Is there any benefit of using a swept wings at low-subsonic speeds?
Birds adjust wing area by pulling in their humerus and ulna so the wing sweeps first forward, and to compensate for that, have to sweep their outer wing backwards which is accomplished by folding the digits backwards. Instead of adjusting lift coefficient, they adjust wing area to not expose more surface to air friction than necessary.
The picture below shows how the wing is opened for maximum lift at low speed. As Zeiss Ikon correctly observes, folding the digits back and fore is the bird's way of pitch control, so the sweep is a consequence of pitch control, not aerodynamics.

Red-tailed Hawk, captive bird, Bacara, Santa Barbara, California. By Steve Jurvetson / CC BY (https://creativecommons.org/licenses/by/2.0)