- A population of Nuttall's cottontails (Sylvilagus nuttallii) in Deschutes County, Oregon, decreased fromm 166 individuals in August 1978 to 76 individuals in August 1979. Survival rates of four annual litter groups in 1978 were 0.74, 0.35, 0.72, and 0.93; survival rates for the same litter groups, respectively, in 1979 were 0.12, 0.28, 0.07, and 0.18. Population density and juvenile survival decreased between the 2 years possibly because of alterations in forage quality and quantity caused by precipitation. Numbers of cottontails on the study area on 30 August were related significantly (P<0.O1) to initial breeding density and to precipitation falling during the breeding season (February - July). Survival in juvenile cottontails from birth
to 30 August was related significantly to precipitation in the first (P<O.05), third (P<O.01), and fourth (p<0.O1) annual litter groups. Vegetative abundance at the end of the growing season (September) decreased by approximately 30% between the 2 years; the decline corresponded with a 45% decrease in crop-year (September - June) precipitation. Succulence of grasses, forbs, and shrubs was greatest in spring, declined in summer, and increased slightly in late summer. Forbs consistently were the most succulent group, except in September when shrubs were most succulent. Grasses exhibited the greatest
variation in succulence in response to precipitation; shrubs exhibited the least. A moisture availability index (MAI) indicated that, despite approximately equal estimates of succulence between the 2 years, moisture was more available to cottontails in 1978 because of greater abundance of vegetation. Cottontails seemingly selected forage groups
in relation to their relative succulence, within limits. Shrubs were avoided, and succulent juniper (Juniperus occidentalis) foliage was eaten only in dry periods, possibly because of low tolerance in cottontails to plant secondary compounds, particularly terpenoids. Juveniles selected forbs in significantly (P<0.05) greater proportions than adults, possibly to aquire additional moisture for growth; diets of younger (<600 g) and older (>600 g) juveniles did not differ. Breeding females selected forbs in significantly (P<0.05) greater proportions than males, possibly because of moisture demands associated with reproduction and lactation. Other nutrients, especially protein, that are associated with moisture content of plants, possibly
were as important as moisture in determining food habits and population fluctuations in Nuttall's cottontails in central Oregon. The
relationship between juvenile survival and precipitation possibly was caused by genotypic variation in efficiency in cottontails to utilize moisture and nutrients.