I realize that the easiest approach is to look at one thing, make a quick decision and attack, but I have to point out a few things.
Songbirds have declined in the last 40 years, so could it be Sparrows? Well they have been here 150 years, so why did they start a decline in their competitors 40 years ago? Starlings, cats, snakes, hawks, Blue Jays, they all take some toll, but thats been going on for 100 years.
What has dramatically increased in the last 40 years is mans obsession with order. We buy an acreage for development and cut every tree and bush, we pave part of it, cover part of it with houses, and whats left we strip of of any bugs or seed producing plants, or seed producing wild plants. We plant nice open trees and shrubs that keep the pesky pooping birds from nesting, kill the bugs so they can't eat, and most of all get rid of any plants that drop seeds or fruits on the nice lawn, even if the birds would clean them up eventually. We like "ornamentals", they look nice but no seeds or fruits like a messy native Crabapple which we can replace with a Flowering Crab that produces no food for critters.
No Meadowlarks on the fences? How about no fence rows, how about Fescue instead of native grasses, how about no bugs or seeds, no food, no where to nest or hide?
On 76 west of Branson they've cleared a ridgetop and the sides down to dirt and rock, its probably a 360 or bigger, and there's nothing left for any wild animal, but it will produce more humans. I'm sure they animals that were pushed out aren't going to find an uninhabited area to populate, so I suppose most will die off taking others with them in the process because of overcrowding in an area nearby.
So my vote for the most detrimental animal to birds is you.