Why Female Red-winged Blackbirds only?

[W. Robert Shade III]

I have as many as 50 or more Red-winged Blackbirds swarming my feeders every morning. Why are they all females? Males do not look like females in winter do they? This means they must spend the winter in different places. If so, what is the rationale for that? I cannot think of any other species that segregates by gender in winter. 

Bob Shade

Green Mountain

Lakewood

[Scott]

My guess is that the birds your seeing are young males. Many males, especially juveniles have a rusty edge to their feathers that will wear off my spring. This may make them appear like females.

 From what I have been seeing near Fort Collins and Loveland, all the Red-wings that I am seeing are males.

Scott Rashid

Estes Park

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[Charlie Chase]

All males here as well,  both along the South Platte in Denver and on the Barr Lake CBC.  Definitely a mix of young and adult plumaged birds.  And try as I might none of them could be converted to Rusty's.

Charlie Chase

Denver

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[Meredith McBurney]

Like Charlie, I did the Barr Lake CBC, and my section was a big open area north and east of the lake.  I found adult males in a couple of marshy areas and then found a tree-full of females (perhaps some young males....) in a yard in a more residential area.  Sibley says they "often segregate by sex".

[Derek Lecy]

Hi All,

I'm out here in the Bay Area, CA but this topic recently surfaced with regards to observers noting that Red-winged Blackbirds appear in single-sex flocks near the coast in winter. Alvaro Jaramillo, who literally wrote the book on New World Blackbirds (icterids), noted the following.

Often they are in all female flocks around here, and only in winter. The males are not as common on the coast as the females, but they are frequent in the Central Valley in winter. Again, they segregate in winter into single sex flocks. 

When asked if Tricolored Blackbirds segregate into single sex flocks, this was Alvaro's response:

It is a classic Red-winged behavior, it seems to me that Tricolored
blackbirds are mixed but in general I see a preponderance of males on the
coast. Yellow-headed Blackbirds also segregate, it would be interesting to
see what Great-tailed Grackles do in areas where they are common. The reason
is that sexual size dimorphism may be one of the factors in sex segregated
flocks. Certainly oropendolas in the Neotropics segregate during the
non-breeding season, and they have the largest size dimorphism of any bird.
Great-tailed grackles are not too far behind.

I hope this info is of interest.

Regards,

Derek Lecy

San Rafael, CA (but born and raised in Boulder)

[davecp...@gmail.com]

This thread caught my eye.  I saw huge flock out north of DIA and noticed the same thing after looking a photos.  Only a few males mixed in as far as I can tell.

See high resolution photo...  https://flic.kr/p/2kntCXj