediterranean-to-Atlantic migration, whereas C and S signatures differed according to the Atlantic wintering area. Our results indicate that isotopic signatures from distant oceanic regions can integrate the feathers of a given bird and can indicate the region in which each feather was grown. This study thus underscores how stable isotope analysis can link marine animals to specific breeding and wintering areas, and thereby shed new light on studies involving assignment, migratory connectivity and carry-over effects in the marine environment.

This paper reviews information from ecological and physiological studies to assess how extrinsic factors can modulate intrinsic physiological processes. The annual cycle of birds is made up of a sequence of life-history stages: breeding, moult and migration. Each stage has evolved to occur at the optimum time and to last for the whole duration of time available. Some species have predictable breeding seasons, others are more flexible and some breed opportunistically in response to unpredictable food availability. Photoperiod is the principal environmental cue used to time each stage, allowing birds to adapt their physiology in advance of predictable environmental changes. Physiological (neuroendocrine and endocrine) plasticity allows non-photoperiodic cues to modulate timing to enable individuals to cope with, and benefit from, short-term environmental variability. Although the timing and duration of the period of full gonadal maturation is principally controlled by photoperiod, non-photoperiodic cues, such as temperature, rainfall or food availability, could potentially modulate the exact time of breeding either by fine-tuning the time of egg-laying within the period of full gonadal maturity or, more fundamentally, by modulating gonadal maturation and/or regression. The timing of gonadal regression affects the time of the start of moult, which in turn may affect the duration of the moult. There are many areas of uncertainty. Future integrated studies are required to assess the scope for flexibility in life-history strategies as this will have a critical bearing on whether birds can adapt sufficiently rapidly to anthropogenic environmental changes, in particular climate change.

Avian migration, which involves billions of birds flying vast distances, is known to influence all aspects of avian life. Here we investigate how birds fit moult into an annual cycle determined by the need to migrate. Large variation exists in moulting patterns in relation to migration: for instance, moult can occur after breeding in the summer or after arrival in the wintering quarters. Here we use an optimal annual routine model to investigate why this variation exists. The modelled bird's decisions depend on the time of year, its energy reserves, breeding status, experience, flight feather quality and location. Our results suggest that the temporal and spatial variations in food are an important influence on a migratory bird's annual cycle. Summer moult occurs when food has a high peak on the breeding site in the summer, but it is less seasonal elsewhere. Winter moult occurs if there is a short period of high food availability in summer and a strong winter peak at different locations (i.e. the food is very seasonal but in opposite phase on these areas). This finding might explain why only long-distance migrants have a winter moult.

Long-distance migration, and the study of the migrants who undertake these journeys, has fascinated generations of biologists. However, many aspects of the annual cycles of these migrants remain a mystery as do many of the driving forces behind the evolution and maintenance of the migrations themselves. In this article we discuss nutritional, energetic, temporal and disease-risk bottlenecks in the annual cycle of long-distance migrants, taking a sandpiper, the red knot Calidris canutus, as a focal species. Red knots have six recognized subspecies each with different migratory routes, well-known patterns of connectivity and contrasting annual cycles. The diversity of red knot annual cycles allows us to discuss the existence and the effects of bottlenecks in a comparative framework. We examine the evidence for bottlenecks focusing on the quality of breeding plumage and the timing of moult as indicators in the six subspecies. In terms of breeding plumage coloration, quality and timing of prealternate body moult (from non-breeding into breeding plumage), the longest migrating knot subspecies, Calidris canutus rogersi and Calidris canutus rufa, show the greatest impact of bottlenecking. The same is true in terms of prebasic body moult (from breeding into non-breeding plumage) which in case of both C. c. rogersi and C. c. rufa overlaps with southward migration and may even commence in the breeding grounds. To close our discussion of bottlenecks in long-distance migrants, we make predictions about how migrants might be impacted via physiological 'trade-offs' throughout the annual cycle, using investment in immune function as an example. We also predict how bottlenecks may affect the distribution of mortality throughout the annual cycle. We hope that this framework will be applicable to other species and types of migrants, thus expanding the comparative database for the future evaluation of seasonal selection pressures and the evolution of annual cycles in long-distance migrants. Furthermore, we hope that this synthesis of recent advancements in the knowledge of red knot annual cycles will prove useful in the ongoing attempts to model annual cycles in migratory birds.

