I've spent hours during Summer evenings watching Bluebirds feed off the power line. It's a perfect spot for them to sit and wait on an insect to show up in the mowed yard below. As you said, I've counted as many as 32 there at one time. By the end of Summer they are used to me as well and never fly away because I'm in the yard. They also like to gather on the line when I'm mowing. Purple Martins enjoy that too. I don't have cats either.
Gotta figure out a way to get rid of the B-Martins or Tree Swallows or I'm afraid I'll not have Bluebirds or Purple Martins. Real Bummer!!!
Tree swallows are good birds. They eat just as many bugs as purple martins. They are in the same family.
Barn swallows are good too, if you don't mind them building mud nests in the tops of any open barns or animal shelters and pooping from the nests..
We have 14 blue bird boxes and it usually ends up about half of them are occupied by tree swallow families.
They are very protective of their young and help keep the sparrows away.
Sparrows love to bust bluebird eggs or kill the babies in the nest. They try that in the boxes having tree swallows and the sparrows
either get killed or driven away by the swallows.
Folks love to talk about purple martins and how they eat so many bugs and mosquitoes. Dunno, we don't have many daytime mosquitoes up here that bite. Lots of big ones in the woods but they don't bite.
The tree swallows fly around in droves while I'm mowing or on their own during the day grabbing bugs out of the air to feed their young.
We have a pond and they patrol it well too.
Just build some more bluebird boxes, put double thick wood around the hole to protect from predators, hang them the same height and enjoy all the birds and the benefits they give. If you see sparrows taking over a nest go into action.