This morning I went down along the river to a small drainage where I found a Marsh Wren building a nest yesterday. Yesterday my assistant demanded we leave before I got to spend much time watching the nest building. Today I was by myself and I spent a couple of hours watching him work.
He spent a good deal of time singing from the cattails in the vicinity. Occasionally he would fly across the path I was sitting on and into the neighbors territory. Then the neighbor would do the same thing back. They would make these short loops through each others territory two or three times with no singing or other interaction other than the invasion of airspace and then they would each get back to the business at hand - nest building and singing.
Nest building centered around gathering two types of nesting materials. First was the fluff from year old cattail heads. This was the most fun to watch. He would dart from the nest area up to the cattail heads and then back to the nest with a mouth full of fluff.
The other source of nest building materials was the old soggy cattail remnants floating on the water. He would fly down to the base of the cattails and pull these long noodles out of the water then muscle them back to the nest location and pull them into the the mass of standing dead cattails where the nest was being constructed. You can just see the wren pulling the dark soggy cattail in the center of the photo.
More fluff. He would alternate nest materials with about 3 trips per each material before switching.