The annual life cycle of many birds includes breeding, moult and migration. All these processes are time and energy consuming and the extent of investment in any one may compromise the others. The output from breeding is of course the ultimate goal for all birds, while the investment in moult and migration should be selected so that lifetime fitness is maximized. In particular, long-distance migrants breeding at high latitudes face severe time pressures, which is a probable reason why natural selection has evolved efficient behaviours, physiological and morphological adaptations allowing the maximum possible migration speed. Optimal migration theory commonly assumes time minimization as an overall strategy, but the minimization of energy cost and predation risk may also be involved. Based on these assumptions, it is possible to derive adaptive behaviours such as when and at which fuel load a stopover site should be abandoned. I review some core components of optimal migration theory together with some key predictions. A review of accumulated empirical tests of the departure rule indicates that time minimization is an important component of the overall migration strategy, and hence gives support to the assumption about time-selected migration. I also briefly discuss how the optimal policy may be implemented by the bird by applying a set of simple rules. The time constraints on migrants increase with increasing body size. Some consequences of this are discussed.

Large and medium-sized alcids have a very intense wing molt wherein many flight feathers are shed in rapid succession and wing surface area is reduced by as much as 40%. Although these birds are rendered flightless during wing molt, they must still use their wings to propel themselves underwater. A molt-induced loss of wing area could simply reduce wing propulsion such that more muscular work would be required to maintain a given speed. Alternatively, molt could reduce drag on the wings, making a bird more penguin-like and actually enhancing diving ability. I addressed this issue by filming captive common guillemots Uria aalge and tufted puffins Fratercula cirrhata using an array of video cameras to plot the birds' movements in three dimensions. From these coordinate data I calculated swimming velocities, angles of descent and absolute depths. These values allowed me to estimate the forces due to drag and buoyancy that must be counteracted by flapping, which in turn yielded estimates of the amount of work generated during each flap as well as the average power and cost of transport. Within-bird comparisons of diving performance when wings were intact and during several stages of wing molt indicated that molt is associated with more frequent flapping, reduced displacement during the flap cycle, and possibly reduced work per flap. These negative effects on diving may explain why primary and secondary molts were offset in the birds I studied such that the period during which all of the flight feathers are effectively missing is minimized.

To understand the hormonal mechanism behind a unique strategy of breeding and molting in Humboldt penguins, six pairs of captive Humboldt penguins kept in an outdoor open display pen were observed and blood collected weekly for a year. They all molted between the middle of June and the middle of August within 10 days except one pair that molted about a month later. The late pair had been rearing a hatchling until July due to the successful second clutch after the first clutch failed. A peak of plasma levels of thyroxine and triiodothyronine, respectively, overlapped a period of molting in both sexes. Plasma testosterone concentrations in the males and females were lowest for two month during a period of pre-molt and molting. Plasma concentrations of estradiol were also lowest during the molt in both sexes. Except for the period of molting, sex steroid hormone concentrations were high although there was great individual variation. During the molt, the birds were forced to fast since they did not enter the pool in the display pen where they usually forage live fish. To compensate this forced fasting, they took more food than usual during pre-molting period and gained body mass to about 20% more than the baseline value. Increased flipper thickness was parallel to increased body mass indicating that the gained body mass attributed to fat reservoir. These data indicate that rapid molting in Humboldt penguins is correlated with a drastic increase and decrease of thyroid hormones during the period of lowest concentrations in sex steroid hormones.

