ON THE PHYLOGENY OF THE TIBETAN SANDGROUSE
By E. V. KOZLOVA
(Communicated by E. N. Pavlovsky, Member of the Academy, 11.X.1945.)
The study of certain anatomical characters of
Syrrhaptes tibetanus Gould made me reconsider the degree of relationship between this species and
S. paradoxus (Pall.).
The following external features are common to both species: 1) absence of hind toe; 2) shape and size of bill; 3) toes feathered to the claws; 4) elongated and pin-shaped middle rectrices.
The most important distinctions are based upon: 1) external structure of foot; 2) structure and shape of primaries; 3) shape of wings; 4) length of proximal (inner) secondaries; 5) some details in the structure of tail; 6) total size of the birds.
Three toes of the foot of
S. paradoxus are externally fused together (in adults as well as in downy young), forming on the inferior side a sort of broad horny sole. Most authors suggest that the foot of
Syrrhaptes paradoxus is in this way perfectly adopted for walking on soft drifting sand. According to my experience, however,
S. paradoxus does never frequent sandy deserts, as a rule, mostly inhabiting dry xerophitic steppes and semi-deserts with firm, solid ground. The structure of the foot in
S. paradoxus is thus of the greatest value to the bird, protecting it from the burning heath of the soil in summer and the severe frost in winter. In any case, the foot of
S. paradoxus shows doubtless a high grade of specialization.
The foot of
S. tibetanus is decidedly more primitive. Its toes are free, joined only at the base by a small web, as in the representatives of the genus
Pterocles.
The first (outer) primary in the male of
S. paradoxus is very much elongated and pointed, whereas in
S. tibetanus it is but slightly longer than the second primary and does not differ from the latter in shape. All the primaries in
S. tibetanus are much softer, as it were looser, and rounder in shape than in
S. paradoxus.
The whole wing of
S. tibetanus exhibits a striking dissimilarity in shape, not only with the exceedingly narrow and sharp-pointed wing of
S. paradoxus, but also with the more rounded wings of
Pterocles orientalis and
P. alchata.
The average ratios of the top of the wing to the total length of the wing in the different species of
Syrrhaptes and in two species of
Pterocles can be expressed by the following indices:
Syrrhaptes tibetanus, index 0.53;
Syrrhaptes paradoxus, index 0.68;
Pterocles orientalis and
P. alchata, index 0.61 (the largest figure denotes the sharpest wing).
In
S. tibetanus the proximal (inner) secondaries are unusually long (as in most Limicolae), extending in a closed wing as far as the tip of the 4th primary; in
S. paradoxus the secondaries are short (as in
Pterocles), not extending beyond the tip of the 7th primary.
The tail of
S. tibetanus is relatively long (if measured from the base to the tip of the 7th pair of rectrices) and not so perfectly wedge-shaped as are the tails of
S. paradoxus,
Pterocles orientalis and
P. alchata. The largest upper and under tail coverts in the three latter species are exceedingly long, the longest extending to the tip of the 5th (upper coverts) and 6th (under coverts) pairs of rectrices, only the tips of one middle pair remaining exposed. The longer tail coverts are also covered by the overlying shorter ones, so that only the tips of the former remain uncovered. The tail coverts form a kind of a series of sheets overlying each other. Numerous at the base of the tail, they gradually decrease in number distally, and are ultimately reduced to one or two, near the tip of the tail. The tail, thick at the base, becomes gradually thinner towards the tip. In the Tibetan sandgrouse the tail is not so perfectly wedge-shaped because of a more abrupt decrease of the number of tail coverts, the longest of which extend scarcely to the tips of the 3rd (under coverts) and 4th (upper coverts) pairs of rectrices. The distal portions of the four middle pairs of rectrices remain thus exposed. The elongated tips of the central rectrices in
S. tibetanus do not end in a filament as in
S. paradoxus. Finally, the total size of
S. tibetanus is much larger than that of the ordinary sandgrouse.
All the features above described characterizing the structure of wing and tail of
S. tibetanus point to its adaptation to a less powerful and rapid flight than that in
S. paradoxus and in the two species of
Pterocles. The wing of the tibetan sandgrouse, differing conspicuously from those of
S. paradoxus,
Pterocles orientalis and
P. alchata, is much similar in shape to the wing of the dove (
Columba livia).
The few observations available on the mode of life of
S. tibetanus fully confirm the above-mentioned statements, based upon the theoretical study of the structure of the flying organs of that bird.
