Gender Dimorphism in Birds

Home to discover more interesting bird facts and information at The Wonder of Birds

Dimorphism is a term originally used by botanists to express the fact that certain plants a difference, whether in form or colour, more or less considerable exists between individuals belonging to the same species, this difference not being attributed to local influences or of the kind called accidental, but yet one that is constantly exhibited.  As analogous cases are observable in animals, the term has been adopted by zoologists, and, disregarding other classes, it will at once be perceived that among Birds there are two kinds of dimorphism - one depending upon gender, in which the characters of the male and female may differ in very many ways, and the other which is apparently quite independent of gender distinction. Of this last kind, which seems to approach most closely to the Dimorphism of botanists, there are not many undisputed instances.

Colour Dimorphism

The best known is that of some species of SKUA, in which a parti-coloured bird may be frequently found mated with one that is (so to speak) whole-coloured-in some cases the former being the male, the latter the female, and in others just the contrary, it rarely happening that both partners are alike in plumage.  Another case is that afforded by the GUILLEMOT, for at nearly everyone of its breeding - resorts a portion of the tenants (perhaps one in a score) will be found to have a white circle round the eye and a white line stretching backward from it - these Ringed or Bridled Guillemots being of either gender and apparently paired with birds of normal plumage (At one time these Ringed or Bridled Guillemots were looked upon as a distinct species, called Uria Lacrymans, but that view has been abandoned.  Similarly the dark, whole-colored examples of the common species of SKUA were originally described as forming a separate species, Lestris richardsoni, but though the name was retained by many writers for some while, by the end of the 19th Century it appears to have been generally understood that this bird was not a distinct species).

Among birds examples of gender Dimorphism are so numerous as to make it almost the rule. Yet, as already stated and as is widely known, this kind of Dimorphism manifests itself in very many ways-the commonest being that of general coloration, instances of which will occur to everyone; but apart from that the coloration of particular parts is scarcely less often divergent in the two genders, while differences of the form or development of certain portions of the plumage are also very abundant, as witness the occipital plumes in the male of many birds, while the extraordinary elongation of the feathers of the lower back in the PEACOCK, of those on the side of the breast in the BIRD OF PARADISE, or of the tail in the BLACKCOCK are notorious.

Passing to characteristics which may be of greater significance, we have spurs on the metatarsus or near the wrist, the former only among the Gallinae, but the latter found in birds of several groups that are not closely allied. These are generally and justly admitted to be weapons, and hardly less effective are the knobs which occupy the like position in other forms, those of the male n the now extinct Rodriguez solitaire (Pezophaps solitaria) being perhaps the most remarkable. Gender Dimorphism of the bill has been already noticed, and it extends in various ways to the head, wattles, frontal plates, protuberances that are permanent or only temporarily erectile, which are far too numerous to mention; but other much more special peculiarities are the sublingual (i.e. under the tongue) bag of the Musk Duck (Biziura lobata), the seasonal pouch of the Bustard, and the inflatable sacs of the Prairie-fowls (GROUSE), while the convolutions and enlargements of the trachea in many birds (e.g. MANUCODE) though not externally visible produce an audible gender Dimorphism.

Size Dimorphism

Gender Dimorphism in size is also manifested among birds-and this in both directions. To humans, it may seem natural that the male would be the stronger and therefore the bigger gender, and among Mammals he generally is.  But in Birds this is by no means so much the rule, the male being very considerably larger than the hen only in certain Gallinaceous and Ratite groups, most of which are polygamous, and hence a possible explanation may be afforded.

On the other hand, although a case in which the female is larger than the male is rarely found among Mammals, instances occur among Reptiles (notably in Tortoises and Snakes) and very frequently among Amphibians and Fishes.  Among Birds it is almost universal with the Accipitres. It also occurs in the Dotterel and several of the Limicolae, such as the GODWIT, PHALAROPE, and Rhynchaea or painted SNIPE, as well as in some of the Turnicidae (HEMIPODE).  No single explanation that will fit all these cases seems possible; but in those of the Limicolae just mentioned, it is to be remarked that the females are not only larger but are more conspicuously coloured than the males, which latter are believed to perform exclusively the duty of incubation. In the lower classes of Vertebrates the production of the often numerous eggs may be the original cause of the greater size of the females.

Home to the Wonder of Birds

This page ©