Animals don’t like to advertise the location of the places where they are raising their young and so they hide them. Often these places are often hidden among the leaves of trees. When winter arrives, the leaves disappear and these animal constructions become pretty obvious to an observer. Of course, in the case of bird nests, it doesn’t really matter since the birds and their offspring have long since migrated south to warmer climes.
I keep spotting the bird nests in places I have walked past countless times during the summer, totally unaware of their existence. Same goes for this wasp nest, which I failed to see until all of the leaves dropped off of the willow that supported it.
Nature has so much going on that we are never aware of, secrecy is one of the reasons why some species have survived for so long.