In many birds, reproduction, molt, migration and other seasonal activities are controlled by endogenous circannual rhythmicity. Under constant conditions, this rhythm persists for many cycles with a period deviating from 12 months. Whether or not the rhythm is expressed depends on day length (photoperiod), which thus represents an important permissive factor in the process of rhythm generation. In nature, circannual rhythms are usually synchronized by the seasonal changes in photoperiod. However, equatorial birds may use daytime light intensity, which changes with the annual cycle of dry and rainy seasons, as a synchronizing zeitgeber. Photoperiod also modulates the rate of progress of the successive phases of the rhythmicity, such that an optimal adjustment to the annual environmental cycle is guaranteed. Populations of a given species may differ in their responsiveness to photoperiod in a manner that can be described as 'adaptive population-specific reaction norms'. In young migratory songbirds a circannual program determines changes in migratory direction and, at least partly, the time course and distance of migration. This circannual mechanism is replaced or supplemented in older birds by mechanisms formed on the basis of learning and memory. In general, circannual rhythms are intimately involved in the seasonal organization of a bird's behavior, providing the substrate onto which seasonal environmental factors act.

Postnuptial molt is the major molt that occurs in most wild and domestic avian species each year. The process is much more than the replacement of feathers. Studies have shown that a significant increase in metabolic rate, increase in whole body protein synthesis, osteoporosis, loss of body fat, and a suppression of the immune system occur during this event of a bird's annual cycle. Several procedures have been developed to initiate feather replacement, and this review addresses hormonal and neuropeptide treatments that effected molt. The administration of thyroxine, progesterone, gonadotropin-releasing hormone agonist, and prolactin were examined. Of the four, the two most effective were thyroxine and prolactin administration. The neural modulators known to release thyroxine and prolactin, respectively, are thyroid hormone releasing hormone (TRH) and vasoactive intestinal polypeptide (VIP). To gain insight into the possible functions of VIP and TRH, the distribution of these neuromodulators and their terminal fields were reviewed in the avian brain. It was found that VIP-containing neurons and fibers identified a neural system in birds comparable to the visceral forebrain system (VFS) described in mammals. The VFS functions to regulate the balance of the autonomic nervous system. The autonomic nervous system and, therefore, the VFS have been proposed to regulate the many behavioral and physiological events in the annual cycle of a bird, including postnuptial molt. In contrast to VIP that has an extensive brain distribution throughout the forebrain and brainstem, TRH is relatively restrictive and has a main concentration in nerve cells in and about the paraventricular nucleus, a key neural component of the VFS. Due to the roll of TRH in regulating the thyroid axis and operating within the framework of the VFS, it is proposed that the peptide functions to shift the balance of the autonomic nervous system in the direction of the sympathetic nervous system. A shift toward the dominance of the sympathetic nervous system appears to be required during this phase of a bird's yearly cycle.

We examined the effect of temperature on resting metabolic rate in seven field-captured laughing kookaburras (Dacelo novaeguineae) during late winter and early spring. Basal metabolic rate averaged 201+/-3.4 ml O(2) h(-1) (0.603 ml O(2) g(-1) h(-1)). Overall thermal conductance (K(o)) declined with ambient temperature ( T(a)) and averaged 0.026 ml O(2) g(-1) h(-1) degrees C(-1) at T(a)s<10 degrees C. Day-night differences in body temperatures (2.6 degrees C) and in alpha-phase versus rho-phase minimum metabolic rates were much greater (33%) than predicted for 340-g nonpasserine birds and suggest that these animals operate as low-metabolic intensity animals in their rest phase, but normal-metabolic intensity animals during their active phase. Metabolic rate was measured in four of the same birds undergoing moult. Thermal conductance increased to 60% above pre-moult values about 6 weeks after moult began. Basal metabolic rate of moulting birds showing peak thermal conductance readings averaged 17 ml O(2) h(-1) higher than pre-moult measurements. Although this increase was not statistically significant, we believe the moult costs of kookaburras are too low to overcome the inherent variability of BMR determination. We suggest that moult costs of kookaburras are only somewhat higher than the measured costs of protein synthesis of other endotherms.

