athers , ▁they ▁drop ▁off ▁when ▁the ▁bird ▁m oul ts . ▁ ▁Another ▁method ▁is ▁imp ing ▁in ▁a ▁bright ly ▁colored ▁false ▁fe ather ▁instead ▁of ▁a ▁natural ▁fe ather . ▁ ▁A ▁pat ag ial ▁tag ▁is ▁a ▁permanent ▁tag ▁held ▁onto ▁the ▁wing ▁by ▁a ▁riv et ▁p unch ed ▁through ▁the ▁pat ag ium . ▁ ▁Radio ▁transmit ters ▁and ▁satellite - track ing ▁▁ ▁Where ▁detailed ▁information ▁is ▁needed ▁on ▁individual ▁movements , ▁tiny ▁radio ▁transmit ters ▁can ▁be ▁fitted ▁on ▁to ▁birds . ▁For ▁small ▁species ▁the ▁transmit ter ▁is ▁carried ▁as ▁a ▁' back pack ' ▁fitted ▁over ▁the ▁wing ▁bases , ▁and ▁for ▁larger ▁species ▁it ▁may ▁be ▁attached ▁to ▁a ▁tail ▁fe ather ▁or ▁loop ed ▁to ▁the ▁legs . ▁Both ▁types ▁usually ▁have ▁a ▁tiny ▁( 1 0   cm ) ▁flexible ▁aer ial ▁to ▁improve ▁signal ▁reception . ▁Two ▁field ▁rece ivers ▁( reading ▁distance ▁and ▁direction ) ▁are ▁needed ▁to ▁establish ▁the ▁bird ' s ▁position ▁using ▁tri ang ulation ▁from ▁the ▁ground . ▁The ▁technique ▁is ▁useful ▁for ▁trac ing ▁individuals ▁during ▁landscape - level ▁movements ▁particularly ▁in ▁dense ▁veget ation ▁( such ▁as ▁tropical ▁for ests ) ▁and ▁for ▁sh y ▁or ▁difficult - to - spot ▁species , ▁because ▁birds ▁can ▁be ▁located ▁from ▁a ▁distance ▁without ▁visual ▁confirm ation . ▁ ▁The ▁use ▁of ▁satellite ▁transmit ters ▁for ▁bird ▁movements ▁is ▁currently ▁restricted ▁by ▁transmit ter ▁size ▁– ▁to ▁species ▁larger ▁than ▁about ▁ 4 0