The wing musculature of the Weka (Gallirallus australis), a flightless rail endemic to New Zealand

The pectoral musculature of the Weka, a flightless New Zealand rail, is almost identical with that of a volant relative, the American coot, even to details of the propatagial complex. Similar observations have been made for other flightless carinates, including ducks and cormorants (Lowe 1928 a , 1934, 1935). Ratites, in contrast, have far fewer wing muscles than carinates, and lack a propatagium (Lowe, 1928 b ; McGowan, 1982). The fact that the loss of flight in carinates is accompanied by such little change in the pectoral musculature, while that of ratites is so radically different, weakens the hypothesis that ratites evolved from carinates through the loss of flight.
An assessment of the relationship between muscles and their bony attachment areas shows how little myological information can be deduced from the surface features of bones. Thus, not even the vaguest of muscle reconstructions could have been attempted by studying the pectoral skeleton of the Weka, and similar findings have been made elsewhere (McGowan, 1979, 1982). This raises serious doubts on the validity of reconstructing the musculature of extinct vertebrates, a matter of much interest to palaeontologists.     

Wing moult and mass change in free‐living mallard Anas platyrhynchos

Captured free‐living male mallard Anas platyrhynchos at Abberton in southern Britain showed peak mass gain immediately prior to simultaneous remex moult. Individuals of both sexes were heavier before shedding wing feathers than when flightless confirming literature accounts that show mallard accumulate fat stores in anticipation of moult to contribute to meeting energy needs during remex re‐growth. Over the course of four seasons, males lost 13 17% of initial body mass on average during re‐growth of flight feathers, females 13 23%. Based on energy expenditure of 1.3 times BMR, male mallard were estimated to be able to fulfil 42 60% and females 41 82% of their energy needs throughout moult from stores. Free‐flying male mallard fed ad libitum in a predator‐free environment did not differ in starting body mass or rate of mass loss during wing moult compared to free‐living Abberton birds, suggesting depletion of fat stores, irrespective of available sources of exogenous energy. Based on this evidence, we reject that the hypotheses that mass loss in moulting mallard is due to 1) simple energy stress and 2) restrictions on feeding and consider that 3) attaining the ability to fly at an earlier stage on incompletely grown flight feathers is not the primary factor shaping this trait. Rather, we consider the accumulation and subsequent depletion of fat stores, together with reductions in energy expenditure, enable mallard to re‐grow feathers as rapidly as possible by exploiting habitats that offer safety from predators, but do not necessarily enable them to balance energy budgets during the flightless period of remex feather re‐growth.     

Phenotypic flexibility of a southern African duck Alopochen aegyptiaca during moult: do northern hemisphere paradigms apply?

Phenotypic flexibility during moult has never been explored in austral nomadic ducks. We investigated whether the body condition, organ (pectoral muscle, gizzard, liver and heart) mass and flight‐feather growth Egyptian geese Alopochen aegyptiaca in southern Africa show phenotypic flexibility over their 53‐day period of flightless moult. Changes in body mass and condition were examined in Egyptian geese caught at Barberspan and Strandfontein in South Africa. Mean daily change in primary feather length was calculated for moulting geese and birds were dissected for pectoral muscle and internal organ assessment. Mean body mass and condition varied significantly during moult. Body mass and condition started to decrease soon after flight feathers were dropped and continued to do so until the new feathers were at least two‐thirds grown, after which birds started to regain body mass and condition. Non‐moulting geese had large pectoral muscles, accounting for at least 26% of total body mass. Once moult started, pectoral muscle mass decreased and continued to do so until the flight feathers were at least one‐third grown, after which pectoral muscle mass started to increase. The regeneration of pectoral muscles during moult started before birds started to gain overall body mass. Gizzard mass started to increase soon after the onset of moult, reaching a maximum when the flight feathers were two‐thirds grown, after which gizzard mass again decreased. Liver mass increased significantly as moult progressed, but heart mass remained constant throughout moult. Flight feather growth was initially rapid, but slowed towards the completion of moult. Our results show that Egyptian geese exhibit a significant level of phenotypic flexibility when they moult. We interpret the phenotypic changes that we observed as an adaptive strategy to minimize the duration of the flightless period. Moulting Egyptian geese in South Africa undergo more substantial phenotypic changes than those reported for ducks in the northern hemisphere.     

