You may need to use a lot of cement blocks to be heavy enough if your bank of nests is very big.
You might consider sinking a post or two or four instead. Do you have a post hole digger? Around here, they can be rented. They are much better than digging the hole with a shovel because so much less of the ground is disturbed. Then tamp dirt around the posts; don't cement them in. I'd go 30" - 3' deep.
It doesn't matter how high above the ground the posts are; you can tie to them at ground level if you want. Or have them high enough to hold a roof and/or siding on three sides (now or as an option for later) if you want more protection for the nests.
You might consider sinking a post or two or four instead. Do you have a post hole digger? Around here, they can be rented. They are much better than digging the hole with a shovel because so much less of the ground is disturbed. Then tamp dirt around the posts; don't cement them in. I'd go 30" - 3' deep.
It doesn't matter how high above the ground the posts are; you can tie to them at ground level if you want. Or have them high enough to hold a roof and/or siding on three sides (now or as an option for later) if you want more protection for the nests.