▁just ▁after ▁a ▁big ▁migration . ▁This ▁is ▁believed ▁because ▁of ▁the ▁long ▁days ▁of ▁flying ▁in ▁migration ▁the ▁thy roid ▁g land ▁sends ▁out ▁more ▁T 4 ▁which ▁help ▁the ▁body ▁co pe ▁with ▁the ▁longer ▁journey . ▁The ▁increased ▁T 4 ▁levels ▁are ▁also ▁associated ▁with ▁increased ▁mus cle ▁mass ▁( hy pert ro phy ) ▁of ▁the ▁breast ▁mus cle , ▁also ▁because ▁of ▁the ▁longer ▁time ▁spent ▁flying . ▁It ▁is ▁believed ▁that ▁the ▁body ▁sends ▁out ▁more ▁T 4 ▁to ▁help ▁the ▁go ose ' s ▁body ▁with ▁this ▁long ▁task ▁by ▁speed ing ▁up ▁the ▁met abol ism ▁and ▁lower ing ▁the ▁temperature ▁at ▁which ▁the ▁mus cles ▁work . ▁Also , ▁other ▁studies ▁show ▁levels ▁of ▁stress ▁h orm ones ▁such ▁as ▁cort ic oster one ▁rise ▁dram atically ▁in ▁these ▁birds ▁during ▁and ▁after ▁a ▁migration . ▁ ▁Sur v ival ▁The ▁lif es pan ▁in ▁the ▁wild ▁of ▁ge ese ▁that ▁surv ive ▁to ▁ad ul th ood ▁ranges ▁from ▁ 1 0 ▁to ▁ 2 4   year s . ▁The ▁British ▁lon ge vity ▁record ▁is ▁held ▁by ▁a ▁spec imen ▁tag ged ▁as ▁a ▁nest ling , ▁which ▁was ▁observed ▁alive ▁at ▁the ▁University ▁of ▁York ▁at ▁the ▁age ▁of ▁ 3 1 . ▁ ▁Pred ators ▁ ▁Kn own ▁pred ators ▁of ▁eggs ▁and ▁g os lings ▁include ▁co y otes , ▁Ar ctic ▁fo xes ▁( V ul pes ▁lag opus ), ▁northern ▁racc o ons ▁( Pro cy