Sunday, November 18, 2012

Sunday's Results: Ageing and Sexing Yellow-rumps

We got rained out today and no banding occurred. 


With all of the Yellow-rumped Warblers we have been getting lately (and will continue to get), I thought it would be appropriate to do a post on how banders determine the age and sex of these ubiquitous winter residents.  Before we can assign a sex to an individual it is best to determine it's age.  To do this we look for a molt limit within the alula feathers - usually between the alula covert (A1) and the lesser alula (A2).  If a molt limit exists the replaced A1 will be darker and have wider edging than the unreplaced A2 indicating a hatch-year (juvenile) individual.  Also, note that that the primary coverts have not been replaced during the first prebasic molt and are paler with narrow grayish edging.     


Yellow-rumped Warbler (hatch-year, female)

Sometimes we will see a individual that has not replaced any alulas making it appear as if there is no molt limit but a quick look at the carpal covert will reveal that a molt limit exists.  In this instance, the alula covert (A1) was not replaced during the first prebasic molt and the molt limit is between the replaced carpal covert and the unreplaced A1. 

Yellow-rumped Warbler (hatch-year, female)

During the adult prebasic molt all wing feathers are replaced therefore no molt limits exists.  Note on the individual below that there is no contrast in color between the alula feathers.  Additionally, note that the primary coverts are dark with moderately wide edging (grayish in males, brownish in females).  See the next two photos.   

Yellow-rumped Warbler (after hatch-year, male)

Yellow-rumped Warbler (after hatch-year, female)

There is much variation in Yellow-rumped Warblers but sex can be determined for most individuals.  There are several characteristics that banders look at when determining sex:  (1) color of lesser coverts, (2) size of black centers on upper tail coverts, (3) color of edging of upper tail coverts, (4) size of black centers on back feathers, and (5) amount of white on outer rectrices (tail feathers).   

1.  Color of Lesser Coverts (see previous two photos): 

         a.  Males will have bluish-gray lesser coverts 

         b.  Females will have brownish lesser coverts.

         c.  However, some adult females and juvenile males can have a mix of brown

              and bluish lesser coverts.    


2.  Size of Black Centers on Upper Tail Coverts (see next three photos):  

         a.  Adult males will have large black centers

         b.  Juvenile males and adult females can have moderate black centers

         c.  Juvenile females will have narrow black centers

3.  Color of Edging of Upper Tail Coverts (see next three photos):

         a.  Adult males will have blue edging

         b.  Juvenile males will have blue edging sometimes with narrow brownish tips

         c.  Adult females will have bluish-gray edging with brownish tips

         d.  Juvenile females will have grayish-brown edging

Yellow-rumped Warbler (Male)

Yellow-rumped Warbler (Female)

Yellow-rumped Warbler (after hatch-year female or hatch-year male)

4.  Size of Black Centers on Back Feathers:

         a.  Adult males will have large black centers

         b.  Juvenile males and adult females can have moderate black centers

         c.  Juvenile females will have narrow black centers

Yellow-rumped Warbler (after hatch-year, male)
Note the large black centers on the back feathers

5.  Amount of White on Outer Rectrices:  

         a.  In general, males average more white than females.

         b.  There can be a lot of variation with tail spots so it is best not to use this

              criteria without support from other characteristics.   

Yellow-rumped Warbler (female)

Yellow-rumped Warbler (male)

However, sex cannot be determined by using just one of these criteria.  Several (or all) of these criteria must agree before sex can be accurately determined.  In addition, wing chord can be helpful if some of these characteristics are conflicting with males averaging a larger wing chord than females.                  

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