ON THE SPURS ON BIRDS’ WINGSBY A. L. RAND

I

N handlingspecimens of birds bearing spurs on their wings, two points

emerged which seem to be little known or new: (a) the horny covering of

the wing spur, in some species, undergoes molt, and (b)the spur in somebirds is a modification of a process already in existence for another purpose.
I n presenting these points, a summary of our knowledge regarding wing
spurs is presented.
In early writings

spurs and claws were confused until Jefferies

pointed out that they are quite different:(1881)

claws are horny sheaths on the tipsof terminal phalanges (for a review of their occurrence see Fisher, 1940) ;spurs are projecting

bony cores with an outer layer of horn, similar to thehorns of cattle. Between the bone core and horn covering is a layer of tissue,
the outer part of which produces new horn material.

The horn must obviously

increase in length from the base, the tip being the oldest (Gadow, 1891).
Well developed spurs occur on the tarsi of many gallinaceous birds, butwing spurs, all borne on the forward edge of the wing in the neighborhood of
the carpal joint, are found only as follows.
ANSERIFORMES
Anhimidae (Screamers).-All
three species of this family have two conspicuous, stout,
smoothly tapering, sharp spurs with a slight radial curve on each wing; the proximal
spur is much the larger. They are both on the fused metacarpals; the proximal spur on
the process of metacarpal I which is for the attachment of the extensor muscles; the distal
spur on the distal end of metacarpal II (as figured by Sclater, 1886:150).
Specimens
examined show fine lines about the base of the spurs indicating growth in layers, and one
specimen had three separate bands of these lines suggesting annual growth. The moltthat occurs complicates this idea, however. Following are descriptions of the spurs in thethree species, as seen in specimens in the Chicago Natural History Museum:
Anhima cornuta: the spur is triangular in cross section, with the side of the spur facing
proximally, somewhat concave, and all the corners as well as the tip sharp; length ofproximal spur, males, 58-61; females, 5&55; distal spur, males, 15, 16; females, 11-17mm.
Chauna torquata: spur nearly oval in cross section but with a sharp-edged flange near
the proximal edge, recalling the triangular, sharp-edged spur of the previous species;
length of proximal spur, males, 3047;
females, 3545;
distal spur, males, 13-20; females, 15-17 mm.
Chauna chavaria (Fig. 1C) : spur smoothly oval in section, sharp only at tip; length ofproximal spur, male, 28; female, 30; distal spur, male, 18; female, 18 mm.
Anatidae (Ducks, Geese, etc.) .-The ducks and their relatives number some 144 species.
A number of genera have a projection near the carpal joint. This projection is conspicuous as a knob especially in Sarkidiornis and Chloephaga in which it projects beyond
the feathering and hears a horny covering. This seems to be the process of metacarpal I.
However, only two genera of the Anatidae, both monotypic, have a single well-developed
spur on each wing. In the two species the spurs differ in position and some details; descriptions follow.

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Plectropterus
gambensis (Fig. 1D) : the spur is borne on the radial carpal bone (skeleton, C.N.H.M.) as shown by &later (1886, p. 300). The spur is stout, strong, with a gradual taper, nearly oval in cross section, but with a tendency toward flanges giving small
sharp edges. There is an area about 5 mm. from the base that suggests a growth ring inall specimens. Length of the spur in males is 20-25 mm., in females, 18-22 mm. In one
specimen there is a small pad of horn on an auxiliary spur that appears to be the tip of
the process of metacarpal I.
Merganetta
armatu (Fig. 1E) : the spur is borne on the basal anterior edge of themetacarpal on the process of metacarpal I (specimen C.N.H.M.). The spurs are stout atthe base, oval in section, and taper abruptly, but with an attenuated, very sharp tip.
They differ from the spurs of Plectropterus
also in that the horny sheath ends abruptly
basally with an abruptly rounded edge indicating thickness of the horny covering to thebase. No suggestion of growth rings is evident. Length of the spur in males is 9-17 mm.;in females, 6-13 mm. The spur of the female usually has a less attenuated and lesssharp point.
CHARADRIIFORMES