Equatorial stonechats (Saxicola torquata axillaris) in Africa are seasonal breeders like their temperate-zone conspecifics (S.t. rubicola). Their annual cycle in gonadal size and function is controlled by an endogenous circannual rhythmicity that has been shown to run for up to 10 years in a constant equatorial photoperiod under laboratory conditions, with a period deviating from 12 months. In nature, however, this rhythm is synchronized with the actual year. Because photoperiod is essentially constant at the equator, it is likely that other environmental factors act as zeitgebers. The authors test whether food availability affects reproductive cycles of free-living East African stonechats. The authors offered supplemental food to the birds 2 months before the regular onset of the breeding season. Supplementally fed males started to sing and display earlier than males of control pairs that did not receive extra food. Although the supplemented food advanced the onset of the breeding season in the pairs that were fed, the onset of the postnuptial molt following the breeding season was not correspondingly shifted. Furthermore, in the year following the experiment, all pairs initiated breeding at the same time. The authors conclude that food availability does not act as a zeitgeber, but rather as a factor that modifies the timing of reproduction without affecting the underlying rhythmicity. The authors propose that this is adaptive under environmental conditions that are relatively constant within a given year but may vary considerably between years. The zeitgeber synchronizing the endogenous rhythmicity remains to be identified.

Year-class differences in reproductive function were investigated in a free-living population of adult male Dark-eyed Juncos, Junco hyemalis, breeding in interior Alaska. Second-year males (SY, entering their first breeding season) were compared with after-second-year males (ASY, entering at least their second breeding season). We measured body mass, size of the cloacal protuberance (CP), testis mass, onset of prebasic molt, and concentrations of plasma luteinizing hormone (LH), testosterone (T), corticosterone (CORT), and prolactin (PRL) throughout the reproductive season (April to mid-July). There were no differences in SY and ASY body weights but SY males had smaller CPs and testis masses than ASY males during gonadal recrudescence and at the end of the breeding season. Plasma LH was elevated from April until mid-June and then decreased in the same way in both year classes. In contrast, plasma T was high from April until mid-May and was lower in SY than in ASY juncos shortly after they arrived on their breeding grounds at the end of April, but not at other times. In July, SY males started to molt earlier, suggesting that they became photorefractory earlier than ASY males. Plasma PRL increased progressively in both year classes between April and early June and decreased in early July. At this time, plasma PRL decreased earlier in SY than in ASY males. Plasma CORT changed seasonally, but did not differ between SY and ASY juncos. Thus, year-class differences in CP sizes and testis mass apparently did not result from SY males secreting less LH or more PRL or CORT than ASY males. It is suggested that differences in reproductive condition in SY and ASY juncos are mediated by interactions with conspecific birds and do not result from an intrinsic effect of age., (Copyright 2000 Academic Press.)

The breeding season of wild starlings is controlled by photoperiod. Full breeding condition is attained during exposure to lengthening days in the spring, and photorefractoriness ensues. The reproductive system of starlings will not subsequently be stimulated by long day lengths until photorefractoriness is dissipated by the short day lengths experienced in the autumn and winter. Unlike most studies on avian photoperiodism, this investigation involved manipulation of light intensity of a fixed photoperiod rather than of photoperiod itself. Photosensitive starlings transferred from short days to long days of different light intensities underwent graded reproductive responses according to the light intensity they experienced. Testes size in the group in the lowest intensity (3 lux) increased faster than that in controls on short days of normal intensity, but they did not become photorefractory. Testes size increased in the groups on 13, 45, and 108 lux and subsequently became photorefractory. However, the 13- and 45-lux groups required more time to become photorefractory than did the 108-lux group. The responses observed were similar to those seen in starlings exposed to different photoperiods (e.g., 11 h light:13 h dark [11L:13D], 13L:11D, 16L:8D, 18L:6D), even though all were on the same 18L:6D photoperiod. Initially, the results appear to challenge the external coincidence model for photoperiodic time measurement, but consideration of the phase response curve of the circadian rhythm of photoinducibility in starlings and the way in which it might be affected by low light intensities refute this challenge.