Young's modulus varies with differential orientation of keratin in feathers

Feathers are composed of a structure that, whilst being very light, is able to withstand the large aerodynamic forces exerted upon them during flight. To explore the contribution of molecular orientation to feather keratin mechanical properties, we have examined the nanoscopic organisation of the keratin molecules by X-ray diffraction techniques and have confirmed a link between this and the Young's modulus of the feather rachis. Our results indicate that along the rachis length, from calamus to tip, the keratin molecules become more aligned than at the calamus before returning to a state of higher mis-orientation towards the tip of the rachis. We have also confirmed the general trend of increasing Young's modulus with distance along the rachis. Furthermore, we report a distinct difference in the patterns of orientation of beta-keratin in the feathers of flying and flightless birds. The trend for increased modulus along the feathers of volant birds is absent in the flightless ostrich.     

An alternate and reversible method for flight restraint of cranes

Flight restraint is important for zoos, safaris, and breeding centers for large birds. Currently used techniques for flight restraint include both surgical and non-surgical approaches. Surgical approaches usually cause permanent change to or removal of tendon, patagial membrane, or wing bones, and can cause pain and inflammation. Non-surgical approaches such as clipping or trimming feathers often alter the bird's appearance, and can damage growing blood feathers in fledglings or cause joint stiffness. We observed microstructure of primary feathers of the red-crowned crane (Grus japonensis) and found that the width of barbs is a determinative factor influencing vane stiffness and geometric parameters. We hypothesized that partial longitudinal excision of barbs on the ventral surface of the primary feathers would reduce the stiffness of the vane and render the feathers unable to support the crane's body weight during flight. Furthermore, we hypothesized that this modification of barbs would also change the aerodynamic performance of feathers such that they could not generate sufficient lift and thrust during flapping to enable the bird to fly. We tested this hypothesis on a red-crowned crane that had normal flight capability by excising the ventral margin of barbs on all 10 primaries on the left wing. The bird was unable to take off until the modified feathers were replaced by new ones. Removal of barbs proved to be a simple, non-invasive, low-cost and reversible method for flight restraint. It is potentially applicable to other large birds with similar structural characteristics of primary feathers.     

Size scaling and stiffness of avian primary feathers: implications for the flight of Mesozoic birds

The primary feathers of birds are subject to cyclical forces in flight causing their shafts (rachises) to bend. The amount the feathers deflect during flight is dependent upon the flexural stiffness of the rachises. By quantifying scaling relationships between body mass and feather linear dimensions in a large data set of living birds, we show that both feather length and feather diameter scale much closer to predictions for geometric similarity than they do to elastic similarity. Scaling allometry also indicates that the primary feathers of larger birds are relatively shorter and their rachises relatively narrower, compared to those of smaller birds. Two-point bending tests indicated that larger birds have more flexible feathers than smaller species. Discriminant functional analyses (DFA) showed that body mass, primary feather length and rachis diameter can be used to differentiate between different magnitudes of feather bending stiffness, with primary feather length explaining 63% of variance in rachis stiffness. Adding fossil measurement data to our DFA showed that Archaeopteryx and Confuciusornis do not overlap with extant birds. This strongly suggests that the bending stiffness of their primary feathers was different to extant birds and provides further evidence for distinctive flight styles and likely limited flight ability in Archaeopteryx and Confuciusornis.     