Przewalsky who saw much of both
Syrrhaptes, gives in his ornithological diary (MS) the following description of the flight of
S. tibetanus: “The flight of the Tibetan sandgrouse differs conspicuously from that of
S. paradoxus; it is relatively slow, much similar to the flight of ordinary dove”. Shäfer (¹) does not compare the flight of the two species, noting only that the flight of
Syrrhaptes tibetanus is dove-like (“taubenartig”).
Being distributed over different areas,
S. paradoxus and
S. tibetanus are confined to different haunts; their habits and behaviour also differ conspicuously.
S. paradoxus inhabits mostly inhospitable countries: wormwood steppes and semi-deserts. Feeding on dry seeds of desert plants, those birds, like all Pteroclidae, are in need of drinking water, and travel twice a day immense distances to quench their thirst. Hence, the adaptation of their flying organs to most powerful, rapid flight must have been indispensable for the survival of the species in its natural surroundings. In the struggle for existence, selection (survival of the fittest) must have been rather severe.
The following details of ontogenic development in
S. paradoxus is worth mentioning. The outer primaries in young birds (in juvenile plumage) are softer and more rounded at the tips than those in adults. Young birds are therefore unable to develop high speed. The necessity rapidly to acquire a perfect adult wing doubtless accounts for the fact that the moult of the juvenile primaries takes place during the first autumn, at a moment when the wings of the young bird are not yet fully grown.
The life and evolution of the Tibetan sandgrouse, as well as the development of its flight organs, have evidently followed a different course.
The Tibetan sandgrouse inhabits stony slopes of river valleys and of lake depressions, occurring mostly in the neighbourhood of water basins, and avoiding desert plateaus.
Syrrhaptes tibetanus needs drinking water no less than its relatives, but its environment seems to have never stimulated distant travels and the development of an extra-rapid flight. The dove-like shape of the wing acquired by this sandgrouse in the process of its evolution was evidently adequate to its requirements. Neither were the young
S. tibetanus ever in need to develop an early and rapid moult of juvenile wing feathers. The young proved able to survive, although they had to use their primitive primaries throughout their first autumn and winter, their moult taking place only in spring. Worth mentioning is the fact that the juvenile primaries in S. tibetanus do not differ much in shape and structure from those of adult birds, as in the case of
S. paradoxus. This points to a more primitive structure of adult primaries in
S. tibetanus, as compared to those of the ordinary sandgrouse.
The above-described differences in the external characters of
S. paradoxus and
S. tibetanus make a direct and intimate relationship between the two species rather doubtful.
Among the numerous African and Indian Pteroclidae we can easily find species which in the structure of their flight have more features in common with
S. tibetanus than has the latter with
S. paradoxus. The species of sandgrouse which, according to some authors, form a separate group constituting the genus
Nyctiperdix, are all adapted to a less strong and speedy flight than the remaining Pteroclidae. The wing in these species is somewhat rounded (not so pointed), the first, second and in some species even the third primaries exhibiting nearly similar dimensions. The tail is also rounded–not wedge-shaped, the tips of several pairs of middle rectrices but slightly extending beyond the neighbouring tail feathers.
It must also be noted that these species, with less specialized flight organs, are active only after sunset and even during the night.
Syrrhaptes tibetanus in northern Tibet, according to Przewalsky's observations (²), was recorded many times flying in parties over his camp cat the dead of night. It seems therefore more probable that the ancestral forms of
S. tibetanus and
S. paradoxus have diverged at a time when among the members of the family Pteroclidae there were no birds with so highly specialized flight organs, as we now see in
P. orientalis,
P. alchata and
S. paradoxus.
The interspecific differences within the genus
Syrrhaptes, along with the mentioned phylogenetic suggestions, clearly indicate that a separation of the genus
Syrrhaptes into two different genera would give a more correct idea of the phylogeny of these birds. Hence I propose to give the name
Przewalskia g. n. to the genus, encluding the Tibetan sandgrouse. I hope that further anatomical investigations, which I am presently unable to perform for lack of material, will confirm the correctness of my view.
Zoological Institute.
Academy of Sciences of the USSR.
Received 11. X. 1945.
REFERENCES
¹ E. Schäfer, J. f. Ornith., Sonderheft (1938)
² Н. М. Пржевальский, Монголия и страна тангутов, II, СПБ, 1876.
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