The contour feathers of water birds exhibit adaptations to the impact forces of diving, plunging and alighting

The contour feathers of water birds are well-known to show structural details in their distal one-third that optimally confer water repellency and resistance to water penetration. In this study, these details were further examined to see if they also provide resistance to the impact forces of diving and alighting. To this end, 49 species representing 37 water bird families were grouped into nine foraging niches before measurement of length, diameter, and spacing of their barbs. Twelve land bird species grouped into two foraging niches were included in this study for comparison. These measurements allowed the calculation of the ranges and medians for barb stiffness and vane deflection for each foraging niche. A phylogenetic ANOVA approach was followed to determine if the foraging niches for water and land birds explain differences in feather microstructure while accounting for phylogenetic relationships. There were no significant group aggregations for water or land birds confirming the statistical reliability of the ANOVA approach. Differences between the deflection parameter medians of water and land bird foraging niches proved significant demonstrating an evolutionary distinction between these groups. No such difference was observed for the two land bird foraging niches indicating similarity in feather structure. For the water birds, significance was found among all aquatic niches showing that differences in feather microstructure are associated with respect to differences in aquatic feeding niches. These findings support the notion that evolutionary adaptations of feather traits are significant across bird species and their respective foraging niches. The observed mechanical and morphological variations of feathers are therefore considered adaptations to different habitats and behavioral patterns.

COMPARATIVE STUDY OF YELLOW-BILLED BABBLER (TURDOIDES AFFINIS) FEATHERS REVEALS UNIFORMITY IN THEIR MICROSTRUCTURES AMONG INDIVIDUALS

Though a few in numbers, investigations on feather microstructures from the early 20th century till date, have contributed immensely to various fields such as phylogeny, palaeontology, archaeology, wildlife forensic, biomechanics and so on. However, existing studies on feather micro-structures of birds endemic to the India/Indian sub-continent are few in number and limited in their scope. Also, no study has ever been conducted to compare feather microstructures of different individuals of a species from India. To address this issue, a comparative feather microstructure study of three individuals of Turdoides affinis, a passerine endemic to the Indian sub-continent was done. Select microstructure parameters for five different types of feathers were studied in detail. The molecular sexing method was used to elucidate the sex of T. affinis individuals for gender based differences if any. Results of the study identified that two of T. affinis individuals were female whereas one of them was male. Morphometrically, tail contour was the longest (9.63±0.76 cm) and bristle were the shortest (1.00±0.07 cm) feather. Semiplume had the longest barb length (1.73±0.04 cm) and shortest barbs (0.16±0.01 cm) were present in bristles. Subpennaceous barbs and knob-shaped villi, characteristic of members of the Passeriformes family, was also observed in all three individuals. This study records no significant difference in feather characteristics amongst the three T. affinis individuals irrespective of the differences in their sex and size. Systematically documented feather micro-characteristics of T. affinis in this study could be used as a species identification tool and would provide baseline data for the feather catalogue of Indian bird species being compiled at SACON.

Multiple components of feather microstructure contribute to structural plumage colour diversity in fairy-wrens

Closely related species often differ in coloration. Understanding the mechanistic bases of such differences can reveal whether evolutionary changes in colour are driven by single key mechanisms or changes in multiple pathways. Non-iridescent structural plumage colours in birds are a good model in which to test these questions. These colours result from light absorption by pigments, light scattering by the medullary spongy layer (a nanostructure found within barbs) and contributions from other structural elements. Fairy-wrens (Malurus spp.) are a small clade of closely related birds that display a large diversity of ornamental structural colours. Using spectrometry, electron microscopy and Fourier analysis, we show that 30 structural colours, varying from ultraviolet to blue and purple, share a similar barb morphology. Despite this similarity, we find that at the microscopic scale, variation across multiple structural elements, including the size and density of the keratin cortex, spongy layer and melanin, explains colour diversity. These independent axes of morphological variation together account for sizeable amounts of structural colour variability (R2 = 0.21–0.65). The coexistence of many independent, evolutionarily labile mechanisms that generate colour variation suggests that the diversity of structural colours in this clade could be mediated by many independent genetic and environmental factors.