Relative size apparently makes no difference when birds are competing for nesting cavities. In this case the Northern Flickers are twelve times larger than the house wren (130 g vs 11 g).
Last week in northern Utah a pair of Swainson’s Hawks mated right in front of me. I’ve seen it happen fairly often over the years but this time they were close, I had good light, an unobstructed view and my camera was ready. I couldn’t believe my good fortune.