In many bird species, males and females look alike. In Antarctica, I once heard a tourist ask one of our naturalists how to differentiate male penguins from females. His response: If you see it lay an egg, it is a female. In other words, they all look alike.
In the next 6 posts, I will show various bird species, all of which have been spotted on my Toronto area nature walks over the years, in which the female and the male do NOT look alike.
Female Gadwalls look somewhat like smaller female Mallards. The gray bill bordered with orange-yellow along the sides is one way of differentiating the 2 species. Another is to see who the duck is swimming with; Mallards or Gadwalls.
In Downy Woodpecker and Hairy Woodpecker, the female lacks the red nape patch of the male.
Downy Woodpeckers are about the size of a House Sparrow while Hairy Woodpeckers are about the size of, or slightly smaller than a Robin.
On occasion, people send me photos of a bird that they have seen but which they cannot identify. Frequently it is the female House Sparrow.