The male and female American Goldfinch usually wait until the trees have adequate foliage to camouflage in before they begin flirting about and nesting, respectively. The male, shown here front and back, is very bright yellow with black tail and wing features and a black crown.
The female (not shown) has no black on its head and has a brownish yellow underbelly and brown wing and tail feathers, but is also very camouflaged in bare branches and Spring leaves of the trees.
While bright red male Cardinals seem purposefully to expose themselves on open branches, possibly to make attracting females easier, the girls stay more hidden in brush and tree branches. In many incidences, that makes them harder to photograph.
In fact, since earlier in the Spring this season, I’ve gotten one good chance at capturing a female in the only place I’m able to photograph — the backyard of our apartment building.
Female Cardinals are a good example of how bright coloration favors the male in the bird world, probably for the reason mentioned above: being the mating aggressor, the male needs to be seen more readily for purposes of necessary attraction.
In the second photo, this same female has gone down closer to the riverbank (note the rocks) in search of nesting materials. She holds a tiny branch in her beak.