Heat loads on reproducing bobwhites in the semiarid subtropics

The cause-effect process leading to boom-bust population behavior in quails of the semiarid subtropics remains obscure. We challenged the heat hypothesis, which states dial heat loads explain variation in quail production among years. Comparison of the 1997 and 1998 breeding seasons in southern Texas revealed the quantity of thermally tolerable space in time (operative temperature less than or equal to 39 degreesC) declined and the intensity of thermal conditions increased. This change was associated with an 86.4 +/- 5.43% decrease in the calling activity of northern bobwhite (Colinus virginianus) males, We observed heat loads for extended periods within days and breeding seasons and over extensive areas that were sufficient to kill embryos in eggs, chicks, and adults; cause premature incubation anti staggered hatching; reduce the length of the laying season and thus inhibit renesting and multiple-brooding: and cause males and females to go out of reproductive condition. Our results supported a necessary condition for the heat hypothesis, namely that thermal conditions severe enough to inhibit quail reproduction occur over extensive areas and times in the semiarid subtropics.