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Birdstrike and Flight Safety


A Common Swift is a small bird of 16 cm length and 40 gr weight. One may think it is inconspicuous, but in the air it is a flying object competitive with airplanes. There are rarely any accidents in bird flights, because size, manoeuvrability and speed are perfectly in tune with each other. Each bird knows how to evaluate the speed of other birds precisely and behaves accordingly. The feathers act as an all-round flexible and adjustable surface, allowing it optimal manoeuvrability within the smallest confines. This is the reason why the air traffic of birds is nearly free of accidents even when they gather in large numbers. Their safeguarding mechanics are not yet completely understood. However, Common Swifts reduce flight speed in situations of higher accident risk (Tigges & Mendelssohn 2005, APUSlist-No. 2854).


Airplanes lag far behind the techniques and morphology of birds that were optimised during evolution. This is the cause of accidents in the air, the bird strikes.


The force of impact with birds jeopardises airplanes in flight. The power of the force is determined by the mass and the speed of the birds and the airplanes. The masses of the flying objects impact with identical power (Newtons’ rule of power and counter power). Additionally the degree of speed multiplies the power (e= ½ mV²). This is why a Common Swift meets an airplane, flying at 450 kph, with the power of 550 kg. If the birds are bigger and heavier, such as a Buzzard the force is 6000 kg. If either bird meets a jet flying at 900 kph, this power multiplies to 2200 kg (2.2 tons) for the Common swift and 25000 (2.5 tons) kg for the Buzzard.


The birds die from the accidents and the airplanes may be damaged to varying degrees. Larger birds, like buzzards or geese can break through the canopies, damage or destroy engines and in extreme cases may kill the pilots.


Birds pose a higher risk in countries with a high density of air traffic. This is why there are efforts worldwide to reduce the danger. In Germany between 1997 and 2001, 47 bird strikes were recorded involving the Common Swift. The actual count will be higher, because bird species are identified in only in 30-50 % of all accidents.


The danger rises in countries crossed by migratory species. In Israel for example, 500 million birds cross the country twice a year. All these birds fly over a small area,  75 km wide on their passage from Africa to Europe and Asia in spring, returning in autumn. Between 1970 and 1998 8 planes were destroyed due to bird strikes and 3 pilots were killed.


This is why the Society for the Protection of Nature in Israel (SPNI) and the Israel Air force (IAF) developed a systematic study of migration across the country. The flight routes of the different species were recorded for several years and the daily time tables and seasonal schedules were collected and noted. The dates resulted in a map called ”Bird Plagued Zone“ (BPZ) which contains all details of passage over Israel. This material became a major part of pilot drills and plane schedules were adapted to the habits of the birds. The motto is: The birds were here first! Since this knowledge was implemented into air traffic in 1984, major accidents have been reduced by 76 % and minor accidents by 81 %.

Fig. 1: Map of Israel with bird plagued zones


To further reduce bird strikes and improve the quality of bird strike statistics, the Air Force and the University of Tel Aviv developed a project of expert bird remains identification. The microscopic structure of feathers holds the key for identification of each species.


Feather characteristics

Fig. 2: Topography of a contour feather. Source: Laybourne & Dove 1994



The major part of the feather is made up of a rachis, with vanes on each side,

and in most species an afterfeather on the underside of the feather. At the basal end of the rachis is the calamus, the part of the feather attached to the bird's body. The vane on each side of the rachis is divided into primary branches off the shaft called barbs. In turn, along each side of the barb are secondary branches called barbules. The distal portion of the vane is pennaceous. Pennaceous barbs and barbules create the flat intricately interlocked surface of the vanes. The proximal portion of vane, closest to the calamus, is plumulaceous or downy. The downy portions of contour feathers are concealed beneath the pennaceous area of overlapping feathers. The microstructure of basal barbules of plumulaceous, or downy, barbs was studied for the purpose of feather identification.


