Actually, take-off is the proper term for that stage of bird flight.
I remember reading about take off angles in wild turkeys and bred turkeys (to stock the forests), that wild turkeys had a steeper angle of incline than those that were raised in sanctuaries. From this, they could tell how much interbreeding was being done in the wild after release (the more wild genes, the steeper).
An article in the Journal of Experimental Biology (Effects of Body Size on Take-off Flight Performance in Phasianidae [Aves]) studied take off angles in Pheasants and other birds in that family.
They cite other studies of bird take offs from hummingbirds to swans...
...this trend will explain aspects of the ecology and evolution of flight. For example, within the Anseriformes, teal (Anas spp.) take off vertically while swans
(Cygnus spp.) take off in a laboured manner, with a shallow initial angle of ascent. Since teal require less room than swans for take-off, teal may utilize smaller ponds with less open surface area...
before getting down to their own work:
To evaluate the mechanisms responsible for relationships between body mass and maximum take-off performance in birds, we studied four species in the
Phasianidae: northern bobwhite (Colinus virginianus), chukar (Alectoris chukar), ring-necked pheasant (Phasianus colchicus) and wild turkey (Meleagris gallopavo).
One of their conclusions (a simple one comparatively):
Take-off power (Pta) scaled in proportion to m^0.75 and pectoralis mass-specific Pta declined with increasing body mass, roughly in direct proportion to wingbeat frequency.
From another study from the scientific literature (this one measuring seasonal differences and take-off angles):
In particular, the ability to initiate flight through jumping is critical to predator avoidance and may be influenced by changes in body mass (Mb). Here we investigate seasonal differences in the jump take-off performance of high Arctic Svalbard rock ptarmigan (Lagopus muta hyperborea)...
Anyway, you get the picture. Apparently ornithologists refer to 'lift off' as take off (with or without the hyphen). It starts with the first downbeat of the wings once the feet are no longer in contact with any surface.