▁Tar ang ire ▁e cos ystem s , ▁populations ▁are ▁mostly ▁mig rat ory , ▁with ▁her ds ▁consisting ▁of ▁both ▁sex es ▁frequently ▁moving , ▁but ▁resident ▁sub pop ulations ▁also ▁exist . ▁During ▁the ▁r ut ting ▁season , ▁the ▁males ▁may ▁form ▁temporary ▁territ ories ▁for ▁a ▁few ▁hours ▁or ▁a ▁day ▁or ▁so , ▁and ▁attempt ▁to ▁gather ▁together ▁a ▁few ▁females ▁with ▁which ▁to ▁mate , ▁but ▁soon ▁they ▁have ▁to ▁move ▁on , ▁often ▁moving ▁ahead ▁to ▁set ▁up ▁another ▁temporary ▁territory . ▁ ▁In ▁the ▁Ma as ai ▁Mar a ▁game ▁reserve , ▁a ▁non - m ig rat ory ▁population ▁of ▁blue ▁w ilde be est ▁had ▁dw ind led ▁from ▁about ▁ 1 1 9 , 0 0 0 ▁animals ▁in ▁ 1 9 7 7 ▁to ▁about ▁ 2 2 , 0 0 0 ▁in ▁ 1 9 9 7 . ▁The ▁reason ▁for ▁the ▁decl ine ▁is ▁thought ▁to ▁be ▁the ▁increasing ▁competition ▁between ▁cattle ▁and ▁w ilde be est ▁for ▁a ▁dimin ishing ▁area ▁of ▁gra zing ▁land ▁as ▁a ▁result ▁of ▁changes ▁in ▁agricult ural ▁practices , ▁and ▁possibly ▁fl uct u ations ▁in ▁ra inf all . ▁ ▁Each ▁year , ▁some ▁East ▁African ▁populations ▁of ▁blue ▁w ilde be est ▁have ▁a ▁long - distance ▁migration , ▁seem ingly ▁tim ed ▁to ▁co inci de ▁with ▁the ▁annual ▁pattern ▁of ▁ra inf all ▁and ▁grass ▁growth . ▁The ▁timing ▁of ▁their ▁migr ations ▁in ▁both ▁the ▁ra iny