Jacanidae (Jacanas).-Of
the seven species in six genera in this family, only two
species have well developed spurs:
Jacana spinosa (Fig. 1G) has a long conspicuous spur, borne on the process ofmetacarpal I (skeleton, C.N.H.M.),
as figured by Sclater (1886:301).
The spur is almost conical, with a slightly attenuated and very sharp tip. Faint lines suggesting growthrings are somewhat evident. The spurs of males measure 7-10; of females, 8-10 mm.
In Hydrophasianus
chirurgus the spur is apparently similarly located and is short andvery sharp; in males it measures 3-5; in females, 4-7 mm.
Another aspect of wing armature in this group is noteworthy in this connection. In
Actophilornis
africana(Fig. lF), A. albinucha, Metopidius indicus, and Irediparra
gallinacea, the radius is flattened and heavy, much heavier than the ulna (Forbes, 1881:646).
In these species, spurs are absent, being represented only by the knob of the process ofmetacarpal I. Jacana spinosa and H. chirurgus, both with sharp spurs, have “normal”
radii (Forbes, 1881:646-7).
Charadriidae, subfamily Vanellinae (Wattled Plovers and Lapwings) .-Of
the 25
species in 19 genera belonging to this subfamily I have examined 241 species in 18
genera and find a conspicuous spur in 10 species in 7 genera, a very small but distinct
sharp spur in 4 species in 4 genera, and a condition in which a spur is represented onlyby the knob formed by the process of metacarpal I in 7 genera (see list below).
The following species have conspicuous, well developed spurs:
LengthSpecies
Belonopterus

chilensis

Xiphidiopterus(Fig. 1H)

albiceps

Female8-14 mm.

8-12
16-22
no specimen
13-14
no specimen
2-5
4-7
7-12
6-15
4-5

18-23

Rogibyxtricolor

15

Lobibyx

novae-hollandiae

16, 17

Lobibyxmiles

15

Afribyx

senegallus

3-11

Hoplopterus

spinosus

5-11

Hoplopterus

armatus9-12

Hoplopterus

duvaucelii

11-13

Hoploxypterus
1

4-9

cayanus

I have not seen Tylibyz

melanocephalusof SpurMalewhich is said to have no spur.

A. L.
Rand

WING SPURS

FIG. 1.

Bony structures in the region of the wrist of birds. Species figured are: A,
B, Haematopus ostralegus; C, Ch,auna chavaria; D, Plectropterus gambensis; E, Merganetta
armata;F, Actophilornis
africana;
G 1, Jacana spinosa;
G 2,chilensis;
I,
Jacana spinosa, another view, showing curve of spur; H, Belonopterus
Pezophaps solitaria(from Newton and Newton, 1868). A and B show “normal” process
of metacarpal I; in C, E, G, and H, this process is elongated into a spur (drawn withhorny sheath in E and G) ; C has an extra spur; D has the spur on a carpal; F has athickened radius; I has a swollen knob on metacarpal and on radius. Abbreviations:
r=radius;
rr=ulna; mc-metacarpal.
Alectoris

rufa;

The following species, all in different genera from those listed above, have short spurs
only a few millimeters long at most, hut usually pointed and apparently horn covered:Hemiparra

crassirostris,

Microsarcops

cinereus,

Lobivanellus

indicus,Ptiloscelys

resplen-dens.

In the three species of Stephanibyx;
the single species of Zonifer, Lobipluvia,
Sarcioand Vanellus; and two species of Chettusia, the knob formed byphorus, Anomalophrys,

