The bug
A parrot summoned with a negative Age has a smaller hitbox, as if it was a baby.
Since MC-148210 has been fixed, this report should be valid as well.
How to reproduce
Enable hitboxes being shown:
F3+BSummon a parrot with a negative
Age/summon parrot ~ ~ ~ {Age:-12000,NoAI:1b}/summon parrot ~ ~ ~ {Age:-12000,NoAI:1b}
Confirmed in 1.14 Release