Re: [SBB] Fwd: Pajaro River grosbeak

[Mike Rogers <[[email protected]]>]

All,

I don't have enough time at the moment to do a detailed analysis of this 
bird, but thought that others may be interested in Joe Morlan's Birding 
article on the subject, available at:

http://fog.ccsf.cc.ca.us/~jmorlan/grbkid.htm

(photos available in the 1991 Birding magazine cited there).

You may note that Joe states "The only totally reliable plumage 
difference is the width of the dark streaks in the middle of the breast" 
- these are unfortunately not visible in the photos of the Pajaro River 
bird. Joe also states "Claimed differences in flank streaking, however, 
are not consistent", so care is needed in using flank streaking as a 
field mark. Also "In the briefly held juvenal plumage, Black-headed 
Grosbeaks can be more heavily streaked on the breast".

As far as the white head stripe mentioned by observers of the Pajaro 
bird, Joe adds /"many Black-headed Grosbeaks have the underparts and 
head as white or whiter than those of Rose-breasted Grosbeaks", /and 
this has been my experience as well.

Regarding bill color, it is not completely clear how reliable this field 
mark is for fall/winter and hatch-year birds - bill color of many 
recently fledged birds differs notably (often being paler) from that of 
adult (e.g. yellow in the bills of young White-breasted Nuthatches etc.).

I am not convinced that an "abnormally fresh plumaged adult" is 
something that would look like a hatch-year bird. Remember (especially 
useful for aging shorebirds), juveniles have feathers that all grew at 
about the same time - they look uniformly fresh. Adults acquire fresh 
feathers by molt in a progression (ducks being an exception when they 
molt all flight feathers at once and become flightless) and evidence of 
this progression is usually visible, especially until the process is 
fully complete much later in the season. Additionally, there are shape 
differences between juvenile and adult feathers (e.g. juvenile tail 
feathers and primary coverts are more pointed and less "truncate" than 
those of adults). The presence of remnant rictal flanges (gape marks at 
the side of the bill) also indicates a hatch-year bird in this case.

I am sure that careful study of the photos could add more to this 
discussion.

Mike Rogers
Sunnyvale


[[email protected]] wrote:
> In a message dated 8/6/2007 8:16:44 A.M. Pacific Daylight Time, 
> [[email protected]] writes:
>
>     Hey Guys,
>
>     I do know of at least one record of red-breasted grosbeak nesting
>     in Marin
>     County in the last five years.  I thought I would just throw that
>     into your
>     discussion.
>
> All,
>  
> In a brief discussion with Rick Fournier he said that he thought that 
> there was a local breeding record for Rose-breasted Grosbeak.  I was 
> wondering if hatch year BHGR ever have pink bills?  Has anyone found 
> any BHGR photos or drawings with streaking as broad as on the bird in 
> question?  Wouldn't any hybrid normally have streaking that would be 
> wider than that on BHGR but narrower than that on RBGR?  If we think 
> that this bird is a hatch year BHGR with abnormally wide streaking 
> (even for a RBGR) then why can't it be an abnormally fresh plumaged 
> adult female RBGR.  I was wondering if one did a write-up on this bird 
> with references to normal field marks (bill color and wide streaking 
> being key and with freshness and color of plumage being subjective) 
> would one accept it as an adult female RBGR?  For what it's worth this 
> bird, when seen, was instantly identified as not being a "normal" BHGR.
>  
> Take care,
> Bob Reiling



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