The Zoologist/4th series, vol 2 (1898)/Issue 681/On Sexual Differences in the Feathering of the Wing of the Sky Lark (Alauda arvensis)

by Arthur Gardiner Butler

 

ON SEXUAL DIFFERENCES IN THE FEATHERING
OF THE WING OF THE SKY LARK (ALAUDA
ARVENSIS).

By Arthur G. Butler, Ph.D., &c.

 

In Volume II. of 'British Birds with their Nests and Eggs' (p. 174), I have noted the fact that bird-dealers recognise cock Sky Larks, by the greater length of the second primary of the wing in that sex. I had intended to illustrate the well-defined sexual differences in this species by a process-block (l.c.), as I did in the case of the Linnet, but unhappily when the wings were needed Mr. Frohawk was utterly unable to obtain examples for illustration, whilst at that time I possessed the male wing only.

I now have before me eight wings, four of which (two right-hand wings of each sex) were secured, mounted, and kindly given to me by Mr. C.H.B. Grant, who shot these and other birds last December in Hampshire.

The wing of the male Sky Lark, as I have already stated elsewhere, is especially adapted for powerful and sustained flight, whereas that of the female is altogether weaker in construction; indeed, so greatly do the wings differ in old birds that a glance would enable the dullest observer to decide their sex; even in young birds the distinctions are well marked.

As is well known, the first primary in the Sky Lark (as in many Passerine birds) is very small; so that by a superficial observer it might easily be confounded with the coverts. The second, third and fourth primaries are, however, the longest in the wing, and in the male Sky Lark these three feathers terminate almost at the same level; thus when superposed there is hardly any noticeable difference, though the third primary is very slightly the longest. In the female the second primary is decidedly the shortest of the three and either the third or the fourth the longest, these three feathers in the female thus either forming an angle of which the extremity of the third primary is the apex, or a regularly graded oblique line. It is therefore not strictly correct to say that the second to fourth primaries of the Sky Lark are almost of equal length, inasmuch as there is frequently (probably always) a considerable discrepancy in the length of those of the female.

The secondaries of the male are distinctly longer than in the female and slightly less contracted towards the tips, which are bilobed (or, more strictly, bispatulate) in both sexes, the lobe or spatula terminating the outer web being longer in proportion to that of the inner web than in the female: this is especially noticeable in the larger and presumably older birds.

There can be little doubt that the slightly greater width at the extremity of these feathers and their more even termination offer better resistance to the air, in flight, than those of the female, and, combined with the increase of length in these feathers, help materially in supporting the bird when soaring.

A comparison of a series of Sky Larks in the flesh shows that the males are distinctly larger than the females, and, comparing the general outline of the expanded wings, it will be at once observed that those of the males are decidedly longer in proportion to their width than those of the females.

When one critically examines the feathering, to see where the principal difference in measurement exists, it at once appears that the uncovered portion of the primaries in the male is disproportionately greater than in the female, and the emargination of the outer web in the third and fourth primaries commences considerably farther from their extremities, about half the edge of the outer web in the exposed portion being emarginate in the males, and about two-fifths in the females.

Later on I hope to publish additional notes on sexual wingstructure in other birds. As a rule, the male wing is specially modified to enable the bird to overtake its female; but sometimes the development seems to serve the purpose of sustained rather than rapid flight; and it must be remembered that in certain birds (such as the Dunlin) in which the wings of both sexes agree in expanse and hardly differ in the structure of the feathering, the inferior size and weight of the body in the male give him a considerable advantage in flight.