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the process of metacarpal I can scarcely be called a spur. Tylibyx is said also to lack a
spur.
I have examined skeletons of but two genera of the plovers with conspicuous spurs,
Hoplopterw
and Belonopterus, and in these the spur is on the process of metacarpal I(Fig. 1H) as in jacanas and screamers (main spur). In the other genera the spur appears to be in a similar location and presumably is also on the process of metacarpal I.
In these plovers with well developed spur, the spurs are usually more or less oval incross section, somewhat flattened in Lobibyx, rather slender, sharply pointed, and more orless distinctly curved radially. Frequently there are faint, wavy lines running around thespurs which are suggestive of growth rings (see under molt), but otherwise the spurs aresmooth at the base. Usually the spurs of the female are slightly smaller than those of
the male.
COLUMBIFORMES
Raphidae (Dodos, Solitaires.-No
pigeons have a wing spur as such, but the wing armature of the extinct solitaire, Pezophaps solitaria (Fig. 1 I) of Rodriguez (Newton and
Newton, 1868)) should be mentioned. In this species large examples, presumably males,
have a considerable enlargement of the distal end of the radius, especially at the base of
the fused carpometacarpus. While apparently not sharp pointed, it may well have beenhorn covered and certainly could have increased the wing’s effectiveness as a weapon.
Other Groups.-The
older literature sometimes mentions spurs on such birds as athrush (Turdus), the knob-winged pigeon (Diduncelus)of Samoa, and the moundbuilders (Megapodius) (Jefferies, 1881). Examination of specimens in the Chicago Natural History Museum showed no wing spurs on any of our specimens of these groups.
However, the rather pronounced projection of the process of metacarpal I might be considered a rudimentary spur, as it could in most flying birds (see Fig. 1 A and B).
Also, as Gadow (1891:501, 502) points out, cornification can ocmr sporadically on theskin of various parts of the bird’s body, producing horny spurs, and to have this happen
occasionally on the wings of birds which normally lack spurs is probable.
In summary, well developed wing spurs occur in: Anhimidae-2
spurs, in all 3 species:
Anatidae-1
spur, in 2 of the 144 species; Jacanidae-1 spur, in 2 of the 7 species (rudimentary in all others) ; Charadriidae, subfamily Vanellinae-1
spur in 10 of the 25 species(present, small and sharp in a number of others).
The location of the spurs is as follows (Thomson, 1923:219), lists spurs as occurringon digits but none of his examples show this condition) :A. On radial carpal: Plectropterus.
B. On carpometacarpus: all othersa. on process of metacarpal I: all except the distal spur of screamers.
b. on distal end of metacarpal II near articulation of digit II: distal spur of
screamers.
Functionally related structures are the enlarged radii of jacanas of the genera Actophilornis,
Metopidius
and Irediparra
and the enlargement of the distal end of theradius and also of the proximal end of the carpometacarpus of a solitaire, Pezophaps.

USESOF WING ARMATURE
Spurs probably have their use in fighting.

In many gallinaceous birds thetarsal spurs, worn only by the male, are definitely secondary sexual charactersused in intraspecific fighting at mating time. Wing-spurs are well developedin both sexes of species in which they occur but those of the female are usual-

A.

L.

WING

ly slightly smaller.
ship and mating.

SPURS

This might imply they were not used primarily

in court-

However, the jacana (J. spinosa) is said to have displays atmating time when with spread wings the birds act as though they were attempting to strike each other with their sharp spurs.
Some birds use their wings in fighting
species. The swans (Cygnus),

off enemies or intruders of otherwithout spurs, make such effective use of theirwings as weapons that they can be dangerous to children.
goose (Plectropterm),with its formidable

The spur-winged

spurs, is said to be extremely ag-

gressive and sometimes to injure other waterfowl with its spurs (Delacour andMayr,torputa)de-fend their nest with strong wing blows in which the spur is brought

1945:28).

Both male and female screamer(Chauna

intoplay (Stoner, 1939:48).
One can assume that the bony enlargement of the carpal area of the solitaire and the thickened radius of certain jacanas serve to render blows more
effective.

ORIGINOF WING ARMATURE
Well developed wing spurs occur in only two families
orders, Anseriformes and Charadriiformes.

of each of two

In one small family (Anhimidae)

all the species have wing spurs, while in the related large family(Anatidae)

about two per cent have spurs.
The diversity of the armature of birds’ wings is apparent from the preceding summary.

Two main effects are achieved: a club effect in two ways; aknife effect in one main and two less frequent ways.
It is interesting to note here that both club and knife or spear motive arepresent in one group, the jacanas, but no species has both.

