Selective Feeding by Black-Tailed Deer: Forage Quality or Abundance?

Abstract

Selectivity of forages by ungulates may be in response to the abundance of forages (forage-abundance hypothesis) or the nutrient quality of palatable forages (selective-quality hypothesis). I examined predictions of both hypotheses by measuring feeding behaviors of free-ranging black-tailed deer ( Odocoileus hemionus columbianus ) at Hopland Field Station, Mendocino Co., California. I observed 98 foraging sessions (foraging bout of 7–10 min) of adult males and females in all seasons and measured the number of bites, time the head was in the feeding position, type of forage, and biomass of plants along foraging paths. Selection (seconds per bite) was strongly correlated with biomass of forages only when deer foraged on dried grass or forbs. Seconds per bite varied considerably among types of forages: green grass-forbs ( X = 1.3 s), leafy browse (3.8 s), dry grasses and forbs (6.1 s), acorns (19.6 s). The head was in the feeding position significantly less when foraging on dry grasses and forbs, and acorns than when deer foraged on green grass-forbs and leafy browse. Selectivity (seconds per bite) of leafy browse and acorns by deer was correlated with Julian date, an index to nutrient content. Number of bites was positively related to time the head was in the feeding position only when deer foraged on green grass-forbs. Selectivity of deer ostensibly varied in response to nutrient content, and searching was probably the dominant process influencing selectivity on most types of forages.