Buy some gutter covers; they are cheap and easy to install.
Gutter guards properly installed will deter birds (and debris) from entering your gutters. Improperly installed (no end caps) will actually invite birds in since it provides shelter that’s secure for them.
Gutter screen guards are pretty easy to install and can be found at the big hardware store. This will solve your problem.
Gutter guards will work. They also have the foam type that goes inside the gutter, which only lets water through. These also don’t have to be cleaned as often as they catch the debris and it sits on top and can be swept or blown off very easily. Both are available at any Lowes, Home Depot etc.
Provide and install gutter covers and regular maintenance of your gutters will insure that birds will not nest in your gutters.
The two best, most effective items: gutter covers, and a fake owl on the roof.
We have that problem out here in california quite often and we use a gutter guard/screen , seems to work well but even if you install the guard you should have your gutters cleaned every year to keep the functioning properly.