Each downy barbule is a row of single cells that are differentiated to some degree. Downy barbules are divisible into a base of fused cells and a slender pennulum of joint cells. Each segment of the pennulum is a single long cell. Cells are either uniformly thick or swollen at their distal end. The distal portion of each cell, or junction of cells, is the node. The main portion of a cell is the internode. One end of the node holds the proximal end of the next internode. Internodes are generally straight, however, in some cases, internodes may occasionally be kinked (fig. 3a).


The downy barbule microstructures described are most developed on the basal barbules of the distal vanule on the basal barbs. The farther distal on either the feather or the barb, as well as on the proximal vanules, the structures are often less specialized, and occasionally lack some of the characteristic features of the group.


Nodal structures and distribution

Nodes may be enlarged to various degrees. In addition to being enlarged, some groups have characteristic features at the nodes. These include: heart-shaped nodes, ring-like nodes (fig. 3b), vase-shaped nodes (fig. 3c), triangular nodes (fig. 3d), quadrilobed nodes (fig. 3e), lobed nodes and bell-shaped nodes (fig. 3f). Prongs of various length may also be found at the nodes. Prongs may be short and cilia-like (fig. 3g), long, asymmetrical (one prong longer then the other, or only one prong at a node), or a combination of the above (fig. 3h). Nodes may also be described as flared when there are multiple very short prongs surrounding the node (fig. 3i).


The distribution of the different nodal structures along the barbule is also of

importance. Nodal structures may be distributed evenly along the length of the barbules, or even gradually decrease in size, in both these cases, nodal distribution is labeled as uniform (fig. 3j). However, certain structures may only be found at the basal nodes (fig. 2d), at the distal nodes (fig. 3a), or occasionally at the terminal nodes (fig. 3j). Hence, one group of birds may have several characteristics simultaneously.


Additional characteristics that may be mentioned include villi at the bases of barbules. Sickle-shaped villi are found at the bases of barbules in the order Piciformes (fig. 3k), knob-like villi are found at the bases of basal barbules of all Passeriformes (fig. 3f).


Node density

The nodes are counted along 1 mm of barbule, giving the node density. Since this number may vary slightly between barbules, barbs, and feather tracts a range is given for each species.



Pigmentation along the downy barbules generally appears black or brown under the microscope. Pigmentation can be concentrated in the nodes (fig. 3f), or stippled. When pigmentation is stippled, it may be stippled evenly throughout the whole barbule (fig. 3h), in the nodes and/or in the internodes (fig. 3g). The degree of pigmentation may vary from none (fig. 3e) to heavy. Often the degree of pigmentation decreases gradually towards the distal end of the barbules.



Figure 3: A key to the different nodal structures, nodal distribution, and pigmentation


a. Distal nodes are heart-shaped and internodes are kinked (Anas angustirostris)



b. Ring-like muliple nodes (Alectoris chukar)



c. Vase-shaped nodes (Calidris minuta)



d. Basal nodes are triangular (Larus ridibundus)



e. Quadrilobed nodes, no pigmentation in barbules (Columbia livia)



f. Bell-shaped and lobed nodes, pigment concentrated in nodes, knob-like villi at bases (Oenanthe oenanthe)



g. Short prongs at slightly enlarged nodes. Pigment heavily stippled in nodes and internodes (Apus affinis)



h. Long, asymmetric prongs and nodes. Pigment evenly stippled in barbules (Pelecanus onocrotalus)



i. Terminal node is flared (Hirundo rustica)



j. Nodes uniformly distributed. Sickle-shaped villi at bases (Jynx torquilla)




x50 (5 mm)                          x200 (1 mm)                         x50 (factor of x1.33) (5 mm)

k. Calibration for LM measurements




The Swift family

The Swift family causes 10 % of all Israeli bird strikes. In Israel there are 4 species of Swifts resident and breeding in the country. These are the Common Swift, which is the largest group, the Alpine Swift, the Pallid Swift and the Little Swift. They also migrate over Israeli territory.


