0 g . ▁They ▁may ▁be ▁attached ▁to ▁mig rat ory ▁birds ▁( ge ese , ▁sw ans , ▁c ran es , ▁p engu ins ▁etc .) ▁or ▁other ▁species ▁such ▁as ▁p engu ins ▁that ▁undert ake ▁long - distance ▁movements . ▁Ind ividual s ▁may ▁be ▁track ed ▁by ▁sat ell ites ▁for ▁immense ▁distances , ▁for ▁the ▁lifetime ▁of ▁the ▁transmit ter ▁battery . ▁As ▁with ▁wing ▁tags , ▁the ▁transmit ters ▁may ▁be ▁designed ▁to ▁drop ▁off ▁when ▁the ▁bird ▁m oul ts ; ▁or ▁they ▁may ▁be ▁recovered ▁by ▁re capt uring ▁the ▁bird . ▁ ▁Field - read able ▁rings ▁▁ ▁A ▁field - read able ▁is ▁a ▁ring ▁or ▁rings , ▁usually ▁made ▁from ▁pl astic ▁and ▁bright ly ▁col oured , ▁which ▁may ▁also ▁have ▁consp ic uous ▁mark ings ▁in ▁the ▁form ▁of ▁letters ▁and / or ▁numbers . ▁They ▁are ▁used ▁by ▁bi olog ists ▁working ▁in ▁the ▁field ▁to ▁identify ▁individual ▁birds ▁without ▁re capt ure ▁and ▁with ▁a ▁minimum ▁of ▁disturb ance ▁to ▁their ▁behaviour . ▁R ings ▁large ▁enough ▁to ▁carry ▁numbers ▁are ▁usually ▁restricted ▁to ▁larger ▁birds , ▁although ▁if ▁necessary ▁small ▁extensions ▁to ▁the ▁rings ▁( leg ▁flags ) ▁bearing ▁the ▁identification ▁code ▁allow ▁their ▁use ▁on ▁slightly ▁smaller ▁species . ▁For ▁small ▁species ▁( e . g . ▁most ▁passer ines ), ▁individuals ▁can ▁be ▁identified ▁by ▁using ▁a ▁combination ▁of ▁small ▁rings ▁of ▁different ▁colours , ▁which ▁are ▁read ▁in ▁a ▁specific ▁order . ▁Most