The Zoologist/4th series, vol 4 (1900)/Issue 708/On Sexual Differences in the Feathering of the Wing, Degen

by Edward Degen

 

THE ZOOLOGIST

 

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No. 708.—June, 1900.

 

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ON SEXUAL DIFFERENCES IN THE
FEATHERING OF THE WING.

By Ed. Degen.

 

Under this heading Dr. A.G. Butler has published (Zool. 1898, pp. 104, 105) his observations made on the Sky-Lark (Alauda arvensis), and more recently again (ante, pp. 74, 75) the results of his investigations in regard to the House-Sparrow (Passer domesticus). Unquestionably any additional knowledge of birds whose plumage, on account of the great similarity existing between the two sexes, offer such great difficulties in distinguishing between them, as the Sky-Lark and many others, will be not only appreciated by the aviculturist, but also by the ornithologist and student, the fuller such information is afforded. That differences in the area of the bird's wings of the two sexes exist, at any rate for the species referred to above, has been shown by Dr. Butler in the respective dimensions of their wings. More important, however, are the deductions he makes therefrom, other than mere sexual differentiation; especially when he calls attention to a possible advantage in power of flight acquired by the male bird over the female, induced by a slight increase in the wing-feather area—in those cases, at least, as he points out, where a reduction in the weight of the body, as in the male Dunlin, does not take place.

As I happen to possess a series of over thirty specimens of the Black-backed Crow-Shrike, or Piping Crow (Gymnorhina tibicen), erroneously, but very persistently, called Magpie by the colonists, collected during my stay in Australia, I decided, after reading with interest Dr. Butler's remarks on the subject, to seize the opportunity for trying to ascertain from such ample material the extent to which this principle might affect this particular species.

Dr. Butler's method for measuring the dried and mounted wing consists in ascertaining its length from the upper head of the upper arm-bone to the extreme tip of the longest of the flight-feathers, which is situated on the hand portion. Since any difference in the measurements can therefore only be the result of this longest of the primaries itself, I came to the inevitable conclusion that in this way no information at all is procurable in regard to the extent to which the rest of the remiges participate on the area of wing for supposed sexual difference, an adjunct of no mean importance in this question. To supply this deficiency necessitated the measuring of all the flight-feathers separately, on one side at least, which in the present case was that of the left-hand wing. This process was rendered somewhat easier from the fact that only three of the specimens examined were dry skins, all the others being preserved in spirit. There are ten primaries and also ten secondaries in the wing of this species. By commencing to count the primaries from the tip of the wing inwards, as Dr. Butler has done, the fourth flight-feather is found to be the longest of all. This makes this remex identical with the corresponding one of the song-birds proper, or Oscines, in which group it has been considered to be the third, owing to the "assumed" absence of the first or outermost remex. For this reason I have here followed the practice now generally adopted by systematists of counting the primaries from within—that is, the carpal joint, outwards to the extreme end. By doing this it will be found that the longest primary in this species is the seventh in number, same as in the case of the song-birds, where there is supposed to be one less, namely, the tenth.

All my measurements are expressed in millimetres. The highest figures for the seventh, or the longest primary, obtained for the twenty-seven specimens examined, are the following:—

For an adult male	219 mm.
For a juvenile male	212„
For an adult female	205„
For a juvenile female	205„

The lowest figures obtained for the same quill-feather were: —

For an adult male	200 mm.
For a juvenile male	200„
For an adult female	190„
For a juvenile female	192„

It will be noticed from the foregoing figures that, as far as difference of sex is concerned, there is a marked excess of length for this longest of the flight-feathers in both the old and the young individual male birds over the females of from 7 to 14 mm. We thus find Dr. Butler's conclusions fully confirmed when confining ourselves to this feather exclusively, but if the same test be applied to a considerable number of individuals we not only get every degree of length possible for this quill amongst the two sexes, but as a matter of fact the same individual variation manifests itself in all the other flight-feathers. For this longest one, then, we get a range from a maximum of 219 mm. to a minimum of 200 mm. for adult males; the latter being smaller by 5 mm. to female individuals, both old and young, which is that of 205 mm.

If the relative lengths range for the adult males between 219 to 200, for the young males from 212 to 200, they do the same in the case of the females, which for the adults is one of from 205 to 190, and one of 205 to 192 in young females.

