I ndian Peafowl Pavo cristatus have been exhibited at Brookfield Zoo since it opened in 1934. As is common in many zoological parks, the birds have been given free range of the zoo grounds during the summer months. However, during the colder months the birds are confined to heated quarters, as winters in the Chicago region are too harsh for them to be at liberty. On nice days in winter the birds have access to a large covered coop from which they can be herded inside for the night.
The fact that the birds must be confined for the winter limits the size of the park's population, as they cannot be too crowded without causing battles among the males. In the past we
have limited the population to the number that could comfortably be housed in the winter quarters by not allowing the peahens to nest. This required searching out each nest, a time-consuming and not always successful strategy.
In 1984 two of our peahens managed to hide their nests, which presented us with five more birds to add to the wintering area. We did not want to stop exhibiting peahens, as we felt that showing the differences between the male and female birds
teaches an important lesson about sexual dimorphism in birds. Having a few peahens also helps to stimulate the males to display. What we needed was a permanent solution to the problem of unwanted reproduction.
After some discussion with the zoos's veterinarians, we decided to perform an ovariectomy (spaying) on each hen. In early 1985 five peahens underwent this surgery, which involved the removal of their ovaries and oviducts. Ovaries were removed along with the oviducts to prevent any future problems with egg peritonitis. All recovered without incident and were released into the park with the males later in the spring. Their behavior was unchanged and they reacted to
the males as they always had - they just did not nest.
The story could end here as an interesting technique to control reproduction in a long-lived and popular zoo animal. However, in 1994 we noticed something unusual in our peahen "Duck" (so named because she tended to stay at our Formal Pool with the waterfowl). Duck molted her plumage as usual in late summer, but when the new feathers on her neck started to break from the sheaths they were blue instead of green. Duck was growing male plumage.
As her molt continued Duck developed a small train of eye feathers instead of the usual short brown tail coverts found on females. Her primaries came in the chestnut color of the male, her white lower abdomen turned blue, and she even exchanged her brown crest feathers for blue ones. When she had completed her molt she looked exactly like a young male, with full adult body plumage and a short train of eye feathers. In successive
years her train has continued to lengthen with each molt, similar to the growth seen with increasing age in males Qohnsgard 1986), and she now resembles a full adult male.
Of the remaining females that underwent the surgery in 1985, three died in accidents unrelated to the surgery while still in female plumage. "Curly Toes" (named for an obvious physical characteristic) did not begin to display any male plumage until early in 1997. We photographed her in the midst of her fall molt when she was showing more male feathers. "Gypsy," a female donated to the zoo in 1989 and spayed in 1990, began to molt into male plumage in 1995.
What causes this phenomenon? A brief refresher in the anatomy of female birds will help. Unlike female mammals, female birds only have one ovary and oviduct, that on the left side (the right fails to develop in the embryo, being present only as a vestige). This reduction is thought to reduce weight for flight, and to protect the developing egg. If there were two mature eggs present in parallel oviducts, a sudden jolt of the bird's body might crack them (Welty I 982).
In many sexually dimorphic birds, such as peafowl, the development of the plumage is estrogen-dependent. That is, the dull plumage found in females is dependent on the presence of estrogen, and the more ornamental male plumage develops in the absence of estrogen (Hegelin & Kimball 1997). This has been demonstrated in a number of species in the Galliformes (the family of chicken-like birds to which the peafowl belong). Ovariectomized females, older females, and females with diseased ovaries have all been found to molt into partial or full male plumage because they do not produce enough estogen (see citations in Hegelin & Kimball 1997 for more information).
Hagel in, Julie C. and Rebecca T. Kimball. 1997.
"A female Gambel's Quail with partial male plumage." The WilsonBulletin 109 544-546.
johnsgard, Paul A. 1986. The pheasants of the world. Oxford: Oxford University Press.
Welty, Joel Carl. 1982. The life of birds, third edition. Philadelphia: Saunders College Publishing. ~