Great photos - I really like them. What camera, lens and shutter speed? How far is the feeder in front of the cardboard? Ruby throated females? I assume the reddish tint comes from the light reflection off of the feeder.
I've had to use some similar tactics. I take down all the feeders but one along two sides of the screened-in porch and take the glass out of the window with the remaining feeder. I also close the blinds except for the one open window. I have to sit back from the opening. I wait to hear them coming and try to get the camera prefocused. The birds don't like seeing me, the camera and tripod, or hearing the sound of the shutter. I haven't tried putting up a piece of cardboard. I like your idea and will try it. Thanks for posting. And I may try it with the deer and turkeys that come around the farm yard.
