During a visit to Tualatin Hills Nature Park, I was watching a Cassin’s Vireo pulling material from tree trunks. Eventually she flew up to the nest site, where we can see her nest under construction.
Below is a typical view of a Cassin’s Vireo in the canopy (backlit and facing away). She was weaving material into the nest, then sat down to assure a good fit.
Ethical Birder 