summer popiiLitioii was sufficiently dense, breed- 

 in": niigiit cease as early as August. The evidence 

 at hand suggests that males determine the breed- 

 ing season since they lose capacity to reju-oduce 

 earlier in the season than the females. 



There were measurable differences in the endo- 

 ci-ine anatomy and physiology of adult males from 

 the two populations of diti'erent densities. As a 

 result of these differences, animals in the denser 

 population indicated a shorter breeding season, 

 thus limiting the growth rate of the population, 

 a demonstrated instance of biological control of 

 a rodent popidation. 



Ecology of prairie graiine on sandhill refuges. — 

 A study of sharp-tailed grouse and prairie chick- 

 ens conducted jointly by the Missouri Unit and 

 the Division of Wildlife Refuges was carried out 

 on the Valentine and Fort Niobrara National 

 Wildlife Refuges in the Nebraska sandhills. The 

 ol)jective was to provide better background infor- 

 mation for management of prairie grouse in the 

 sandhill region. 



Mowed wetlands were important habitat for 

 booming and dancing grounds. Seventy-six per- 

 cent of 51 active breeding grounds on the Valen- 

 tine were on wetland sites in 1903, yet this land 



In re<-ent years the sage ^oiLse has regained status as an important game species in several Western States. Like 

 all plains and prairie grouse, the si>e(ies is polygamous, making it possible to obtain g(K)d estimates of local 

 populations on "strutting" grounds. Two displaying males (left) on a Colorado strutting groinid, and male and 

 female (right) on an Idaho strutting ground. (Photos by Colorado Game. Fish, ;ind I'.irks Department, left: 

 Paul D. Dalke, right) 



Two species ol pniiric grouse were involved in .studies comideted in 11(64 by the .Missouri I'nit. working with the 

 Bureau's Divisiim of Wildlife Kefuges. The proj<H( dealt with the basic ecology of the sharp-tailed grouse (left) 

 and the prairie chicken (right) on the Saiidliills .\;Uional Wildlife Refuge in Nebraska. (Photo.s by Denver 

 Wildlife Research Center, left; Colorado Game. Fish, and Parks Department, right) 



37 



