In my experience Parsons are disturbers, rather than aggressors......as are Longtails, but to a slightly lesser degree. My Masks are ten-fold more timid by comparison.
I have just re-organised and housed what I believe to be "aggressive/territorial" species with my Parsons........That is, my Stars (which I rate as territorial) with two pairs of Parsons and a pair of Purple Crowned Lorikeets.........and three pairs of Parsons with a pair of Crimsons.....which I rate as "tough"....and "territorial" and Little Lorikeets......neither of these species will take a backward step when a Parson decides to interfere
After moving things around my Yellow rumps are nesting again

.........they are now with White Eared Masks......My Parsons would parasitise the Yellow Rumps nest.....my Yellow Rumps were good foster parents

.....raising 4 Parsons
If I had the space I wouldnt have any other species with Parsons........they do not drive or hurt other birds, they just interfere

...........and I have learnt the value of removing young Parsons.....they too interfere and have an impact on production......but thats a general rule we all learn......and break
I dont have longtails at the present.....they'tre on the list for later.....But I did have them and bred them in mixed collection in the 80's.........showy birds, great mannerisms.....but also interfering, not killers..........but my other birds still bred in the company of Longtails, whereas I rate Parsons as having more impact on other species breeding performance..........IMO................And why have I picked Parsons to specialise in
