It depends on how one defines singing.

I have heard seen females that were doing weak songs
routinely in the winter. I once called a female into
my campsite in the central Cascades by imitating the
a Varied Thrush which responded with weak songs.

Ian Paulsen wrote:
>
> HI ALL:
> In New York, there's a Varied Thrush identified as a female, but it is
> singing. Has anyone heard a female Varied Thrush sing?
>
> Ian Paulsen
> Bainbridge Island, WA, USA
> ipaulsen at krl.org
> A.K.A.: "Birdbooker"
> "Rallidae all the way"

--
Mike Patterson
Astoria, OR
celata at pacifier.com

A child who becomes acquainted with the birds about him
hears every sound and puzzles out its meaning with a cleverness
that amazes those with ears who hear not.

-Neltje Blanchan

http://www.pacifier.com/~mpatters/bird/bird.html