Kill pattern suggests coons (several) did that. Typical this time of year with momma teaching this year's kits to hunt.
If you decide you want to try an electric fence, and don't have house current nearby, electric fencer can always run on batteries or even solar. Couple strands of hot wire...
You didn't say the nature of how the birds were killed....bite marks, etc. But for every bird killed by a weasel, hundreds and hundreds are killed by coons. More information would help clarify.
Most folks don't realize it, but a solid covered roof.....one that enables enclosure on all four...