Feather structure reflects flight behaviour and habitat


Our manuscript – mentioned here – about how feather structure adapts to meet flight- and habitat-related requirements in European birds was accepted for publication in Functional Ecology. This is the first time we make our way into this great journal. Below, EvolEcol members are typed with boldface. Note that SK, BL and AM were undergrad students during the time of the study and manuscript writing.

Pap PL, Osváth G, Sándor K, Vincze O, Bărbos L, Marton A, Nudds RL and Vágási CI 2015. Interspecific variation in the structural properties of flight feathers in birds indicates adaptation to flight requirements and habitat. Functional Ecology (in press).


Summary

1. The functional significance of intra- and inter-specific structural variations in the flight feathers of birds is poorly understood. Here, a phylogenetic comparative analysis of four structural features (rachis width, barb and barbule density, and porosity) of proximal and distal primary feathers of 137 European bird species was conducted.

2. Flight type (flapping and soaring, flapping and gliding, continuous flapping or passerine type), habitat (terrestrial, riparian or aquatic), wing characteristics (wing area, S and aspect ratio, AR), and moult strategy were all found to affect feather structure to some extent. Species characterized by low wing-beat frequency flight (soaring and gliding) have broader feather rachises (shafts) and feather vanes with lower barb density than birds associated with more active flapping modes of flight. However, the effect of flying mode on rachis width disappeared after controlling for S and AR, suggesting that rachis width is primarily determined by wing morphology.

3. Rachis width and feather vane density are likely related to differences in force distribution across the wingspan during different flight modes. An increase in shaft diameter, barb density and porosity from the proximal to distal wing feathers was found, and was highest in species with flapping flight indicating that aerodynamic forces are more biased toward the distal feathers in flapping flyers than soarers, and gliders.

4. Habitat affected barb and barbule density, which was greatest in aquatic species, and within this group, barb density was greater in divers than non-divers, suggesting that the need for water repellency and resistance to water penetration may influence feather structure. However, we found little support for the importance of porosity in water repellency and water penetration, because porosity was similar in aquatic, riparian and terrestrial species, and among the aquatic birds (divers and non-divers). We also found that barb density was affected by moult pattern.

5. Our results have broad implications for the understanding of the selection pressures driving flight feather functional morphology. Specifically, the large sample size relative to any previous studies has emphasised that the morphology of flight feathers is the result of a suite of selection pressures. As well as routine flight needs, nutrition, habitat (particularly aquatic) and migratory requirements also affect flight feather morphology. Identifying the exact nature of these trade-offs will perhaps inform the reconstruction of the flying modes of extinct birds.

Key-words: barb density, barbule density, flight feathers, flight, functional morphology, moult, vane porosity, rachis width, water repellence, wing morphology


Graphical abstract

graphical abstract

Four representatives of the entire species pool: common rosefinch Carpodacus erythrinus (a), red-footed falcon Falco vespertinus (b), Eurasian wryneck Jynx torquilla (c), and common kingfisher Alcedo atthis (d). Photographs by Csongor I. Vágási.


Lay summary

The size and structure of primary feathers appears to vary greatly between species. The primary feathers form the outer wing and are used to propel birds through the air. Feathers consist of a central shaft called the rachis. Attached to and aligned perpendicular to the rachis are the barbs and attached to, and, again perpendicular to the barbs, are the barbules. Barbs and barbules together make the feather vane, which gives the feather its shape and surface area. Feathers become damaged over time so birds replace them through a process called moult, replacing old feathers with new ones.

Birds differ in the way they fly (different flight types), particularly in how much and how fast they beat their wings. Birds also have differing life history traits, i.e. living in different habitats, moulting feathers at different times of year and some migrate over long distances. Also, wing shape differs between species (e.g. long and narrow versus short, and broad wings). In this study, we examined the primary feathers of 137 European species to determine whether feather structure was related to life history traits and wing shape.

Flight type, habitat, wing morphology, and moult strategy all affected feather structure. Species characterized by low wing-beat frequency flight (soaring and gliding flight) had broader rachises and feathers with a lower density of barbs than birds associated with more active flapping flight types (high wing-beat frequency). Rachis width was primarily determined by wing shape. Our results suggest that species that flap their wings most vigorously during flight require denser feather vanes. The forces created by the air increase with flapping frequency and more dense feathers are likely to reduce the chance of air being forced through the feathers.

Barb and barbule density was highest in aquatic species, peaking within diving birds. Hence, the need for water repellency and resistance to water penetration may also influence feather structure.

In conclusion, the optimum feather morphology for flight and habitat for some species may conflict resulting in a compromise structure, for example, gliding and soaring flight selects for low barb density, while aquatic habitats select for high density.

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