Increase of feather quality during moult: a possible implication of feather deformities in the evolution of partial moult in the great tit Parus major

Here we investigate the change in feather quality during partial post‐juvenile and complete post‐breeding moult in great tit Parus major by measuring the change in the number of fault bars and feather holes on wing and tail feathers. Feathers grown during ontogeny usually are of lower quality than feathers grown following subsequent moults at independence. This is reflected by higher number of fault bars and feather holes on juveniles compared to adults. Fault bars are significantly more common on tail and proximal wing feathers than on the distal remiges, indicating a mechanism of adaptive allocation of stress induced abnormalities during ontogeny into the aerodynamically less important flight feathers. On the contrary, feather holes produced probably by chewing lice have a more uniform distribution on wing and tail feathers, which may reflect the inability of birds to control their distribution, or the weak natural selection imposed by them. The adaptive value of the differential allocation of fault bar between groups of feathers seems to be supported by the significantly higher recapture probability of those juvenile great tits which have fewer fault bars at fledging on the aerodynamically most important primaries, but not on other groups of flight feathers. The selection imposed by feather holes seems to be smaller, since except for the positive association between hatching date, brood size and the number of feather holes at fledging, great tits' survival was not affected by the number of feather holes. During post‐juvenile moult, the intensity of fault bars drops significantly through the replacement of tail feathers and tertials, resulting in disproportional reduction of the total number of fault bars on flight feathers related to the number of feathers replaced. The reduction in the number of fault bars during post‐juvenile moult associated with their adaptive allocation to proximal wing feathers and rectrices may explain the evolution of partial post‐juvenile moult in the great tit, since the quality of flight feathers can be increased significantly at a relatively small cost. Our results may explain the widespread phenomenon of partial post‐juvenile moult of flight feathers among Palearctic passerines. During the next complete post‐breeding moult, the total number of fault bars on flight feathers has remained unchanged, indicating the effectiveness of partial post‐juvenile moult in reducing the number of adaptively allocated fault bars. The number of feather holes has also decreased on groups of feathers replaced during partial post‐juvenile moult, but the reduction is proportional with the number of feathers moulted. In line with this observation, the number of feather holes is further reduced during post‐breeding moult on primaries and secondaries, resulting in an increase in feather quality of adult great tits.     

The physiology of life-history trade-offs: experimental analysis of a hormonally induced life-history trade-off in Gryllus assimilis

Abstract Adult Gryllus assimilis given an analog of juvenile hormone exhibited reduced flight muscles and enlarged ovaries similar to those found in naturally occurring flightless individuals of species that are polymorphic for dispersal capability. Control and hormone-treated (flightless) G. assimilis did not differ in the amount of food consumed or assimilated on any of three diets that differed in nutrient quantity. Thus, enhanced ovarian growth of flightless individuals resulted from increased allocation of internal nutrients to reproduction (i.e., a trade-off) rather than from increased acquisition of nutrients. Compared with flight-capable controls, flightless G. assimilis also had reduced whole-organism respiration, reduced respiration of flight muscles, and reduced lipid and triglyceride (flight fuel) reserves. These differences are remarkably similar to those between naturally occurring flightless and flight-capable morphs of other Gryllus species. Results collectively suggest that the increased allocation of nutrients to ovarian growth in flightless G. assimilis and other Gryllus species results from reduced energetic costs of flight muscle maintenance and/or the biosynthesis or acquisition of lipids. Reduction in these energetic costs appears to be an important driving force in the evolution of flightlessness in insects. Respiratory metabolism associated with flight capability utilizes an increasing proportion of the energy budget of crickets as the quantity of nutrients in the diet is decreased. This leads to a magnification of greater ovarian growth of flightless versus flight-capable individuals on nutrient-poor diets.     

