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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  female 
  of 
  the 
  original 
  pair 
  had 
  died 
  and 
  the 
  male 
  had 
  deserted 
  on 
  

   that 
  account. 
  In 
  order 
  to 
  test 
  this 
  I 
  shot 
  the 
  females 
  of 
  three 
  pairs 
  

   on 
  three 
  near-by, 
  and, 
  to 
  me, 
  well-known 
  territories. 
  In 
  none 
  of 
  

   these 
  cases 
  did 
  the 
  males 
  desert; 
  they 
  remained 
  and 
  soon 
  found 
  other 
  

   mates. 
  I 
  can 
  attribute 
  this 
  desertion 
  to 
  no 
  cause 
  other 
  than 
  the 
  

   diminishing 
  potency 
  of 
  the 
  territorial 
  instinct 
  with 
  the 
  passing 
  of 
  

   time 
  between 
  the 
  establishment 
  and 
  the 
  utilization 
  of 
  the 
  territory 
  

   in 
  question. 
  Another 
  bit 
  of 
  evidence 
  in 
  this 
  connection 
  was 
  gathered 
  

   at 
  the 
  height 
  of 
  the 
  breeding 
  season 
  of 
  the 
  species 
  (January) 
  at 
  Santa 
  

   Elena 
  in 
  Entre 
  Rios, 
  eastern 
  Argentina. 
  I 
  was 
  studying 
  a 
  bay- 
  

   winged 
  cowbird's 
  nest, 
  making 
  daily 
  notes 
  of 
  everything 
  concerned 
  

   with 
  it. 
  On 
  January 
  8, 
  1924, 
  I 
  noted 
  that 
  a 
  pair 
  of 
  screaming 
  cow- 
  

   birds 
  flew 
  into 
  the 
  tree 
  in 
  which 
  the 
  nest 
  was 
  and 
  stayed 
  around 
  in 
  

   the 
  near-by 
  branches 
  but 
  were 
  kept 
  away 
  from 
  the 
  nest 
  by 
  the 
  bay 
  

   wings. 
  Suddenly 
  another 
  pair 
  of 
  screaming 
  cowbirds 
  flew 
  into 
  the 
  

   tree 
  and 
  joined 
  the 
  first 
  pair. 
  A 
  minute 
  later 
  the 
  second 
  pair 
  (the 
  

   newly 
  arrived 
  birds) 
  flew 
  over 
  to 
  the 
  nest 
  and 
  were 
  chased 
  back 
  by 
  

   the 
  bay-wings. 
  They 
  flew 
  back 
  to 
  the 
  first 
  pair 
  and 
  a 
  little 
  while 
  

   later 
  both 
  pairs 
  flew 
  off 
  together, 
  screaming 
  as 
  they 
  flew. 
  It 
  seemed 
  

   that 
  in 
  this 
  case 
  one 
  of 
  the 
  two 
  pairs 
  of 
  screaming 
  cowbirds 
  was 
  

   encroaching 
  on 
  a 
  nest 
  in 
  the 
  territory 
  of 
  the 
  other. 
  The 
  birds 
  being 
  

   strictly 
  monogamous, 
  only 
  one 
  pair 
  would 
  occur 
  in 
  any 
  one 
  territory, 
  

   and 
  the 
  other 
  pair 
  must 
  have 
  just 
  recently 
  come 
  in. 
  Screaming 
  cow- 
  

   birds 
  were 
  not 
  very 
  plentiful 
  in 
  that 
  district, 
  and 
  there 
  was 
  plenty 
  

   of 
  land 
  available 
  for 
  the 
  other 
  pair 
  to 
  use. 
  There 
  were 
  also 
  plenty 
  of 
  

   bay 
  wings 
  scattered 
  over 
  the 
  country. 
  This 
  looked 
  as 
  though 
  the 
  

   second 
  pair 
  had 
  not 
  yet 
  established 
  themselves 
  in 
  a 
  breeding 
  terri- 
  

   tory 
  although 
  it 
  was 
  very 
  late 
  in 
  the 
  season. 
  It 
  is 
  hardly 
  possible 
  

   that 
  this 
  pair 
  had 
  not 
  attempted 
  to 
  do 
  so 
  before; 
  it 
  seems 
  likely 
  that 
  

   they 
  were 
  birds 
  that 
  had 
  deserted 
  a 
  territory 
  and 
  were 
  not 
  yet 
  settled 
  

   in 
  a 
  new 
  one. 
  

  

  POPULATION 
  PRESSURE 
  AS 
  A 
  MODIFIER 
  OF 
  BREEDING 
  HABITS 
  

  

  In 
  the 
  case 
  of 
  the 
  shiny 
  cowbird, 
  the 
  numercial 
  abundance 
  of 
  the 
  

   species 
  usually 
  modifies 
  or 
  hides 
  the 
  true 
  state 
  of 
  affairs. 
  The 
  sexual 
  

   and 
  territorial 
  relations 
  of 
  this 
  species 
  are 
  easily 
  overridden 
  by 
  the 
  

   pressure 
  of 
  population, 
  resulting 
  in 
  undue 
  competition 
  for 
  breeding 
  

   areas. 
  In 
  this 
  species 
  the 
  factors 
  influencing 
  the 
  extent 
  of 
  the 
  in- 
  

   dividual 
  territories 
  are 
  not 
  associated 
  directly 
  with 
  the 
  food 
  supply, 
  

   but 
  with 
  the 
  abundance 
  of 
  nests 
  in 
  which 
  to 
  deposit 
  the 
  eggs. 
  The 
  

   denser 
  the 
  small-bird 
  population, 
  the 
  smaller 
  the 
  territory 
  of 
  each 
  

   cowbird. 
  Where 
  the 
  cowbirds 
  are 
  very 
  abundant 
  the 
  territories 
  as 
  

   such 
  become 
  almost 
  impossible 
  of 
  definition 
  and 
  demarkation. 
  The 
  

   results 
  of 
  a 
  protracted 
  study 
  of 
  this 
  species 
  indicate 
  the 
  following 
  

   iacts. 
  In 
  areas^^ 
  where 
  the'| 
  birds, 
  are 
  not^extremely 
  abundant, 
  they 
  

  

  