My understanding is that starlings and English sparrows were introduced to the US by a Shakespeare society. Apparently they thought every bird mentioned in his plays should be flitting around Central Park in NY.
In 1890 about 60 starlings were imported to the United States by a group who wanted to introduce all the birds mentioned by William Shakespeare in his plays.Due to the starling’s ability to mimic human speech Shakespeare chose to include the starling in Henry IV, “The king forbade my tongue to speak of Mortimer. But I will find him when he is asleep, and in his ear I’ll holler ‘Mortimer!’ Nay I’ll have a starling shall be taught to speak nothing but Mortimer, and give it to him to keep his anger still in motion.” This is the only instance where Shakespeare mentions starlings. The starlings were released in New York City’s Central Park. Another 40 starlings were released a year later. The birds multiplied rapidly and spread into surrounding areas.
Right up there with Kudzu (brought over for erosion control).
If you can’t nest because European starlings and European sparrows are in all your nests, you can’t reproduce at the rate that’s ideal, and eventually you die. The Cornell Lab is too scientific-oriented to spell it out like that, but it’s all clearly implied.
Yes, they’re increasing now because of increased human planting of appropriate nest boxes. In other words, their natural nesting places are over-run with European starlings and European sparrows. In addition, I have witness this phenomenon first-hand. It’s not that difficult to understand.
The difference, which I guess you’re not seeing (?) is that House Sparrows are invasive species, whereas the bluebirds are native species being threatened by invasives.
“All” the bluebirds are not being killed by House sparrow and Starlings. The reasons for decline of the species, bluebirds, is much more complicated than the Cornell Lab website discusses. It includes pesticide poisoning, habitat loss in addition to competition with other native cavity nesters, not just Starlings and House Sparrows. But bluebirds will likely always need human intervention to maintain a healthy population size.