Feather holes and flight performance in the barn swallow Hirundo rustica

Feather holes are small (0.5–1?mm in diameter) deformities that appear on the vanes of flight feathers. Such deformities were found in many bird species, including galliforms and passerines. Holey flight feathers may be more permeable to air, which could have a negative effect on their ability to generate aerodynamic forces. However, to date the effects of feather holes on flight performance in birds remained unclear. In this study we investigated the relationship between the number of feather holes occurring in the wing or tail feathers and short term flight performance traits – aerial manoeuvrability, maximum velocity and maximum acceleration – in barns swallows, which are long distance migrating aerial foragers. We measured short-term flight performance of barn swallows in a standardized manner in flight tunnels. We found that acceleration and velocity were significantly negatively associated with the number of holes in the wing flight feathers, but not with those in the tail feathers. In the case of acceleration the negative relationship was sex specific – while acceleration significantly decreased with the number of feather holes in females, there was no such significant association in males. Manoeuvrability was not significantly associated with the number of feather holes. These results are consistent with the hypothesis that feather holes are costly in terms of impaired flight. We discuss alternative scenarios that could explain the observed relationships. We also suggest directions for future studies that could investigate the exact mechanism behind the negative association between the number of feather holes and flight characteristics.     

Positive Relationship between Signalling Time and Flight Capability in the Texas Field Cricket, Gryllus texensis

A trade‐off between dispersal ability and reproduction is generally thought to explain the persistence of wing dimorphism in insects, although this trade‐off has received minimal attention in male insects. Research on male sand cricket, Gryllus firmus, supports the trade‐off hypothesis insofar as flight capable cricket’s spend significantly less time signalling for potential mates than their flightless counterparts. By contrast, here I show that this expected trade‐off between signalling time and wing dimorphism does not exist in a male congener, the Texas field cricket (Gryllus texensis). In G. texensis, flight capable males signal twice as often as flightless males. Thus, unless male G. texensis express trade‐offs between dispersal ability and other, presently unmeasured components of reproduction, the trade‐off hypothesis may not explain the persistence of wing dimorphism in all male insects.     

The Evolution of Feathers*

Existing hypotheses on the evolution of feathers are reviewed with the assumptions that feather evolved from reptilian scales and that pennaceous feathers evolved before downy feathers. Observations with a scanning electron microscope demonstrate that basic to the structure of pennaceous feathers is the lamelliform structure of barbules, the planes of which are oriented at right angles to the plane of the feather vane. Thus the structure of the vane is more open than generally realized. The airtight vane of flight feathers is assumed a later specialization. Most of the existing hypotheses assume that the feather acts as a relatively solid barrier between the skin of the bird and the exterior and they are therefore not in agreement with the actual structure of feathers. A hypothesis is needed which explains the adaptive value of a pennaceous feather being porous. The hypothesis is put foward that feathers evolved due to selection for a water-repellent integument. For purely physical reasons a porous surface repels water drops more strongly than does a solid surface of the same material. Physicists have pointed out that the structure of feathers conforms closely with the theoretical requirements for water-repellency. Possibly feathers started to evolve on reptiles living at the seashore, where the main advantage of increased water-repellency was to reduce cooling from evaporation of water off a wet integument.     

Adventitious feather replacement favours a more rapid regeneration of primaries over rectrices in two passerine bird species

There is increasing evidence of adaptive preferential investment during moult in those feather tracts that are more advantageous for fitness. In this study, we assessed whether, after the manual removal of two functionally different flight feathers (one primary and one rectrix), birds from two common passerine species (Eurasian Blackcap Sylvia atricapilla and European Robin Erithacus rubecula) favoured the regeneration of primary (supposedly the most functionally important feathers) over rectrix feathers. Our results did not show differences between replaced primary and rectrix feathers in their final length, but demonstrated that the gap left by the loss of the primary feather was filled earlier, suggesting that a rapid repair of the most essential feather tracts is also evolutionarily advantageous during the adventitious replacement of plumage.     

The metabolic basis of life history variation: genetic and phenotypic differences in lipid reserves among life history morphs of the wing-polymorphic cricket,Gryllus firmus