To reduce bird strikes the species that cause such accidents must be identified so that knowledge of their behaviour may help the air force to minimize accidents. Bird and feather remains found on and along runways or on aircraft are collected and sent to the Laboratory for Feather Remains Identification at the University of Tel Aviv. Here the feathers were cleaned so that they gain their natural shape and colour back. Sometimes one can identify the bird species, when large or very characteristic bird remains are collected just by the naked eye, but in general macroscopic comparison is not sufficient. Most feather remains, particularly small remains require microscopic examination of downy barbule structure. The microstructure of feathers from 175 species was examined, described, photographed and compilet into an atlas, which now makes the whole process of identification easier. Further developments in the field of feather identification include the development of a multimedia expert system on CD-ROM (The Bird Remains Identification System), developed in cooperation between Tel Aviv University, University of Amsterdam, The Israel Air Force, The Royal Netherlands Air Force and the Expert Center for Taxonomic Identification (for further information see:



The feather characteristics of the Swift family


מין: סיס חומות              Apus apus                

משפחה: סיסיים            Apodidae

סדרה: סיסאים              Apodiformes

נוכחות בארץ: מאמצע פברואר עד ספטמבר

תפוצה גיאוגרפית בארץ: כל הארץ

תפוצה גיאוגראפית יחסית לבסיסי ח"א: כל הארץ

נפיצות: מקיץ וחולף מצוי

משקל גוף: עד 40 גר'


Length of barbules: Short/medium

Nodal structures and distribution: Uniformly barely enlarged. Occasionally, minute prongs at nodes

Node density: 20-30 nodes per mm

Pigmentation: Uniformly heavily concentrated in nodes and heavily stippled in internodes




SEM x 1000: Basal nodes, basal barbules





Light microscope x50: Basal barbules, distal vanule




Light microscope x200: Basal nodes, basal barbules, distal vanule




מין: סיס חומות              Apus melba              

משפחה: סיסיים            Apodidae

סדרה: סיסאים              Apodiformes

נוכחות בארץ: כל השנה

תפוצה גיאוגרפית בארץ: כל הארץ

תפוצה גיאוגראפית יחסית לבסיסי ח"א: כל הארץ

נפיצות: מקיץ וחולף מצוי, חורף נדיר

משקל גוף: עד 92 גר'


Length of barbules: Medium

Nodal structures and distribution: Uniformly barely enlarged with minute prongs at basal nodes

Node density: 20-30 nodes per mm

Pigmentation: Uniformly heavily stippled in nodes and internodes, decreasing distally




SEM x500: Basal barbules, proximal vanule






SEM x1000: Basal nodes, basal barbules, proximal vanule






Light microscope x50: Basal barbules, distal vanule





Light microscope x200: Basal nodes, basal barbules, distal vanule




Light microscope x200: Terminal nodes





מין: סיס הגליל               Apus affinis              

משפחה: סיסיים            Apodidae

סדרה: סיסאים              Apodiformes

נוכחות בארץ: כל השנה

תפוצה גיאוגרפית בארץ: כל הארץ

תפוצה גיאוגראפית יחסית לבסיסי ח"א: כל הארץ

נפיצות: יציב נדיר, מקיץ מצוי

משקל גוף: עד 28 גר'



Length of barbules: Short

Nodal structures and distribution: Uniformly barely enlarged, with several minute prongs at the nodes

Node density: 20-30 nodes per mm

Pigmentation: Heavily stippled in nodes and internodes



SEM x500: Basal barbules, proximal vanule




SEM x1000: Basal nodes, basal barbules, proximal vanule




Light microscope x50: Basal barbules, distal vanule




Light microscope x200: Basal nodes, basal barbules, distal vanule



Judy Shamoun-Baranes and Ulrich Tigges



All photographs by Judy Shamoun-Baranes; all color paintings by Gerald Driessens, published with kind permission of the artist.



APUSlist Nos. 0179, 1410, 1915, 2624,  2767, 2780, 2799, 2800, 2853, 2854, 2864, 2865

For further information view also [with movies]


© APUSlife No. 2863

ISSN 1438-2261


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