﻿SOCIAL 
  PARASITISM 
  IN 
  BIRDS 
  FEIEDMANN 
  377 
  

  

  Fortunately 
  we 
  liave 
  a 
  clue 
  to 
  the 
  waj- 
  in 
  which 
  the 
  male 
  lost 
  his 
  protecting 
  

   instinct. 
  In 
  order 
  to 
  fully 
  appreciate 
  its 
  significance 
  it 
  is 
  necessary 
  to 
  digress 
  for 
  

   a 
  moment 
  and 
  consider 
  the 
  evolution 
  of 
  the 
  screaming 
  cowbird 
  from 
  the 
  bay-wing 
  

   stock. 
  As 
  already 
  indicated 
  in 
  a 
  previous 
  section 
  the 
  screaming 
  cowbird 
  is 
  obvi- 
  

   ously 
  a 
  direct 
  offshoot 
  of 
  the 
  stock 
  of 
  which 
  the 
  bay-winged 
  cowbird 
  is 
  the 
  living 
  

   example. 
  The 
  range 
  of 
  the 
  screaming 
  cowbird 
  is 
  wholly 
  contained 
  within 
  that 
  of 
  

   the 
  bay 
  wing 
  and 
  in 
  general 
  the 
  habitat 
  or 
  type 
  of 
  country 
  occupied 
  by 
  the 
  two 
  

   species 
  is 
  the 
  same. 
  I 
  always 
  found 
  both 
  species 
  in 
  the 
  same 
  type 
  of 
  environment. 
  

   It 
  seems 
  then 
  that 
  there 
  could 
  have 
  been 
  no 
  geographical 
  or 
  ecological 
  isolation 
  

   in 
  this 
  case 
  to 
  preserve 
  and 
  differentiate 
  the 
  budding 
  form 
  which 
  in 
  its 
  present 
  

   state 
  we 
  call 
  the 
  screaming 
  cowbird. 
  Consequently 
  the 
  isolation 
  necessary 
  to 
  

   preserve 
  the 
  distinctness 
  of 
  the 
  newly 
  arisen 
  species 
  must 
  have 
  been 
  physiological. 
  

   The 
  physiological 
  isolation 
  was 
  probably 
  that 
  of 
  differential 
  breeding 
  seasons. 
  

   Probably 
  the 
  original 
  rufo-axillaris 
  was 
  an 
  early 
  breeding 
  bird 
  (badius 
  is 
  a 
  later 
  

   breeder). 
  Although 
  rufo-axillaris 
  to-day 
  is 
  a 
  late 
  breeder, 
  the 
  facts 
  that 
  its 
  court- 
  

   ship 
  season 
  comes 
  early 
  in 
  the 
  spring, 
  and 
  that 
  it 
  establishes 
  its 
  territories 
  very 
  

   early, 
  point 
  to 
  the 
  conclusion 
  that 
  it 
  once 
  was 
  an 
  early 
  breeding 
  bird 
  as 
  bonariensis 
  

   and 
  ater 
  are 
  to-day. 
  Inasmuch 
  as 
  the 
  bay 
  wing 
  is 
  nonparasitic 
  and 
  inasmuch 
  as 
  

   the 
  screaming 
  cowbird 
  is 
  a 
  direct 
  offshoot 
  of 
  this 
  stock, 
  it 
  seems 
  probable 
  that 
  

   originally 
  the 
  latter 
  species 
  was 
  also 
  nonparasitic. 
  In 
  other 
  w^ords, 
  the 
  change 
  

   between 
  the 
  normal 
  and 
  the 
  parasitic 
  mode 
  of 
  reproduction 
  occurred 
  within 
  the 
  

   racial 
  history 
  of 
  M. 
  rufo-axillaris. 
  Assuming 
  that 
  in 
  most 
  ways 
  the 
  original 
  habits 
  

   of 
  the 
  screaming 
  cowbird 
  were 
  similar 
  to 
  those 
  of 
  the 
  bay 
  wnng, 
  we 
  should 
  expect 
  

