[EBB Sightings] Some Swamp Sparrow info

Stephen Long wrote:
 > ...
 > The bird is working hard on its molt into alternate plumage;
 > it still has a good deal of buff in the gray cheek patches,
 > and the crown hasn't yet attained its unstreaked reddish color,
 > but the scapulars are broadly tipped with white.  This was a
 > county bird for me.

I have been a little confused about this Swamp Sparrow in Livermore, in 
particular about its plumage, plumage differences between the sexes (if 
any), and molting as it relates to it.

Stephen's comments finally prompted me to do a very cursory "research" 
on the web last night, and I came across the following two published 
papers on Swamp Sparrow, both written by Russell Greenberg, that contain 
some interesting information on Swamp Sparrows, including answers to my 
own questions. I'd like to mention them here in case anyone else find 
them useful:

[1] Greenberg, R. 1990. Adaptations to tidal marshes in breeding 
populations of the swamp sparrow. The Condor 92:393-404
http://nationalzoo.si.edu/ConservationAndScience/MigratoryBirds/About_us/Greenberg/Papers/greenberg_swsp_adaptations.pdf

[2] Greenberg, R. 1988. Seasonal plumage dimorphism in the swamp 
sparrow. J. Field Ornithol., 59(2):149-154
http://elibrary.unm.edu/sora/JFO/v059n02/p0149-p0154.pdf

Basically, my personal confusion centered around the issue of telling 
the sexes apart in the field (based on plumages), and if the bright 
rusty crown patch in adult breeding plumage had anything to do with it.

For example, "A Guide to the Identification and Natural History of The 
Sparrows of the United States and Canada" (Rising and Beadle, 1996) says 
"sexes similar in coloration" in the Swamp Sparrow section. This seems 
to suggest trying to guess a sex based on plumage might be futile.

But "The Sibley Guide to Birds" (David Allen Sibley, 1st ed. 2nd 
printing, 2000) depicts a male and a female breeding adult separately, 
although it's not clear to me exactly which differences depicted there 
are meant to be due to sex and which are just individual variations. (It 
shows a bright rusty crown on male and darker streaked crown for female, 
and it also includes a caption "clean gray" pointing to the male's nape 
area, among other more subtle differences.)

On the topic of molting pattern in sparrows in general, Rising gives the 
following information: "The breeding (or alternate) plumage and winter 
(or basic) plumage are similar in most species, and the prebasic molt, 
which takes place after breeding, though variable in the group, is 
generally complete. The spring, prealternate, molt is absent or 
incomplete. In dimorphic species, much of the brightness of the male 
breeding plumage is acquired through wear."

(I was aware about the wear creating breeding adult plumages in, for 
example, Snow Bunting. But, in the case of Swamp Sparrow, is the spring 
molt absent or incomplete? Does it acquire the rusty crown through wear 
- didn't seem likely - or through an incomplete molt? These were some of 
my questions.)

In short, the above two papers seem to suggest the following:

1) Swamp Sparrow goes thorough an incomplete spring molt mostly in the 
head, nape, and throat feathers. (But not the flight feathers, tail 
feathers, etc.)

2) Swamp Sparrow exhibits a rather unusual (for sparrow) _seasonal_ 
sexual dimorphism in its crown coloration, created by the spring molting 
process. (The sex differences are strong in the breeding/alternate 
plumages, much less so in the winter/basic plumages)

3) With regard to the Livermore Swamp Sparrow (a 1st winter bird), it 
appears to be in the process of this spring molt as Stephen mentioned. 
(For example the nape area looks pretty gray already, not buffy. But the 
face is still not quite gray. Crown is dark brown with no rusty.)

4) An unstreaked bright rusty crown patch often mentioned in various 
field guides, etc. for the breeding adult plumages may or may not 
clearly emerge in this particular sparrow, depending on, in large part, 
the sex. (Due to the aforementioned seasonal sexual dimorphism, it would 
be more likely if it turns out to be a male; it would be less likely if 
it turns out to be a female.)

Lastly, here are some relevant excerpts from these two papers:

- "Swamp Sparrows complete a molt in the autumn (late August-October; 
Dwight 1900, pers. observ.) replacing most of the contour feathers in 
juveniles and, additionally, the flight and tail feathers in adults. A 
prenuptial molt involves primarily the head, nape, and throat feathers 
(pers. observ. of captive Swamp Sparrows). Therefore, summer specimens 
have relatively worn body, wing, and tail feathers, yet have relatively 
fresh plumage over the head region." [1]

- "Swamp Sparrows (Melospiza georgiana) have crowns that range from dull 
brown to bright rust. Although most descriptions state correctly that 
Swamp Sparrows in the non-breeding season generally have duller crowns 
than in the breeding season, within seasons the variation in crown color 
has been almost universally ascribed to differences in age ... Field 
identification guides have often attributed most variation in crown 
color to age." [2]

- "Breeding season specimens show strong dimorphism (Fig. 1): 85% of 
males and only 10% of the females had completely rusty crowns (crown 
class 4)." [2]

- "Swamp Sparrows show significant dimorphism in crown color in all 
plumages, but only in the breeding season is the overlap in the 
distribution of the two sexes sufficiently small to be helpful in the 
determining the sex of an individual. This dimorphism results primarily 
from males acquiring complete rusty caps." [2]


Hope this helps. Good birding!

-Akira
Pleasanton, CA
(Alameda County)

Disclaimer: I didn't even check to see if any newer researches existed, 
so above may not reflect the latest findings. Please feel free to add to 
this or correct me if anyone has better info. Thanks.