The flight-capable morph of the wing-polymorphic cricket, Gryllus firmus, accumulated a substantially greater quantity of total lipid and triglyceride, compared with the obligately flightless morph, during the first five days of adulthood. Increased lipid accumulation in the flight-capable morph was genetically based, and was produced when ovarian growth is substantially reduced in that morph. Temporal changes in lipid levels suggest that the higher triglyceride reserves in the flight-capable morph fed a high-nutrient diet were produced by elevated lipid biosynthesis. By contrast, on a low-nutrient or high carbohydrate diet, increased lipid levels in the flight-capable morph appeared to result primarily from decreased lipid utilization. Increased biosynthesis or retention of triglyceride (the major flight fuel in Gryllus) by the flight-capable morph may significantly divert nutrients from egg production and hence may be an important physiological cause of its reduced ovarian growth. The obligately flightless morph allocated a greater proportion of total lipid to phospholipid than did the flight-capable morph. No functionally-significant differences in total lipid or triglyceride were produced between morphs during the last nymphal stadium. A second flightless morph, derived from the flight-capable morph by histolysis of flight muscles during adulthood, also had reduced amounts of total lipid and triglyceride but increased ovarian growth compared with the flight capable morph on the standard (high-nutrient) diet. Important qualitative and quantitative aspects of lipid metabolism differ genetically between the flight-capable and flightless morphs of G. firmus and likely contribute importantly to their respective adaptations for flight capability vs. reproduction. This is the first study to document genetically-based differences in energy reserves between morphs of a complex (phase, caste, flight) polymorphism in which morphs also differ genetically in key life history traits.     

Evolutionary shifts in the melanin‐based color system of birds

Melanin pigments contained in organelles (melanosomes) impart earthy colors to feathers. Such melanin‐based colors are distributed across birds and thought to be the ancestral color‐producing mechanism in birds. However, we have had limited data on melanin‐based color and melanosome diversity in Palaeognathae, which includes the flighted tinamous and large‐bodied, flightless ratites and is the sister taxon to all other extant birds. Here, we use scanning electron microscopy and spectrophotometry to assess melanosome morphology and quantify reflected color for 19 species within this clade. We find that brown colors in ratites are uniquely associated with elongated melanosomes nearly identical in shape to those associated with black colors. Melanosome and color diversity in large‐bodied ratites is limited relative to other birds (including flightless penguins) and smaller bodied basal maniraptoran dinosaur outgroups of Aves, whereas tinamous show a wider range of melanosome forms similar to neognaths. The repeated occurrence of novel melanosome forms in the nonmonophyletic ratites suggests that melanin‐based color tracks changes in body size, physiology, or other life history traits associated with flight loss, but not feather morphology. We further anticipate these findings will be useful for future color reconstructions in extinct species, as variation in melanosome shape may potentially be linked to a more nuanced palette of melanin‐based colors.     

Isotopic evidence for endogenous protein contributions to greylag goose Anser anser flight feathers

Feather stable isotope composition may not reflect local isoscapes in which they were grown if supplemented with protein of endogenous origin. Thus, feather isotope analysis, combined with knowledge of local isoscapes can be used to infer endogenous nutrient composition to feathers in cases where birds travel to moult. We investigated this possibility in a study of flightless moulting greylag geese Anser anser on the Danish island of Saltholm, which are known to mobilise endogenous protein stores (acquired at previous terrestrial staging locations in Sweden) to reconstitute muscle blocks and organs whilst feeding on a saltmarsh (i.e. marine-influenced) diet with contrasting stable isotope ratios. We used stable isotope (δ¹³C, δ¹⁵N) measurements to test the prediction that new-grown flight feathers would have stable isotope values intermediate between those derived from a purely terrestrial C₃ diet and one composed purely of saltmarsh plants. Feather δ¹³C and δ¹⁵N values were intermediate between those expected for feather material derived from local saltmarsh (i.e. exogenous) food items and Swedish terrestrial (endogenous muscle) sources, suggesting a mixing of endogenous and exogenous sources. These results confirm that moult migrant Anatidae exploit body stores to meet specific protein needs during the flightless period of remige regrowth and caution against the use of feather stable isotope ratios as direct indicators of the isotopic environment in which they were regrown, where endogenous contributions may occur.     

