Wings of barn owls (Tyto alba pratincola Linnaeus) and pigeons (Columba livia Linnaeus) were prepared to carry out the morphological investigations. Barn owl wings were obtained from specimens of the institute's own colony that had been used in other experiments and were killed by perfusion under a permit of the local authorities (Landespräsidium für Natur, Umwelt und Verbraucherschutz Nordrhein Westfalen, Recklinghausen, Germany (LANUV)). Five wings of three different animals were prepared. Four Pigeons were received from a breeder and killed specifically for this study under a permit from LANUV. Thus, eight pigeon wings were obtained.
The morphological investigations included six remiges and six coverts for each species from different positions. For the barn owl, five feathers from position p10, and two, each from p9, p5, p1, s4, s8, gpc10, gpc9, gpc5, gpc1, gsc5 and gsc9 were acquired. Eight feathers from position p10 and two each from p9, p5, p1, s4, s8, gpc10, gpc9, gpc5, gpc1, gsc5 and gsc10 for the pigeon (Fig. 1A).
All feathers were removed from the wings and photographed with an 8-megapixel digital camera (Canon EOS 350D, Canon Inc., Tokio, Japan) with a 50 mm macro lens. Additionally, they were scanned by an Epson flat-bed scanner (Epson Perfection 3490 Photo, Seiko Epson Corporation, Tokio, Japan) with a resolution of 800 dpi from dorsal and ventral side.
Measurements took place at every 10% of the vane length. Parameters such as the depth of vane or the length of barbs were then normalised by the whole length of the vane for each single feather. The following parameters were extracted from the photos (Fig. 1B):
- Length of the rachis (whole length of the shaft including calamus).
- Depth d of the outer (ov) and inner vane (iv). From measurements between 10% and 90% of vane length (in remiges, resp. 20–80% in the coverts) an asymmetry index AI was derived, defined as:
AI may vary between -1 and 1 and is 0, if the inner and outer vanes have the same depth. It is positive (negative), if the outer vane is narrower (wider) than the inner vane.
- Length of barbs of the outer and inner vane. In this measurements the existence or lack of serrations and fringes on the outer as well as on the inner vane were investigated. If such structures were found their typical mean size and spacing was measured and indicated by a dotted line in the Figures 1B and 4. To reduce the influence of the plumulaceous barbs and abrasions at the feather tips, the mean size and spacing was calculated between 20–90% of the vane length for the remiges and between 20–80% for the coverts.
- Size of the outer and inner vane (the area was measured by counting the pixels and multiplying them by the size of one pixel). A similar asymmetry index as for the depth was calculated, defined as:
- Angle between barbs and rachis on the inner and outer vane (the angle at the base of each barb was measured; the arms of the angle were placed in the middle of the rachis, resp. of the barb).
- Number of barbs (all barbs were counted and the density [nb/cm] was calculated).
The barbs of the feathers p10, s8 and gpc1 of both species were examined by using scanning electron microscopy in order to measure the fine structures. Barbs from the inner and outer vane were taken at four different positions of the vane (at 20%, 40%, 60% and 80%) (Fig. 1B). This results in eight barbs per feather that were analysed. Every barb was cut off at the rachis and placed with a glue pad on an aluminium specimen stub. Afterwards, the specimens were gold coated with a sputter coater (model: Hummer, Technics Inc., Alexandria, Virginia, USA, 10 mA, 1000 V, 7–9 min). Pictures were taken with a Cambridge Stereoscan 604 scanning electron microscope (Cambridge Instruments, Cambridge, UK) at 20%, 40%, 60% and 80% of the barb length. With respect to the much shorter barbs of the outer vane of feather p10, the positions of the pictures were changed to 25%, 50% and 75% of the barb length.
The following parameters were extracted (Fig. 1C):
- Number of hook and bow radiates per mm (nhr; nbr).
- Angle (α) under which the barbules are attached to the barb (αbr; αhr).
- Number of hooklets at the hook radiates (nh).
- Length of the barbules (base and pennulum).
The data was obtained from photographs using Adobe Photoshop CS (Adobe Systems, San Jose, California, USA) and ImageJ (National Institutes of Health, USA).