McGILL BIRD OBSERVATORY

PHOTO LIBRARY

Brown-headed Cowbird / Vacher à tête brune (Molothrus ater)

 Seasonal status at MBO:

JAN
       
FEB
       
MAR
       
APR
       
MAY
       
JUN
       
JUL
       
AUG
       
SEP
       
OCT
       
NOV
       
DEC
       
  common
  fairly common
  uncommon
  rare
  occasional
  no records
 
QUICK TIPS:

1) Consider overall plumage - males are glossy black with a brown head, while females are uniformly grayish-brown.

2) Examine the underwing coverts - HY/SY cowbirds can be recognized by moult limits within tracts, while cowbirds lacking moult limits should be considered AHY in all seasons.

3) Check the tail - some HY/SY cowbirds may retain one or more brownish juvenile rectrices, while uniformly adult tails provide no useful information for ageing.

Species account updated March 2009

Ageing and sexing guidelines:

January - July:

AHY - M
Glossy black body and brown head; all feathers uniformly adult.

AHY - F
Grayish brown body; all feathers uniformly adult.

SY - M
Glossy black body and brown head; some juvenile feathers retained among underwing coverts and occasionally on the tail.
SY - F
Grayish brown body; some juvenile feathers retained among underwing coverts and occasionally on the tail.
     

-

July - December:

AHY - M
Glossy black body and brown head; all feathers uniformly adult.
     
AHY - F
Grayish brown body; all feathers uniformly adult.
     
HY - M
Glossy black body (may have some brown patches) and brown head; some juvenile feathers retained among underwing coverts and occasionally on the tail.
     
HY - F
Grayish brown body; some juvenile feathers retained among underwing coverts and occasionally on the tail.
     
 

Ageing and sexing details:

JAN - JUL:  after-hatch-year male

Male Brown-headed Cowbirds are glossy black with brown heads; age can usually not be determined without examining the bird in the hand, and as some individuals may undergo a complete preformative moult, any in spring that have uniformly adult feathers should be considered after-hatch-year.


Photo by Marie-Anne Hudson, McGill Bird Observatory (QC), May 2008


The upper wing is rarely informative for Brown-headed Cowbirds, as all ages typically replace all flight feathers and coverts during their preformative or prebasic moult.  On the underwing, there may be contrast between feather tracts, but not within any of them.


Photo by Marie-Anne Hudson, McGill Bird Observatory (QC), May 2008


Photo by Peter Pyle, Howell Woods (NC), May 2006


The rectrices on after-hatch-year cowbirds are uniformly adult.


Photo by Marie-Anne Hudson, McGill Bird Observatory (QC), May 2008

RETURN TO AGE/SEX OVERVIEW

JAN - JUL:  after-hatch-year female

Female Brown-headed Cowbirds are a relatively pale grayish-brown; age can usually not be determined without examining the bird in the hand, and as some individuals may undergo a complete preformative moult, any in spring that have uniformly adult feathers should be considered after-hatch-year.


Photo by Marcel Gahbauer, McGill Bird Observatory (QC), May 2005


 
Photo by Barbara Frei, McGill Bird Observatory (QC), May 2007


The upper wing is rarely of any value in ageing cowbirds, as all individuals typically replace all feathers during their preformative or prebasic moult. Note in the photo of the underwing that there are subtle differences between feather tracts, but no moult limits within any of them.


Photo by Barbara Frei, McGill Bird Observatory (QC), May 2007


Photo by Peter Pyle, Spokane (WA), May 2005

 


The rectrices on after-hatch-year cowbirds are uniformly adult.


Photo by Marie-Anne Hudson, McGill Bird Observatory (QC), April 2006

RETURN TO AGE/SEX OVERVIEW

JAN - JUL:  second-year male

Male Brown-headed Cowbirds are glossy black with brown heads; age can usually not be determined without examining the bird in the hand, and as some individuals may undergo a complete preformative moult, only those in spring that retain any juvenile feathers should be considered second-year.


Photo by Marie-Anne Hudson, McGill Bird Observatory (QC), May 2008


Photo by Barbara Frei, McGill Bird Observatory (QC), May 2007


The upper wing is usually of no value in ageing Brown-headed Cowbirds. Rather, note the moult limits within tracts of the underwing coverts that indicate the retention of juvenile feathers and identify these as SY individuals.


Photo by Marie-Anne Hudson, McGill Bird Observatory (QC), May 2008


Photo by Marcel Gahbauer, McGill Bird Observatory (QC), May 2006


Photo by Peter Pyle, Spokane (WA), May 2005 


Typically cowbirds replace all rectrices during their preformative or prebasic moult, but occasionally some HY birds retain one or more through their preformative moult, as in the example below where the contrast between the brownish juvenile rectrix and the replaced black adult rectrices on either side is easily visible.


Photo by Marie-Anne Hudson, McGill Bird Observatory (QC), May 2008

RETURN TO AGE/SEX OVERVIEW

JAN - JUL:  second-year female

 


Moult limits among the underwing coverts are much more difficult to spot on females, as differences in shades of brown are subtle; in this case, the very loos quality of the juvenile coverts is an indication that this is a second-year female.


Photo by Peter Pyle, MerryLea (IN), May 2005



RETURN TO AGE/SEX OVERVIEW

JUL - DEC:  after-hatch-year male

 

 


 


RETURN TO AGE/SEX OVERVIEW

JUL - DEC:  after-hatch-year female

 

 


 


 

RETURN TO AGE/SEX OVERVIEW

 

JUL - DEC:  hatch-year male

 

 


 


 

RETURN TO AGE/SEX OVERVIEW

JUL - DEC:  hatch-year female

 


 


RETURN TO AGE/SEX OVERVIEW 

2002- The Migration Research Foundation Inc.