http://www.ucmp.berkeley.edu/diapsids/dinomm.html
One important dinosaurian synapomorphy is the perforate [completely open] acetabulum, simply a "hip bone" (actually three connected bones, together called the pelvis) with a hole in the center where the head of the femur ("thigh bone") sits. This construction of the hip joint makes an erect stance (hindlimbs located directly beneath the body) necessary — like most mammals, but unlike other reptiles which have a less erect and more sprawling posture. Dinosaurs are unique among all tetrapods in having this perforate [completely open] acetabulum.
Notice that I have added the words "completely open" within square brackets. I add that because as the quote says, it is:
"actually three connected bones, together called the pelvis with a hole in the center where the head of the femur ("thigh bone") sits".
In other words, it is completely open.
But birds do not have a completely open acetabulum. So it is misleading to say they both have a "perforate" acetabulum.
One needs to distinguish between actual dinosaurs (eg. tyrannosaurids) on the one hand and flying (and secondarily flightless) feathered Paraves with long bony tail (eg. long tailed Scansoriopterygids) on the other.
