▁and ▁dry ▁seasons ▁can ▁vary ▁consider ably ▁( by ▁months ) ▁from ▁year ▁to ▁year . ▁At ▁the ▁end ▁of ▁the ▁wet ▁season ▁( May ▁or ▁June ▁in ▁East ▁Africa ), ▁w ilde be est ▁migr ate ▁to ▁dry - season ▁areas ▁in ▁response ▁to ▁a ▁lack ▁of ▁surface ▁( dr inking ) ▁water . ▁When ▁the ▁ra iny ▁season ▁begins ▁again ▁( month s ▁later ), ▁animals ▁quickly ▁move ▁back ▁to ▁their ▁wet - season ▁ranges . ▁Fact ors ▁sus pected ▁to ▁affect ▁migration ▁include ▁food ▁abund ance , ▁surface ▁water ▁avail ability , ▁pred ators , ▁and ▁ph osph orus ▁content ▁in ▁grass es . ▁Ph osph orus ▁is ▁a ▁cru cial ▁element ▁for ▁all ▁life ▁forms , ▁particularly ▁for ▁la ct ating ▁female ▁b ov ids . ▁As ▁a ▁result , ▁during ▁the ▁ra iny ▁season , ▁w ilde be est ▁select ▁gra zing ▁areas ▁that ▁contain ▁particularly ▁high ▁ph osph orus ▁levels . ▁One ▁study ▁found , ▁in ▁addition ▁to ▁ph osph orus , ▁w ilde be est ▁select ▁ranges ▁containing ▁grass ▁with ▁relatively ▁high ▁nit ro gen ▁content . ▁ ▁Aer ial ▁phot ography ▁has ▁revealed ▁that ▁a ▁level ▁of ▁organisation ▁occurs ▁in ▁the ▁movement ▁of ▁the ▁her d ▁that ▁cannot ▁be ▁apparent ▁to ▁each ▁individual ▁animal ; ▁for ▁example , ▁the ▁mig rat ory ▁her d ▁exhib its ▁a ▁w avy ▁front , ▁and ▁this ▁suggests ▁that ▁some ▁degree ▁of ▁local ▁decision - making ▁is ▁taking ▁place . ▁N umer ous ▁document aries ▁feature ▁w ilde be