Penguins are highly adapted to marine life. Their hydrodynamic efficiency depends on feathers which wear with age and need to be replaced regularly. During molting, penguins can not enter the sea to forage and are forced to fast. Therefore the duration of molting is necessarily brief. To better understand molting in penguins, we collected plasma samples from 16 (8 pairs) Humboldt penguins kept in an open display pen at Tokyo Sea Life Park from May to September, 1994 and estimated circulating concentrations of LH, testosterone, estradiol, thyroxine (T4), triiodothyronine (T3) and corticosterone. Body mass was also measured at each blood sampling. Throughout the year, reproductive activities (egg laying, incubation, hatching and rearing) and molting were observed and recorded. Humboldt penguins maintained reproductive activity from January to December except during molting. Each pair started molting between the end of July and early August; usually males started earlier. The duration of molting was 13.4 +/- 0.8 days for males and 12.9 +/- 0.3 days for females. Body masses were highest just before the start of molting in both sexes. Plasma concentrations of LH were high, (> 2 ng/ml) in May in both sexes, then gradually decreased, to 0.53 +/- 0.38 ng/ml in males and 0.72 +/- 0.11 ng/ml in females by the end of July. Testosterone and estradiol concentrations in plasma decreased and were lowest during molting. On the other hand, plasma concentrations of T4 were low until early July (ca. 20 ng/ml) and then doubled within 10 days; the high levels were maintained for one month and then decreased greatly in males and slightly in females. When the plasma concentrations of T4 started to decrease, plasma concentrations of LH increased. Changes in plasma T3 were not consistent with molting. These results indicate that the decrease of plasma levels of sex steroid hormones and the sharp increase of T4 induced molting, which lasted only for a short period.

Seasonal breeding in many birds, including the European starling, is terminated by the development of absolute reproductive photorefractoriness, followed by a postnuptial molt, when photo-induced PRL secretion is at its seasonal maximum. To determine whether this photo-induced increase in PRL secretion has a causal role in the development of photorefractoriness or molt, European starlings were actively immunized against vasoactive intestinal polypeptide (VIP), the PRL releasing hormone in birds, or against PRL, during a photo-induced breeding cycle. In half of the VIP-immunized birds, the photo-induced increase in PRL was completely suppressed. Although these birds became photorefractory, the rate of gonadal regression was markedly slowed. These birds did not molt. In the remaining VIP-immunized birds, the photo-induced increase in PRL was inhibited but not completely suppressed. In these birds, and in those immunized against PRL, gonadal regression was also slowed, but molt progressed as normal. There were no significant differences in concentrations of plasma thyroxine between treatment and control groups, indicating that the effects of immunization on gonadal regression were not mediated by the induction of hypothyroidism. These results are consistent with the view that in the European starling the seasonal photo-induced increase in PRL accelerates gonadal regression during the onset of photorefractoriness but does not itself cause photorefractoriness. Further, the seasonal increase in PRL is required for the induction of the postnuptial molt.

In previous studies, we withdrew thyroid hormones by thyroidectomy before, at, or after the onset of photostimulation and showed that male American tree sparrows (Spizella arborea) are programmed for seasonal reproduction and postnuptial molt by or before Week 3 on long days. In this corollary study, we withdrew long days before or after the control circuits had been programmed. After 1 day to 4 wk on long days, groups of thyroid-intact males were returned to short days until Week 7, when they were moved to constant light and evaluated for photosensitivity or photorefractoriness and postnuptial molt. Long-day controls held 7 wk on long days showed robust testicular growth through Week 6 and then spontaneous testicular regression. Testes of short-day controls and of males photostimulated for 1 day remained small. In all other groups, photostimulation induced testicular growth, which gave way to regression during exposure to short days. Long-day controls tested photorefractory at Week 7 and initiated molt by Week 10. All other groups tested photosensitive and did not molt. Our demonstration that long days are required for expression of seasonal reproduction and postnuptial molt in thyroid-intact male tree sparrows previously programmed for these events suggests that long days create a milieu that is permissive for expression.

