﻿366 
  ANNUAL 
  EEPOKT 
  SMITHSONIAN 
  INSTITUTION, 
  192 
  9 
  

  

  tory. 
  It 
  is, 
  if 
  anything, 
  even 
  more 
  strictly 
  monogamous, 
  as 
  it 
  is 
  

   found 
  in 
  pairs 
  all 
  year 
  round, 
  even 
  in 
  the 
  middle 
  of 
  the 
  Argentine 
  

   winter. 
  So 
  closely 
  are 
  the 
  members 
  of 
  each 
  pair 
  united 
  to 
  each 
  other 
  

   that 
  it 
  is 
  quite 
  the 
  exception 
  to 
  see 
  a 
  single 
  individual 
  at 
  any 
  time. 
  

   On 
  a 
  few 
  occasions 
  I 
  have 
  found 
  single 
  birds 
  or 
  groups 
  of 
  three, 
  but 
  

   in 
  an 
  overwhelming 
  majority 
  of 
  the 
  cases 
  the 
  birds 
  were 
  in 
  twos. 
  

   Like 
  the 
  bay-winged 
  cowbird, 
  it 
  is 
  a 
  very 
  late 
  breeder, 
  the 
  season 
  

   for 
  eggs 
  being 
  from 
  December 
  to 
  the 
  end 
  of 
  February 
  for 
  these 
  

   two 
  species, 
  whereas 
  most 
  small 
  birds 
  in 
  Argentina 
  breed 
  from 
  Sep- 
  

   tember 
  to 
  January, 
  and 
  the 
  geographical 
  and 
  ecological 
  ranges 
  of 
  

   the 
  two 
  species 
  are 
  entirely 
  coincident. 
  The 
  eggs 
  and 
  young 
  of 
  the 
  

   two 
  are 
  practically 
  identical, 
  but 
  the 
  eggs 
  can 
  -be 
  told 
  apart 
  with 
  

   considerable 
  certainty 
  by 
  one 
  who 
  has 
  studied 
  them 
  intently. 
  The 
  

   screaming 
  cowbird 
  is, 
  however, 
  parasitic 
  in 
  its 
  breeding 
  habits 
  and 
  

   confines 
  its 
  parasitic 
  attentions 
  to 
  one 
  species, 
  the 
  bay-winged 
  

   cowbird. 
  

  

  It 
  is 
  interesting 
  to 
  note, 
  in 
  passing, 
  the 
  ironical 
  aspect 
  of 
  this 
  

   situation. 
  As 
  far 
  as 
  the 
  available 
  evidence 
  indicates, 
  the 
  main 
  

   factor 
  coincident 
  with 
  the 
  late 
  breeding 
  of 
  the 
  bay-winged 
  cowbird 
  

   is 
  the 
  abundance 
  and 
  availability 
  of 
  nests 
  built 
  and 
  since 
  deserted 
  

   by 
  other 
  species. 
  In 
  other 
  words, 
  there 
  seems 
  to 
  be 
  some 
  correlation 
  

   between 
  the 
  apparent 
  dislike 
  for 
  nest-building 
  on 
  the 
  part 
  of 
  the 
  

   bay 
  wing, 
  and 
  its 
  late 
  breeding 
  season. 
  By 
  breeding 
  late 
  it 
  avoids 
  

   the 
  task 
  of 
  building. 
  The 
  screaming 
  cowbird 
  also 
  is 
  a 
  late 
  breeder 
  

   but 
  is 
  parasitic, 
  and 
  inasmuch 
  as 
  no 
  other 
  birds 
  are 
  breeding 
  so 
  late 
  

   in 
  the 
  season, 
  the 
  bay 
  wing 
  automatically 
  becomes 
  the 
  chief, 
  if 
  not 
  

   the 
  only, 
  victim. 
  In 
  this 
  way 
  additional 
  work 
  devolves 
  upon 
  the 
  

   species 
  that 
  originally 
  adopted 
  a 
  late 
  laying 
  season 
  to 
  avoid 
  work. 
  

  

  The 
  shiny 
  cowbird 
  is 
  a 
  far 
  more 
  widely 
  ranging 
  species 
  than 
  either 
  

   of 
  the 
  two 
  already 
  mentioned, 
  and 
  occurs 
  from 
  Patagonia 
  to 
  southern 
  

   Darien 
  (Panama), 
  while 
  the 
  bay- 
  wing 
  and 
  the 
  screaming 
  cowbirds 
  

   are 
  found 
  only 
  in 
  the 
  northern 
  part 
  of 
  Argentina, 
  Uruguay, 
  southern 
  

   Brazil, 
  Paraguay, 
  and 
  Bolivia. 
  It 
  is 
  migratory 
  in 
  the 
  southern 
  part 
  

   of 
  its 
  range 
  and 
  is 
  less 
  monogamous 
  than 
  either 
  of 
  the 
  other 
  two 
  species 
  

   found 
  in 
  South 
  America. 
  In 
  general 
  it 
  tends 
  toward 
  monogamy, 
  but 
  

   in 
  localities 
  where 
  it 
  is 
  very 
  numerous 
  its 
  sexual 
  relations 
  seem 
  unable 
  

   to 
  maintain 
  themselves 
  unchanged 
  in 
  the 
  face 
  of 
  the 
  pressure 
  of 
  popu- 
  

   lation 
  numbers 
  and 
  the 
  birds 
  become 
  more 
  or 
  less 
  promiscuous, 
  or 
  

   possibly 
  polyandrous, 
  in 
  their 
  mating 
  habits. 
  This 
  species 
  (and 
  also 
  

   the 
  screaming 
  cowbird) 
  has 
  a 
  very 
  different 
  type 
  of 
  courtship 
  display 
  

   and 
  song, 
  differing 
  in 
  this 
  respect 
  from 
  the 
  most 
  primitive 
  species 
  of 
  

   the 
  group. 
  The 
  shiny 
  cowbird 
  presents 
  a 
  new 
  feature, 
  that 
  of 
  a 
  

   differential 
  sex 
  ratio, 
  the 
  males 
  being 
  much 
  in 
  excess 
  of 
  the 
  females, 
  

   while 
  in 
  the 
  first 
  two 
  species 
  the 
  sexes 
  are 
  about 
  equal 
  in 
  numbers. 
  

   This 
  excess 
  of 
  males 
  almost 
  seems 
  a 
  result 
  of 
  the 
  parasitic 
  habit, 
  as 
  it 
  

  

  