Subject: [Tweeters] Black-headed Grosbeak aging
Date: Jun 30 19:47:55 2005
From: Michael Hobbs - birdmarymoor at verizon.net


Tweets - Doris Erickson asked a seemingly innocent question. On my latest
Marymoor Park report I noted that we'd seen a hatch-year Black-headed
Grosbeak. She asked how we knew.

In this case, we were telling on the basis of a retained juvenile
structure - specifically there was still a visible gape (the thick, colored
"lips" at the corner of the mouth). Such structural clues are often present
on newly fledged young. For instance, down can sometimes be seen,
especially on the head, on young birds, giving visual indication of the age.

Another thing which can help (with some species more than others) is the
amount of wear on the feathers - species which molt once a year, in the
fall, are in fairly worn plumage by the time they have fledged young,
whereas the young have just grown their feathers, and they are all bright
with sharp edges. For species which molt twice a year (where they will
typically have a body molt in the spring to gain their breeding plumage),
you have to compare just the flight feathers (primaries, secondaries, and
rectricies) which only molt once a year.

But Doris had asked if there were plumage pattern differences that could be
used to separate hatch-year BHGR from adult females.

This is often a problematic question; there just aren't enough pictures of
juveniles in field guides. I'd love a field guide to ducklings, for
instance.

However, for Black-headed Grosbeak, there *is* some info. In the big
Sibley, there is a special section on aging Black-headed Grosbeak (p. 467).
It starts with a rather weird drawing of a Juvenile male (Jul-Aug). Of
note, and perhaps useful in the field, is that he shows the wing bars to be
buffy, not white.

But examining this section raises at least as many questions as it answers.
Sibley doesn't even agree with himself. Specifically, for both the 1st
winter male (Aug-Mar) and the Adult male breeding (Mar-Aug), he shows
different drawings than he uses in the main presentation of BHGR on page
466. In both cases, the drawings are significantly different.

It is unclear whether the differences indicate a range of possible plumages,
geographic variation, an error in labeling (perhaps the 1st winter male on
page 467 is actually a 1st winter female?), or something else. Do the
differences in the Adult male breeding (Mar - Aug) drawings indicate that
2nd summer males are distinguishable from 3rd summer+ males?

I'd be interested if anyone can shed some light on this question. And you
know I'll be checking out the Black-headed Grosbeaks at Marymoor a little
more closely.

== Michael Hobbs
== Kirkland, WA
== http://www.scn.org/fomp/birding.htm
== birdmarymoor at verizon.net

> Michael,
>
> I noted your report of the first juvenile Black-headed Grosbeak
> of the year, so I would like to know: How can you tell a
> juvenile from an adult female? We have them constantly at our
> feeders, but I haven't found a bird book that differentiates
> between them.
>
> Thanks for any help you can give me.
>
> Doris Erickson