The extent to which circadian rhythms are involved in photoperiodic time measurement in quail is enigmatic, and earlier investigations have produced results consistent with an hourglass clock or one involving damped circadian oscillators. To address the problem further, the present authors carried out a direct comparison between the clocks in quail and those in starlings. Starlings possess strongly self-sustaining circadian oscillators. In Experiment 1, comparisons of testicular growth were made between the two species when birds were exposed to light:dark (LD) 6:30, LD 6:18, and LD 18:6. Starlings grew their testes rapidly under both LD 6:30 and LD 18:6, and they became photorefractory (under LD 6:18, the testes remained undeveloped). Quail grew their testes rapidly under LD 18:6 but did not do so under LD 6:30 or LD 6:18. In Experiment 2, entrainment of the activity rhythm under cycles of LD 6:30 was investigated by measuring the phase of the rhythm after release into constant darkness (DD). Birds were exposed to either 10 cycles or 11 cycles of LD 6:30 prior to DD. Starlings maintained their 24-h rhythmicity under LD 6:30 and always free ran from the phase of the subjective day. By contrast, quail showed circadian activity approximately 24 h after every light pulse and free ran from the phase of the last light pulse received. In Experiment 3, phase response curves (PRCs) were generated to 6-h light pulses. The species were strikingly different; starlings produced a Type 1 PRC, whereas quail produced a Type 0 PRC. More important, in quail the 6-h light pulse had the same effect regardless of circadian time and in almost every case activity free ran from the position of the 6-h light pulse. The results in quail are consistent with the photoperiodic time measurement system being based on a weakly self-sustaining (rapidly damping) circadian system that is invariably reset by 6 h of light, whereas in starlings the pacemakers are strongly self-sustaining. The results support the notion that hourglass pacemakers can be highly damped circadian pacemakers.

Transfer of intact, photosensitive starlings from short to long days causes an increase in plasma thyroxine and gonadal maturation and later induces photorefractoriness. Thyroidectomy of starlings prevents the induction of photorefractoriness. This study investigated whether the long-day-induced increase in plasma thyroxine is necessary for the induction of photorefractoriness. Photosensitive starlings were thyroidectomised, given thyroxine in their drinking water at concentrations that result in plasma thyroxine at short-day physiological concentrations or lower, and transferred to long days. Plasma thyroxine and prolactin, gonadal size, and moult were monitored. The group with short-day concentrations of plasma thyroxine became photorefractory at the same time as intact controls transferred to long days. The other groups, with lower plasma thyroxine, also became photorefractory, but the onset of photorefractoriness was delayed. The increase in plasma prolactin following photostimulation was proportional to plasma thyroxine concentrations. The onset of moult was also related to plasma thyroxine. We conclude that the long-day concentrations of plasma thyroxine observed in the plasma of intact starlings are not necessary for the induction of photorefractoriness. This suggests that thyroxine acts as a permissive factor rather than actively driving the photorefractory process.

Male European starlings held under constant 12-h days (LD 12:12) undergo circannual rhythms of gonadal maturation and molt. This study examined the temporal relationship of the different physiological phases of these rhythms and the associated endocrinology. The majority of males and about half of the females in this study showed evidence of rhythmicity in gonadal maturation under constant LD 12:12. These gonadal cycles were different from those under natural changes in day length in that the period of photorefractoriness, when birds are unresponsive to an increase in day length, was always much less and in some cases very short indeed. The relationship between molt and gonadal regression was similar to that during natural cycles: every bird that showed gonadal regression began to molt during regression, and molt did not occur except during gonadal regression. The plasma-luteinizing hormone showed rhythmicity related to gonadal maturation, although less clearly in females and much less pronounced than during natural annual rhythms. Plasma prolactin remained low in most males but varied erratically in females. There was no evidence of rhythmicity in prolactin in either sex. High prolactin was not related to gonadal regression or molt. The results suggest that II endogenous circannual rhythms have a role in the expression of natural annual cycles in this species, then they are highly modulated by ambient day length throughout the year rather than simply being synchronized by day length.

