﻿SOCIAL 
  PARASITISM 
  IN 
  BIRDS 
  FRIEDMANN 
  375 
  

  

  Another 
  possibility 
  suggested 
  is 
  that 
  if 
  a 
  bird 
  got 
  into 
  the 
  habit 
  

   of 
  breeding 
  in 
  old 
  nests 
  of 
  other 
  birds, 
  it 
  would 
  be 
  easy 
  

  

  * 
  * 
  * 
  to 
  imagine 
  that 
  the 
  bird 
  might 
  not 
  discriminate 
  between 
  a 
  newly 
  

   completed 
  nest 
  and 
  one 
  recently 
  abandoned. 
  The 
  result 
  would 
  be 
  that 
  if 
  the 
  

   intruder 
  laid 
  in 
  the 
  new 
  nest, 
  its 
  rightful 
  owners 
  would 
  resent 
  the 
  intrusion 
  and 
  

   prevent 
  the 
  repetition 
  of 
  the 
  act, 
  even 
  though 
  they 
  had 
  themselves 
  to 
  bring 
  up 
  

   the 
  unwelcome 
  addition. 
  It 
  is 
  likely, 
  too, 
  that 
  the 
  greater 
  abundance 
  of 
  nev/ 
  

   than 
  of 
  deserted 
  nests 
  would 
  favor 
  the 
  frequency 
  of 
  such 
  mistakes 
  until 
  the 
  para- 
  

   sitic 
  habit 
  would 
  have 
  become 
  established. 
  

  

  This 
  suggestion 
  seems 
  well 
  founded 
  and 
  possesses 
  the 
  virtue 
  of 
  

   being 
  simple. 
  However, 
  even 
  in 
  this 
  case, 
  the 
  actual 
  origin 
  of 
  the 
  

   parasitic 
  habit 
  is 
  not 
  explained. 
  A 
  possible 
  method 
  of 
  evolution 
  of 
  

   the 
  parasitic 
  habit 
  is 
  suggested 
  but 
  no 
  indication 
  is 
  given 
  as 
  to 
  why 
  

   the 
  birds, 
  if 
  repulsed 
  at 
  a 
  new 
  nest, 
  do 
  not 
  continue 
  nest 
  hunting 
  until 
  

   they 
  find 
  an 
  unoccupied 
  one. 
  Furthermore, 
  birds 
  that 
  breed 
  in 
  old 
  

   nests 
  of 
  other 
  species 
  do 
  not 
  normally 
  lay 
  the 
  first 
  egg 
  on 
  the 
  same 
  

   day 
  that 
  they 
  first 
  occupy 
  the 
  nest, 
  but 
  usually 
  the 
  possession 
  of 
  a 
  

   nest 
  seems 
  to 
  provide 
  the 
  stimulus 
  necessary 
  for 
  egg 
  production. 
  

   From 
  this 
  it 
  follows 
  that 
  if 
  a 
  bird 
  of 
  such 
  breeding 
  habits 
  did 
  try 
  to 
  

   occupy 
  a 
  new 
  nest 
  it 
  would 
  either 
  be 
  repulsed 
  by 
  the 
  owners 
  before 
  it 
  

   had 
  a 
  chance 
  to 
  lay, 
  or 
  by 
  the 
  time 
  it 
  did 
  lay 
  an 
  egg, 
  the 
  owners 
  

   would 
  have 
  forsaken 
  the 
  nest, 
  leaving 
  the 
  new 
  occupant 
  to 
  care 
  for 
  its 
  

   eggs, 
  just 
  as 
  if 
  it 
  had 
  originally 
  gone 
  to 
  an 
  old 
  nest 
  to 
  breed. 
  

  

  In 
  order 
  to 
  explain 
  the 
  origin 
  of 
  the 
  parasitic 
  habit 
  we 
  must 
  first 
  

   decide 
  whether 
  it 
  is 
  the 
  result 
  of 
  a 
  change 
  from 
  a 
  normal 
  nesting 
  habit 
  

   or 
  whether 
  it 
  is 
  a 
  phylogenetically 
  original 
  one. 
  All 
  the 
  evidence 
  

   derived 
  from 
  a 
  study 
  not 
  onl}^ 
  of 
  the 
  cowbirds, 
  but 
  of 
  birds 
  in 
  general, 
  

   points 
  unmistakably 
  to 
  the 
  conclusion 
  that 
  parasitism 
  is 
  an 
  acquired 
  

   habit 
  and 
  not 
  an 
  original 
  one. 
  It 
  is 
  inconceivable 
  to 
  think 
  of 
  a 
  long 
  

   line 
  of 
  parasitic 
  birds 
  having 
  no 
  origin 
  in 
  normal 
  nesting 
  types. 
  

   Again, 
  for 
  the 
  evidence 
  behind 
  this 
  statement 
  I 
  must 
  refer 
  the 
  reader 
  

   to 
  my 
  book 
  (23) 
  as 
  space 
  does 
  not 
  permit 
  of 
  its 
  inclusion 
  here. 
  It 
  seems 
  

   entirely 
  safe 
  and 
  justifiable, 
  then, 
  to 
  assume 
  that 
  parasitism 
  was 
  not 
  

   the 
  original 
  condition 
  m 
  the 
  cowbird 
  stock. 
  The 
  problem, 
  then, 
  is 
  

   not 
  whether 
  the 
  cowbirds 
  were 
  or 
  were 
  not 
  always 
  parasitic, 
  but 
  how 
  

   they 
  lost 
  their 
  ojiginal 
  habits 
  and 
  acquii-ed 
  theii' 
  present 
  ones. 
  

  

  LOSS 
  OF 
  PROTECTING 
  IiNlSTINCTS 
  AS 
  A 
  FACTOR 
  IN 
  THE 
  ORIGIN 
  

  

  OF 
  PARASITISM 
  

  

  To 
  quote 
  again 
  from 
  my 
  book: 
  

  

  We 
  have 
  seen 
  that 
  all 
  five 
  species 
  of 
  cowbirds 
  establish 
  breeding 
  territories 
  and 
  

   that 
  the 
  distinctness 
  or 
  the 
  definiteness 
  of 
  the 
  territory 
  is 
  most 
  pronounced 
  in 
  the 
  

   most 
  primitive, 
  nonparasitic, 
  bay 
  wing, 
  while 
  it 
  is 
  least 
  definite, 
  and 
  at 
  times, 
  

   almost 
  imaginary, 
  in 
  the 
  shiny 
  cowbird 
  and 
  the 
  North 
  American 
  Molothrus. 
  In 
  

   the 
  bay 
  wing, 
  and 
  its 
  offshoot, 
  the 
  screaming 
  cowbird, 
  the 
  birds 
  are 
  practicall}' 
  

   always 
  strictly 
  monogamous 
  and 
  only 
  one 
  pair 
  is 
  to 
  be 
  found 
  in 
  u 
  given 
  territory. 
  

  

  