: American ▁aut obi ograph ers ▁Category : American ▁mem oir ists ▁Category : J azz - infl uen ced ▁classical ▁compos ers ▁Category : 2 1 st - century ▁American ▁compos ers ▁Category : The ▁Put ney ▁School ▁al umn i ▁Category : S ong writ ers ▁from ▁Pennsylvania ▁Category : Progress ive ▁big ▁band ▁mus icians ▁Category : 2 0 th - century ▁American ▁compos ers ▁Category : J azz ▁mus icians ▁from ▁Pennsylvania ▁Category : 2 0 th - century ▁American ▁conduct ors ▁( music ) ▁Category : 2 1 st - century ▁American ▁conduct ors ▁( music ) ▁Category : O ber lin ▁Conserv atory ▁of ▁Music ▁al umn i ▁Category : Ge orge ▁Washington ▁University ▁al umn i ▁Category : Man h attan ▁School ▁of ▁Music ▁al umn i <0x0A> </s> ▁H ack les ▁are ▁the ▁ere ct ile ▁pl um age ▁or ▁hair ▁in ▁the ▁neck ▁area ▁of ▁some ▁birds ▁and ▁m amm als . ▁▁ ▁In ▁birds , ▁the ▁hack le ▁is ▁the ▁group ▁of ▁fe athers ▁found ▁along ▁the ▁back ▁and ▁side ▁of ▁the ▁neck . ▁The ▁hack les ▁of ▁some ▁types ▁of ▁ch icken , ▁particularly ▁ro ost ers , ▁are ▁long , ▁fine , ▁and ▁often ▁bright ly ▁col oured . ▁These ▁hack les ▁may ▁be ▁used ▁in ▁fly ▁fish ing ▁as ▁l ures . ▁ ▁In ▁m amm als , ▁the ▁hack les ▁are ▁the ▁hair s ▁of ▁the ▁neck ▁and ▁back ▁which ▁become ▁ere ct ▁when ▁the ▁animal ▁is ▁fear ful