There is convincing evidence that euthyroid male American tree sparrows are already programmed for seasonal reproduction and postnuptial molt by Week 4 of photostimulation. To explore more precisely when, during early photostimulation, thyroid-dependent programming of seasonal events occurs, photosensitive male tree sparrows were radiothyroidectomized or sham thyroidectomized on the first day of photostimulation (Week 0) or at Weeks 1 or 3 thereafter. Birds were monitored for testicular growth and regression over 8 or 12 weeks and then tested for photosensitivity or photorefractoriness by exposing them to constant light and exogenous L-thyroxine for 4 weeks. Molt of the primary flight feathers was scored periodically, and at the end of the experiment, hypothalami were saved for cGnRH-I (chicken gonadotropin-releasing hormone I) assay. Because (1) the thyroid is already dysfunctional by Day 4 after radiothyroidectomy, (2) thyroid-dependent photoinduced gonadal growth is programmed growth, and as demonstrated here, (3) testis length at Week 6 on long days (i.e., maximum or near-maximum testis size) is independent of the time of thyroidectomy at or after the onset of photostimulation, we conclude that male American tree sparrows were programmed for photoperiodic testicular growth during the first week of photostimulation. Based on measurements of testis length and hypothalamic cGnRH-I content, only 2 of 11 birds thyroidectomized at Week 1 were already programmed for photorefractoriness by Week 1, whereas all birds thyroidectomized at Week 3 were so programmed by Week 3. Marked differences in molt scores between photorefractory (thyroidectomized and euthyroid) and photosensitive (thyroidectomized) birds argue that postnuptial molt also was programmed roughly between Weeks 1 and 3. To explain these results, we ascribe organizational-like actions directly or indirectly to endogenous thyroid hormones and argue that the onset of photostimulation opens a window, during which time euthyroid male American tree sparrows are programmed for seasonality. A clear dissociation of photorefractoriness from photoperiodic testicular growth in birds thyroidectomized at Week 1 suggests that separate control circuits mediate these two annually periodic events.

Prompted by evidence of a linkage between the thyroid gland and seasonal reproduction in obligately photoperiodic American tree sparrows (Spizella arborea), we measured circulating T4 (L-thyroxine), hypothalamic cGnRH-I (chicken gonadotropin-releasing hormone, variant I), pituitary and plasma LH (luteinizing hormone), and ovarian mass in euthyroid females moved from short to long days. Our purpose was to correlate temporal changes in T4 and cGnRH-I with each other and with pituitary and plasma LH and ovarian mass. T4 increased early during photostimulation and peaked at Week 1 or 2 on 20L:4D (20 hr light:4 hr dark per day). Hypothalamic cGnRH-I also increased during photostimulation and peaked between Weeks 2 and 4. Pituitary and plasma LH peaked at Weeks 4 and 1, respectively, and ovarian mass increased significantly by Week 6. In a separate study, we charted the progression of postnuptial molt. Molt of the primary remiges began at Week 9, proceeded rapidly through Week 15, and was complete by Week 18. In view of a demonstrated role for the thyroid gland in the control of seasonal reproduction in female tree sparrows and the time-dependent effects of thyroidectomy after photostimulation in male tree sparrows, the observation that circulating T4 increases early during photostimulation suggests the possibility of a causal relationship between the thyroid gland and the hypothalamus-pituitary-ovarian axis, hence a thyroid-hypothalamus-pituitary-ovarian axis.

