A Bird Catcher has Smoother transition from Idle to full throttle. Witch makers it easy to drive and tune at lower RPM’s ( See Below) Here is some picture of my Bug Catcher on a 871
http://www.***boat.com/image_center/...38PIC00055.JPG
http://www.***boat.com/image_center/...38PIC00044.JPG
In diagram A, below the machine surface on a Bird Catcher extends deeper into the hat.
http://www.***boat.com/image_center/...04/738hata.jpg
In diagram B on the Bug Catcher the surface drops off in to the casting, so as soon as the top side off the bug catchers butterflies get to that point it will flow more air then the Bird Catcher. This makes for a quicker transition from low RPM's to to higher RPM's, and it also gives you less resolution in the throttle at lower RPM's.
http://www.***boat.com/image_center/...04/738hatb.jpg