   that 
  the 
  birds 
  tried 
  to 
  breed 
  in 
  nests 
  of 
  ovenbirds, 
  wood 
  hewers, 
  etc., 
  but 
  tiled 
  to 
  

   do 
  so 
  early 
  in 
  the 
  season. 
  As 
  elsewhere 
  indicated, 
  the 
  struggle 
  for 
  nests 
  is 
  much 
  

   greater 
  early 
  in 
  the 
  season 
  than 
  later 
  on, 
  and 
  the 
  screaming 
  cowbird, 
  handicapped 
  

   hereditarily 
  by 
  a 
  weakened 
  territorial 
  instinct, 
  probably 
  could 
  not 
  succeed 
  in 
  this 
  

   struggle. 
  We 
  have 
  seen 
  that 
  sometimes 
  screaming 
  cowbirds 
  establish 
  territories 
  

   in 
  the 
  spring, 
  occupy 
  them 
  for 
  considerable 
  periods, 
  and 
  then 
  desert 
  them 
  with- 
  

   out 
  ever 
  having 
  utilized 
  them. 
  This 
  indicates 
  very 
  strongly 
  that 
  the 
  weakened 
  

   territorial 
  instinct 
  of 
  the 
  male 
  is 
  often 
  insufficient 
  to 
  maintain 
  its 
  influence 
  long 
  

   enough 
  to 
  "make 
  connections" 
  with 
  the 
  somewhat 
  more 
  vernal 
  development 
  of 
  

   the 
  egg-laying 
  instincts 
  of 
  the 
  female. 
  In 
  this 
  lack 
  of 
  attunement 
  between 
  the 
  

   territorial 
  instincts 
  of 
  the 
  male 
  and 
  the 
  egg-laying 
  instincts 
  of 
  the 
  female 
  the 
  para- 
  

   sitic 
  habit 
  probably 
  had 
  its 
  origin. 
  This 
  lack 
  of 
  attunement 
  seems 
  to 
  have 
  been 
  

   caused 
  by 
  the 
  diminution 
  of 
  the 
  protecting 
  territorial 
  instincts 
  of 
  the 
  male 
  and 
  

   this 
  diminution 
  seems 
  in 
  turn 
  to 
  have 
  been 
  started 
  by 
  the 
  reversal 
  of 
  the 
  territorial 
  

   and 
  nest-building 
  instincts 
  in 
  the 
  stock 
  from 
  which 
  the 
  screaming 
  cowbird 
  evolved. 
  

  

  So 
  much, 
  then, 
  for 
  the 
  cowbirds. 
  In 
  the 
  other 
  groups 
  of 
  parasitic 
  

   birds, 
  other 
  factors 
  seem 
  to 
  have 
  been 
  instrumental 
  in 
  bringing 
  about 
  

   the 
  parasitic 
  breeding 
  habit. 
  Too 
  little 
  is 
  definitely 
  known 
  of 
  their 
  

   biology 
  to 
  attempt 
  an 
  explanation, 
  but 
  probably 
  the 
  habit 
  arose 
  dif- 
  

   ferently 
  in 
  each 
  of 
  the 
  five 
  families 
  containing 
  parasitic 
  species. 
  

  

  HOST 
  SPECIFICITY 
  IN 
  THE 
  CUCKOOS 
  

  

  In 
  the 
  cuckoos, 
  we 
  have 
  one 
  clue 
  which 
  indicates 
  that 
  territory 
  had 
  

   little 
  to 
  do 
  with 
  the 
  inception 
  of 
  parasitism. 
  This 
  is 
  furnished 
  by 
  

   the 
  peculiar 
  feature 
  of 
  host 
  specificity 
  shown 
  by 
  some 
  of 
  the 
  species, 
  

   especially 
  in 
  Europe 
  and 
  Asia, 
  but 
  to 
  a 
  lesser 
  extent 
  in 
  Africa 
  and 
  

   Australia 
  as 
  well. 
  In 
  the 
  classic 
  case 
  of 
  the 
  European 
  cuckoo, 
  

   Cuculus 
  canonis 
  canoruSy 
  it 
  is 
  now 
  well 
  established 
  that 
  generally 
  each 
  

  

  