Those species

with spurs do not have thickened radii.
Though wing armature is varied, the spur is the most common and in all but
one species the spur (only the proximal spur in screamers) appears to be a continuation and modification of the process of metacarpal I, a process that in birds
serves for the attachment of the extensor muscles. This process is easily felt
through the skin of specimens of many, perhaps most, flying birds as a distinct
point or knob. It gives the impression of a rudimentary

spur. The size, shape,and position of this process apparently varies with the type of flight of the bird,
as Fisher (1946:559)

has demonstrated for some hawks and their relatives. Al-

most surely this process arose in connection with the attachment of extensor
muscles. But once present, it made the wing more effective as a weapon when
dealing a blow, as do some birds without spurs. Already useful as a weapon,
a new set of selection factors, connected with fighting ability, could haveoperated to elongate the process and give it a horny coating to add to its efPresumably the process can maintain its first func-

fectiveness as a weapon.

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66. No. 2tion as a point of muscle attachment and take over its new function as a weapon. This seems to be a good example of a structure arising for one purpose
through the action of one set of selective factors becoming useful in quite another way when a certain point of development was reached.

At this point

the structure comes under the influence of another set of selective factors, andthe direction of its evolution is changed. The present-day structure is the result of two sets of selective processes acting at different times.
Assuming that the process of metacarpal I is a ready made knob capable ofbeing turned into a spur, it seems strange that this knob has been ignored, so
to speak, in some species in which wing armature was achieved in other ways,
as, for example, the spur on the radial(Plectropterus).

carpal of the spur-winged

goose

It seems just as strange that in the jacanas certain species

developed spurs while in certain other species a heavy radius was developed
suitable for use as a club. No species has both.
It seems that wing armature originated separately a number of times; inthe majorityof cases a bony process already in existence was modified into aspur, but in some cases this process was not used and a weapon evolved alongother lines.

SPURMOLT
Molt of feathers usually occurs at least once a year. Molt of other epidermal
structures is well known, but its occurrence is less general.

Molt has beenrecorded for such structures as: the nails of the red grouse, Lugopus scoticus(Witherby, et al, 1941:227) ; the pectinations on each side of the toes thatserve for “snowshoes” for the ruffed grouse, Bonasa umbellus (figured, Forbush, 1927:27) ; the outer sheath of the puffin’sserves as a nuptial adornment (Witherby,(Fratercula arctica)

et al, 1941:172)

bill that; and the “knob” on

the bill of the white pelican, Pelecanus erythrorhynchos, that is worn during
the breeding season (Baird, 1869). App arently the spurs of domestic fowl
at least do not molt, and I find only one mention of wing spur molt. Chapin(1939:86),

writing of the plover Xiphidiopterus

ulbiceps, says of a pair thatwere molting their remiges and rectrices, “Theirwing-spurs were likewise

about to shed the outer sheath, which could be lifted off, leaving a perfect
new horny point beneath.”
I have found further evidence of molt of wing spurs in three species:(a) Screamer. Chauna chavariu.--ln
screamers there is characteristicallya series of fine lines near the base of the spur which I assume are growthlines due to horn being laid down in layers.

In one specimen there were

three such series of lines separated by intervals. These lines I assumed to beannual growth lines. Hence it came as a surprise in handling a specimen of
C. chavariu that an outer layer of one spur separated at the “growthring”

and slipped off as a cap, leaving the spur about 3 mm. shorter but in ap-

A. L.
Rand

WING

SPURS

pearance much as it was before. This bird showed molt of flight feathers.
(b)

Jacana.

Jacana spinosa.-A

specimen had long, pointed spurs (13mm.) that showed irregular growth lines and a scaly appearance about thebase. A gentle pull on each spur caused an outer layer of the horn of thespurs to slip off like a cap, leaving a pair of clean, pointed, shorter spurs(9 mm.)

with less attenuated points. This specimen was also molting its re-miges.
(c)

Spur-winged

Plover.