A reproduction of the table of measurements of all the twenty flight-feathers in the wings of these twenty-seven birds would serve no useful purpose, as may be gleaned from the following example, applying to the first specimen composing the list of birds measured: —

Primaries.
	X	IX	VIII	VII	VI	V	IV	III	II	I
mm.	105	176	209	212	210	195	169	159	150	145

Secondaries.
	I	II	III	IV	V	VI	VII	VIII	IX	X
	140	138	132	128	121	118	112	105	95	70

Sum total, 2889. Formula for index, 2889/20 (remiges) = index, 144·4.

Not only does this index afford a means for a comparison of the specimens, but it should have the advantage also of giving a more accurate estimate of the real flight-feather area.

Table I. has been compiled from the indices thus obtained by placing them side by side, according to the respective sex and approximate age of the specimens, and in serial order of diminishing figures of these indices.

On the whole, the result of this investigation for eliciting additional evidence in proof of the establishment of a clearly defined line between males and females from the point of difference in the length of the flight-feathers cannot be said to have been a very satisfactory one—at any rate, not for this particular species. The same may very reasonably be inferred from the genus as a whole, to which this species belongs.

On the other hand, and which Table I. shows also, it has been productive of supplying an approximate estimation of the rates of percentages at which the two sexes participate in the maxima and the minima of excess or reduction respectively. Thus 18·5 per cent., or five males only, absolutely exceed in greatest length of flight-feathers; the 59·3 per cent.=nineteen individuals, indiscriminately represent males and females, as well as every range of length; these are Nos. 6 to 21 of Table I., or considerably over the half of this large series of birds. Of females, six only are absolutely inferior in length, forming 22·2 per cent, of the whole. By dealing, however, separately with the sexes, the five males just referred to, out of a total of sixteen, with absolutely higher indices, form 31·2 per cent. Similarly, of the eleven individuals constituting the female portion, six of them show indices absolutely inferior to the remaining seven, or 54·5 per cent., a little over the half for females.^[1]

The measurements obtained for the seventh or longest primary, when compared with the indices derived from the aggregates of all the flight-feathers in the wing, do not coincide always exactly with the rank which has been allotted to the specimen on the list, the latter being the outcome of the index received. As an instance, the first specimen on the list (Table I.), whose index is 144·4, has its longest remex shorter by 2 mm. than the corresponding one of No. 4, whose index of 141·4 is so much lower. The adult male No. 2, with its inferior index of 143·4, exceeds the young male No. 1 by 7 mm. in the longest primary, which measures 219 mm., and is the longest of all. These examples should suffice for showing the great individual variations obtaining throughout alike. Not more than five males exceeding the others by highest indices, also exceed them by having the longest measures for the seventh primary.

It is obvious, then, that as a means of diagnosis for the determination of sexes, a really characteristic difference in the lengths of the wing, or wing-feathers, found to exist in the species dealt with by Dr. Butler, has almost broken down in this particular case.

In addition to the indices on Table I., I have appended a further column, in which the length and the girth of the beaks are given. A comparison of these dimensions in their present order of indices—with which they in no way agree, any more than amongst themselves—does not exactly afford a ready means for discriminating between them. To achieve this latter purpose better, I have dealt with this character separately on Table II. by numbering the specimens according to the diminishing lengths of their beaks, but otherwise adhering to the plan adopted in Table I.

The arrangement of Table II. therefore shows that, apart from the differences in the colour of the plumage on the backs between adult males and females of this species, which does not enter into the scope of this work, a very good character for sexual difference, and of greater reliability than we have seen to exist for dimensions of wings, lies in the length of the beak.

With one exception only, namely, that of an adult male (No. 25 of Table II.), which is conspicuous by having an abnormally short beak (like the two young females at the bottom of the list), all females have their beaks inferior in length to the males; but nevertheless there is, as might be expected, great individual variation perceptible also in both sexes.

Very little positive value should be attached to the girth measurements taken at the base of the beaks, as may be inferred by a consultation of the columns devoted to these on Table II., which may be noticed particularly in the case of the three dry skin specimens distinguished from the others by an asterisk, and mentioned before. They prove in a striking manner that considerable shrinkage takes place on drying out, which, if ignored when admitting this measurement for a comparison of fresh with cabinet specimens, may lead to erroneous deductions.

 

TABLE I.

Indices of Flight-feathers.

 

 

 

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1. These birds are a non-migratory species, and the greater proportion of males to females in a collection made in the same locality as this one was, may be taken as a fair estimate for the proportion of the sexes in a free state. This probably accounts also for the great number of them existing still, although they were considerably disturbed by the influx of the white population settling everywhere in the country.