wildbird
It's not necessarily their behavior at feeders which is the concern.
It is their behavior at their nesting sites.
I'm about to get graphic. Here is why HOSPs are brutal aggressive predators.
HOSPs are cavity nesters. So are bluebirds and Purple Martins. But HOSPs are not EXCLUSIVE cavity nesters. They can nest jut about anywhere.
HOSPs are the rats of the bird world - they lay up to 5 clutches of 4-7 eggs every season. They start breeding earlier than native birds so that's one problem - they occupy many available nest cavities before other birds are even preparing for nesting.
Secondly they do not tolerate neighbors. They will attack and kill the adult parents, any hatchlings, and destroy eggs - then occupy that cavity and build another nest RIGHT ON TOP of the corpses of its previous occupants. They don't just protect their own nest; they are territorial about the entire AREA. They will kill any other bird nesting in their area if they can, and they can kill many birds larger than themselves .
If you put up nesting boxes for cavity nesters like bluebirds and purple martins, yes, you need to be ready to check them regularly and kill any HOSPs that attempt to use the nesting boxes. HOSPs are dangerous even to very small birds even when the nest opening is too small for them to enter as they will reach through even a very small nest opening and harass and kill its occupants even if the HOSPs cant get in themselves. They will hang around the opening and drive off a parent bird trying to return to the nest with food, or trap a parent inside. They are persistent and aggressive.
HOSPs will never be welcome at any feeder of mine. They will mob feeders that are not HOSP-proofed and drive off all the other birds. Sometimes you can get away with it and sometimes you can't. In MO none of my feeders were actually mobbed, but here in NV, I had so many HOSPs at my feeder once they found it that they emptied it in minutes, and drove off every other bird, including quail, pigeons, and the much larger mountain and Stellar jays. Also all my hummingbirds, who never came back.
Hawks do eat native songbirds, but they don't eat them exclusively and they don't prevent the nesting of hundreds of thousands of birds every year. That is a natural part of our native songbird's environment that they have evolved to deal with over tens and even hundreds of thousands of years.
Bluebirds were driven to near extinction by this kind of pressure - until wild bird lovers started putting up MONITORED nesting boxes for them. You have to check the nests and kill any encroaching HOSPs or these nesting sites, also, are taken over and bluebirds driven out.
http://www.daggerpress.com/2010/07/03/l ... -the-open/
https://www.purplemartin.org/forum/view ... hp?t=18616
HOSPs are not evil. They are just doing what they have evolved to do - but they are doing it in areas where the native birds have NOT evolved along with them. So they cause great ecological damage.
They all seem to hang out together.
