LOL. I just figured Mr. Robin was making a nest for his pregnant wife! And with all the hormones raging, I can only imagine her response when he came home to tell her his nest-in-progress was on the ground.
Nest building is done by females in certain species and males in other species. Upon researching this after Mom emailed me, I found out that female robins indeed build the nest and incubate the eggs, but both parents search for nesting materials and feed the nestlings and fledglings. I didn't have a very good view of the bird building the nest, so I'm thinking it was actually Mrs. Robin that I was watching.
If you're wondering why a robin is building a nest in the dog days of summer, I also found that in most North American locations, robins nest two times each year, and in some places three times.
Check out this account of a robin's nest...