How do I prevent birds from nesting on top of my downspouts?

If this is a constant problem use Gutter Stuff EZ 5-Inch K Style Foam Gutter Filter and 3" and 4" Round Wire Strainers for the downspouts. You can purchase them at Home Depot or Lowes or online. Easy to install.

i solved this problem with screen gutter covers from the local hardware store. They are not very expensive your you can just cover your downspouts with chicken wire. The rain get through and the birds will not be able to nest down in.

You could put something difficult to build a nest on there.

Buy a plastic owl and perch near area or hang a rubber/plastic bat from the corners of the spouts. Works well for woodpeckers that like to peck holes in wood siding.