, ▁fish , ▁re pt iles , ▁am ph ib ians , ▁insect s , ▁and ▁cr ust ace ans . ▁The ▁trigger ▁for ▁the ▁migration ▁may ▁be ▁local ▁climate , ▁local ▁avail ability ▁of ▁food , ▁the ▁season ▁of ▁the ▁year ▁or ▁for ▁m ating ▁reasons . ▁To ▁be ▁counted ▁as ▁a ▁true ▁migration , ▁and ▁not ▁just ▁a ▁local ▁dispers al ▁or ▁ir ruption , ▁the ▁movement ▁of ▁the ▁animals ▁should ▁be ▁an ▁annual ▁or ▁season al ▁occurrence , ▁such ▁as ▁Northern ▁Hem is phere ▁birds ▁migr ating ▁south ▁for ▁the ▁winter ; ▁w ilde be est ▁migr ating ▁ann ually ▁for ▁season al ▁gra zing ; ▁or ▁a ▁major ▁habitat ▁change ▁as ▁part ▁of ▁their ▁life , ▁such ▁as ▁young ▁Atlantic ▁sal mon ▁or ▁sea ▁lamp rey ▁leaving ▁the ▁river ▁of ▁their ▁birth ▁when ▁they ▁have ▁reached ▁a ▁few ▁inches ▁in ▁size . ▁ ▁Over view ▁ ▁M igration ▁can ▁take ▁very ▁different ▁forms ▁in ▁different ▁species , ▁and ▁as ▁such ▁there ▁is ▁no ▁simple ▁accepted ▁definition ▁of ▁migration . ▁One ▁of ▁the ▁most ▁commonly ▁used ▁definitions , ▁proposed ▁by ▁Kennedy ▁is ▁ ▁M igration ▁en comp ass es ▁four ▁related ▁concepts : ▁persistent ▁straight ▁movement ; ▁re location ▁of ▁an ▁individual ▁on ▁a ▁greater ▁scale ▁( both ▁spatial ly ▁and ▁tempor ally ) ▁than ▁its ▁normal ▁daily ▁activities ; ▁season al ▁to - and - f ro ▁movement ▁of ▁a ▁population ▁between ▁two ▁areas ; ▁and ▁movement ▁leading ▁to ▁the ▁red ist ribution ▁of ▁individuals ▁within ▁a ▁population .