Hoplopterm

armatus.-A

specimen had clean,fresh looking spurs 12 mm. long. A gentle tug on each spur caused the outerlayer to slip off like a cap, leaving a pair of spurs similar to the old ones but
about two mm. shorter. This specimen also showed molt of its remiges.
The data are scanty for generalization. The similarity of the spur before
and after shedding makes it necessary to see the shed cap to realize what has
happened. But f rom the three examples described above it appears that moltof the outer layer of the horn covering of spurs takes place in at least one
species of screamer, one jacana, and two species of plovers. The correlationof this spur cover molt with wing molt indicates the former may be a regularpart of the annual molt.
The structure of the spurs of birds has been compared to that of horns of
cattle by Gadow. It is interesting to compare the molt of the covering of thespurs on wings of birds with the annual molt of the covering of the bony coreof the horns of the Pronghorn, Antibcapra,

which is similar.

SUMMARY
The occurrence of wing spurs is noted for all species of screamers, someplovers, two jacanas, and two ducks. Additionally, knob or club-like wingarmature is listed for several jacanas and a pigeon. These specialized structures occur on different parts of the wing and involve the radius, the radial
carpal, or the fused metacarpals depending on the species. The structures areapparently used in fighting.
The process of metacarpal I (for the attachment of the extensor muscles)
has been modified into the spur in a great majority of cases. This process
presumably arose in connection with the insertion of muscles, and its size andlocation are influenced by the type of flight.

But another result was that thisknob made the wing more effective as a striking organ.

This knob was thenacted on by a new set of selective factors, those involved in providing thebird with better weapons, and a spur was produced. However, in a few cases
wing armature developed independently from other parts of the wing.
Molt of an outer cap-like layer of the horny covering of the spur in a single
piece is recorded in four species. In each case this molt was correlated withwing molt. It is possible that the annual molt may regularly include the outer
covering of the spur.

THE WILSONLITERATURE
BAIRD,

BULLETIN

Vol.

June 1954
66, No. 2

CITED

S. F.

1869

[Letter re white pelicans1 Ibis, 1869, p. 350.

CHAPIN,J. P.
1939 The birds of the Belgian Congo. Bull. Amer. Mus. Nat. Hist., 75:1-632.
DELACOUR,
J., AND E. MAYR
1945 The family Anatidae. Wilson Bull., 57:1-55.
FISHER, H. I.
1940 The occurrence of vestigial claws on the wings of birds. Amer. Midl. Nat.,

23 :234-243.
1946 Adaptations and comparative anatomy of the locomotor apparatus of New
World vultures. Aner. Midl. Nat., 35~545-727.
FORBES,W. A.
1881 Notes on the anatomy and systematic position of the jacanas (Parridae).
Proc. 2001. Sot. London, 1881, pp. 639647.
FORBUSH, E. H.
1927 Birds of Massachusetts, etc. Vol. 2. Mass. Dept of Agriculture.
GADOW, H.
1891 Vogel: Aves, [in] Bronn’s Klassen und Ordnungen des Thier-Reichs. Bd. 6.
C. F. Winter’sche Verlagshandlung, Leipzig.
JEFFERIES,J. A.
1881 On the claws and spurs on birds’ wings.

Proc. Boston Sot. Nat. Hist.,

21:301-306.
NEWTON, A., AND E. NEWTON
1868 On the osteolo,gy of the solitaire or didine bird of the Island of Rodriguez,
Pezophapssolitaria (Gmel.) . Philos. Trans. Roy. Sot. London, 159:327-362.
&LATER, P. L.
1886 On the claws and spurs of birds’ wings. Ibis, 1886, pp. 147-151; 300-301.
STONER, C. R.
1939 Notes on the breeding habits of the Common Screamer (Chauna torquata).
Ibis, 1939, pp. 45-49.
THOMSON, J. A.
1923 The biology of birds. The Macmillan Co., New York.
WITHERBY, H. F., et al.
1941 The handbook of British birds. Vol. 5. Witherby, London.

CHICAGO NATURAL HISTORY MUSEUM, ROOSEVELT ROAD AND LAKE SHORE
DRIVE, CHICAGO 5, ILLINOIS, MARCH 26, 1953