The time course of thyroid dysfunction after injecting 0.30 mCi Na131I was charted in female American tree sparrows (Spizella arborea) transferred to long days on Day 0 and challenged with 30 micrograms bovine thyroid-stimulating hormone on Postinjection Days 4, 7, 46, and 105. Serum T4 (L-thyroxine) was not detectable in thyroidectomized birds, indicating complete thyroid dysfunction by Day 4 and no restoration of thyroid function by Day 105. By contrast, serum T4 concentrations in similarly challenged thyroid-intact controls greatly exceeded assay sensitivity. To determine whether a single injection of T4 can program thyroidectomized female tree sparrows for seasonal reproduction and postnuptial molt, as it can thyroidectomized male tree sparrows, thyroidectomized females were injected with T4 (100 micrograms) or alkaline vehicle (V) either on the first day of photostimulation or 1 week before photostimulation (during which time injected T4 likely was cleared metabolically). Females injected with T4 on the first day of photostimulation showed robust ovarian growth similar to that reported for photostimulated euthyroid females. Both groups of V-injected females, as well as females injected with T4 1 week before photostimulation, showed slow, but reliable, ovarian growth, indicating that thyroidectomized birds can detect an increase in day length. When birds were moved at Week 7.5 or 9 to constant light and given T4 in drinking water (a qualitative assay for absolute photorefractoriness), only one tested photorefractory and molted. The remaining birds, including those injected with T4 1 week before photostimulation and both groups of V-injected controls, tested photosensitive and did not molt. Taken together, these results indicate that T4, or one of its metabolites, programs thyroidectomized female tree sparrows for ovarian growth early during photostimulation. A single T4 injection (100 micrograms) on the first day of photostimulation usually also programs thyroidectomized male tree sparrows for photorefractoriness and molt. Such an injection is far less effective in programming females for these two late-season events.

Subspecies of the Houbara bustard are resident in North Africa (Chlamydotis undulata undulata) or are partial latitudinal migrants in Asia (Chlamydotis undulata macqueenii). Houbara breed in arid or semiarid zones, and the reproductive biology of wild Houbara is largely unknown. Body mass, molt, courtship display, laying, and plasma concentrations of LH, testosterone, and progesterone were measured in captive-bred birds of both subspecies, housed in outdoor cages under natural conditions, over a period of 1 year and 6 months, at Taif, Saudi Arabia (21 degrees 15' N). Captive Houbara bustards show a seasonal breeding pattern with a laying period from January to July. Males began displaying well in advance of any eggs laid and display long after the termination of laying. In females, gonadal activity and molt were mutually exclusive; however, in males, slight overlap between sexual display and molt was observed. In females, seasonal variation of LH and progesterone was synchronous with laying activity. In males both testosterone and LH plasma concentrations were highest in January (testosterone approximately 8 ng/ml; LH approximately 3 ng/ml) at the beginning display period. This could correspond, in the wild, to the establishment of territories. Levels dropped significantly before the females started laying but stayed at an intermediate level throughout the breeding season (testosterone approximately 3 ng/ml; LH approximately 1.5 ng/ml). In the middle of August, during molt, and following heavy rainfall, LH and testosterone peaked (testosterone approximately 2.3 ng/ml; LH approximately 1.8 ng/ml), suggesting a possible impact of rainfall on Houbara gonadal activity. Houbara stopped laying before the maximum ambient temperature reached 32 degrees, indicating that high temperature could be a factor terminating laying activity. No major differences were found between the two subspecies in the timing or duration of the reproductive period; only the annual variation of the body mass varied slightly between the subspecies.

Plasma levels of prolactin and luteinizing hormone have been examined in intact and pinealectomized European starling (Sturnus vulgaris) during termination and onset of photorefractoriness. Photorefractory starlings exposed to long (18L:6D) daily photoperiods for 10 weeks were pinealectomized. One week after pinealectomy these birds, along with the intact and sham-operated birds, were transferred to short (8L:16D) daily photoperiods. After 10 weeks on short day lengths the daily photoperiod was switched to long (18L:6D) and the birds remained on this regime for a further 10 weeks. Plasma prolactin and luteinizing hormone were determined prior to pinealectomy and then weekly throughout the experiment. Molt scores were also recorded. Profiles of both the hormones and the molt pattern were similar in pinealectomized, sham-operated, and intact control starlings. In European starlings the pineal gland does not seem to exert significant effects on the photoinduced rise in plasma prolactin and luteinizing hormone seen in photosensitive birds. It is also suggested that pinealectomy has no significant effect on the dissipation and onset of photorefractoriness assessed from plasma luteinizing hormone secretion in this bird.