Quail Eggs and Clover

photograph of California Quail by Rohan Kamath
The rate of quail reproduction in the more arid portions of their ranges is closely related to the amount of rainfall. For example, in very dry years on the King Ranch of Texas, most Northern Bobwhites do not breed and populations drop to low levels. When rains are ample, however, the birds reproduce normally. In semiarid parts of California, those years with enough rain to produce spectacular displays of wild flowers also tend to be good years for the reproduction of California Quail.

But how do the rains regulate the breeding of the quail? They seem to do so, in part, by influencing the chemistry of the plants, especially legumes (members of the pea family) that the quail eat. A diet containing an extract from subterranean clover ("subclover" a low-growing clover that buries its seed heads in the soil) has been
shown to greatly reduce egg productionby quail. That plant, among others, produces "phytoestrogens," chemical compounds that are similar to the hormones involved in regulating reproduction in birds and mammals. The phytoestrogens may play a role in protecting the plants against predation by herbivores. Their importance in regulating the reproduction of animals that eat them was first noted when subclover was found to inhibit the breeding of Australian sheep.

Stunted plants of drought years have higher phytoestrogen content than do plants with the luxuriant growth of high-rainfall years. Much higher levels of those compounds were found in samples of food removed from the crops of California Quail in 1972; a low-rain, low-reproduction year, than in samples from 1973, a high-rain, high-reproduction year.

Other factors may also help to regulate quail breeding. One is the quantity of nutritious legume seeds available (they were almost three times as abundant in the diet in 1973 as in 1972). Another is rainfall itself, which could contribute some direct stimulus for breeding. But diet appears to play the major role, and that long-evolved role may, ironically, end up depressing the size of some quail populations. Subclover, an exotic species probably originating in the Mediterranean region, is being used increasingly as a forage crop in California. It is possible that this human-induced change will reduce breeding in California Quail even in places and years with more than adequate rainfall.
SEE: Coevolution.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.