Keeping your roof gutter free of debris is hard enough when all you're dealing with is errant leaves and uncooperative wind. Add in birds and their nests, and you have a whole new chapter of trouble to get through. That is, unless you add a gutter screen. You can't stop birds from deciding to set up a nest on your roof or gutter, but you can prevent them from building it actually in the gutter and creating a blockage.
Rounded Gutter Screens May Help
Most gutter guards or screens lie flat or have the tiniest bulge to them, but you can get gutter screens that are more rounded. These may help ward away some birds. You might have some intrepid ones try to set up a nest in the space between the roof and one side of the rounded screen — it can form a nice little wedge-shaped space — but at least that nest won't interfere with the flow of water through the gutter. And, because the nest would likely be off-center, it wouldn't block rain from falling straight into the gutter.
Birds Nests Get a Break
One of the reasons that birds' nests can be so annoying to deal with (other than the fact that the nest creates a blockage) is that in many cases, you can't remove the nest until the young birds have left and the parent birds abandon the nest. That's particularly destructive if the bird's nest is inside a gutter, so having a screen on top is really the best way to avoid that problem.
By the way, even if the nest is off to the side of the gutter or sitting on top of the screen, bits and pieces of the nest can make their way down into the gutter through the holes in the screen. That means you'll have to very carefully brush off the top of the screen periodically. Be extremely careful not to disturb the nest.
A Word About Gutter Spikes
The next time you're in an area known for pigeons, look at the upper stories of the buildings in the area, particularly the ones that have ledges. You'll likely see a bunch of spikes meant to prevent the birds from sitting on those ledges. If you have a persistent problem with birds, you can find similar spikes meant to be installed on the gutters. Be careful with these, as working around roof gutters is already kind of dangerous due to the fact that you have to be on a ladder to reach them. Adding spikes could make cleaning gutter screens more difficult.
Start with adding the screens and see how that goes. Your first concern is to prevent the gutter from becoming blocked. Contact a gutter company to arrange for gutter screen installation as soon as possible.