Molting and Seasonal Bill-Plate Shedding in the Whiskered Auklet (Aethia pygmaea)

Feather molting and bill-late shedding were studied because of the unique features of the whiskered auklet biology; i.e., they continue to visit the colony after departure of their young. Like other auklets, the whiskered auklets begin to molt during breeding and do not lose their capacity for flight. The molt pattern of different wing feathers is adaptive and allows new feathers to be protected (when they are soft and could be easily injured) by old or full-grown new feathers during flight or feeding (diving) due to the different timing of the molt of primary feathers and their coverts. The possibility of combining breeding with molt appears to be related to the feeding features of the species. The species that feed on abundant and highly aggregated plankton are able to molt during breeding. The pattern of bill-plate shedding in the whiskered auklet is similar to that in the crested auklet.     

Genetic and diurnal variation in the juvenile hormone titer in a wing-polymorphic cricket: implications for the evolution of life histories and dispersal

The wing-polymorphic cricket, Gryllus firmus, contains (1) a flight-capable morph (LW(f)) with long wings and functional flight muscles, (2) a flightless morph with reduced wings and underdeveloped flight muscles (SW), and (3) a flightless morph with histolyzed flight muscles but with fully developed wings (LW(h)). The LW(f) morph differed genetically from the SW morph and phenotypically from the LW(h) morph in the size of flight muscles, ovarian growth during the first week of adulthood, and the hemolymph titer of juvenile hormone (JH). This is the first study to document that phenotypes that differ genetically in morphological aspects of dispersal capability and in ovarian growth also differ genetically in the titer of a hormone that potentially regulates those traits. The JH titer rose 9-100-fold during the photophase in the flight-capable LW(f) morph but did not change significantly during this time in either flightless morph. Prolonged elevation of the in vivo JH titer in flight-capable females, by topical application of a hormone analogue, caused a substantial increase in ovarian growth and histolysis of flight muscles. The short-term, diurnal rise in the JH titer in the dispersing morph may be a mechanism that allows JH to positively regulate nocturnal flight behavior, while not causing maladaptive histolysis of flight muscles and ovarian growth. This is the first demonstration of naturally occurring, genetically based variation for diurnal change in a hormone titer in any organism.     

Feathers of birds of prey as indicators of mercury contamination in southern Finland

Mercury levels in feathers of nestling (162 broods) and fully-grown individuals (n = 48) were studied. Within feather variation was considerable in many individual flight feathers. In juv. ospreys the mercury levels in distal parts of secondary coverts were significantly higher than those in proximal parts (p<0.01). In total feathers, the mercury levels were higher and their variations were larger in ad. than in juv. accipiters. Various irregularities in the mercury levels in different parts of the plumage of a bird seem to support the hypothesis that the mercury in the food eaten during feather growth considerably affects the mercury levels of the feathers, while the regular trends mainly support the hypothesis that the amount of mercury stored in body tissues is a determining factor of plumage levels. The importance of these factors seems to vary depending particularly on the age of the birds. From the result of this study, feathers as indicators of environmental mercury pollution should preferably be from nestlings. Small feathers are preferable to larger ones, and total feathers should be analysed rather than only parts of them.     

Adjustments to nitrogen metabolism during wing moult in Greylag Geese, Anser anser

1. This study examined the nitrogen balance of free-living flightless moulting Greylag Geese, Anser anser , in relation to food quality, nitrogen absorption, food retention time and nitrogen excretion rates.
2. Food intake rates during moult were the same as those before and after the flightless period, but total daily time spent foraging fell by 58% from 9·45 h to 3·96 h. Dropping production during moult was 43%, and mean dropping mass 42% of that before and after moult, suggesting a considerable increase in food passage time through the gut during moult. Nitrogen absorption increased from 25% prior to moult to 47% during moult.
3. At the same time, excreted dry mass uric acid in faecal material fell by 68%, such that the proportion of nitrogen absorbed and retained in the body as a proportion of the nitrogen ingested in food rose from 16% prior to moult to 42% during moult.
4. Based on these significant increases in nitrogen absorption and decreases in nitrogen excretion, geese were able to compensate for reduced food intake and derive sufficient nitrogen from their diet to re-grow flight feathers.