▁The ▁tail ▁fe athers ▁flutter ▁at ▁high ▁frequency ▁and ▁hit ▁each ▁other ▁to ▁produce ▁the ▁bu zz ing ▁sound . ▁ ▁Tem per ature ▁reg ulation ▁ ▁The ▁high ▁met abol ic ▁rate ▁of ▁cal li ope ▁hum ming bird s ▁– ▁especially ▁during ▁rapid ▁forward ▁flight ▁and ▁hover ing ▁– ▁produces ▁increased ▁body ▁heat ▁that ▁requires ▁special ized ▁mechan isms ▁of ▁temperature ▁reg ulation ▁for ▁diss ip ating ▁heat , ▁which ▁becomes ▁an ▁even ▁greater ▁challenge ▁in ▁hot , ▁hum id ▁clim ates . ▁As ▁the ▁smallest ▁North ▁American ▁hum ming bird ▁species , ▁cal li op es ▁appear ▁to ▁adapt ▁their ▁relatively ▁higher ▁surface - to - volume ▁ratio ▁to ▁improve ▁con ve ct ive ▁cool ing ▁from ▁air ▁movement ▁by ▁the ▁wings . ▁When ▁air ▁temper atures ▁rise ▁above ▁, ▁thermal ▁grad ients ▁driving ▁heat ▁pass ively ▁by ▁con ve ct ive ▁diss ip ation ▁from ▁around ▁the ▁eyes , ▁shoulders , ▁and ▁feet ▁are ▁reduced ▁or ▁elimin ated , ▁requiring ▁heat ▁diss ip ation ▁mainly ▁by ▁ev ap oration ▁and ▁ex hal ation . ▁ ▁All ▁hum ming bird s ▁diss ip ate ▁heat ▁partially ▁by ▁ex h aled ▁air , ▁and ▁from ▁body ▁structures ▁with ▁thin ▁or ▁no ▁fe ather ▁covering , ▁such ▁as ▁around ▁the ▁eyes , ▁shoulders , ▁under ▁the ▁wings ▁( pat ag ia ), ▁and ▁feet . ▁ ▁While ▁hover ing , ▁cal li ope ▁hum ming bird s ▁do ▁not ▁benefit ▁from ▁the ▁heat ▁loss ▁by ▁air ▁con ve ction ▁during ▁forward ▁flight