Are you sure that your question reflects what it is that you wish to know? For a start, it depends on which bird you have in mind. I leave it as an exercise for the advanced student to excogitate, explicate, and rationalise the case for the ratites and Spheniscidae, but what makes you think that the Trochilidae cannot do VTOL? They even are able to fly backwards, and do so more routinely and insouciantly than any VIFF-capable aircraft, such as a Harrier jet. And although the humming of their wings gives those birds their popular name, it is a lot quieter than the jet in any mode.
As for why other birds don't do it (though many, such as partridge, routinely exhibit an unnerving capacity for abrupt and noisy STOL), you will notice instantly of course, that the mode of take-off is associated with their mode of flight, which in turn is associated with their need for and application of flight. Not at all surprisingly the same applies to the insects, by the way, many of which, such as the dragonflies, bees, and hover flies, can rival the hummingbirds at take-off and in flight in any direction. Insects with no such need tend to fly forwards, much as most birds do, and for much the same reason.
Birds however, have larger Reynolds numbers than most insects, and nearly all flying birds fly by far the most efficiently and effectively when flying forward. Hovering is expensive and exhausting, and so is take-off. VTOL is worst case because it not only is not efficient for most birds (though many of them can manage it in a pinch) but leaves them in mid air with no forward velocity and therefore with half the job of getting stably airborne still to do. For them it simply is an expensive, dangerous and unrewarding action, much as it is for a Harrier jet as well, by the way! Some of the smaller ducks, the dabblers such as wigeon and teal for example, can explode off the water as vertically as they like, by adding a tail flap to their first wingstroke, so as to use the water beneath as reaction mass. But they do not normally do so, for the same lack of incentive. They use the tail flap to help them off stilll water without a long take-off run.
For most birds and most insects, flying is a very utilitarian thing. They do what they need, and they adapt as they need. Or they die out. Sometimes they scrap abilities that they can do without, when they can substitute something else instead. For example, certain bees in Southern Africa at least, have lost the ability to hover, whereas most bees are great exponents of hovering for all sorts of purposes. These non-hovering bees are excellent at forward flight istead. So what do these bees do when males wish to stay in one spot when waiting for female company? The fly upwards and relax, fall back for several centimetres, then repeat the process, possibly for hours. And these bees bouncing up and down over the veld advertise their presence very selectivley to passing females. VTOL means nothing in their lives.
Birds are not great exponents of hovering either, unless it is a big thing in their lives for special purposes. For example, scavenging species of the smaller gulls oftn hover when inspecting their options above floating carrion. Terns and kingfishers hover when preparing to dive on fish. Kestrels and kites hover in a very similar manner when hunting mice or lizards. In fact, kestrels commonly are known as "windhovers", or equivalently as "windvalkies" in Afrikaans. But such behaviour, so closely related to VTOL, is atypical. Often when you see a bird hanging more or less motionless in the air, it really is doing dynamic soaring in a ripple of wind over a ridge.
Some birds go to the other extreme and certainly cannot do even STOL. Conspicuous examples include heavy species that have to stay aloft for long periods, sometimes for high journeys, sometimes sleeping on the wing, such as geese, swans and albatrosses that can hardly take off at all without a long horizontal run or help from passing waves. Other examples include those magical, breathtaking fliers, the swifts. Many cannot take off at all, unless they have a start getting into the air. They also cannot even hover, but fly. . . Oh they can fly!!!
And as for landing, most birds that have a lot of work to do in taking off, also have to take care in landing, or the energy that they had put into getting off the ground and up to speed may do them a mischief when they shed it again so as to come to rest at ground level. The way in which say, a large pelican water-skis to a halt is a beautiful display: Joe Cool showing off to the admiring multitude.
The way in which some albatrosses routinely crash-land is positively unnerving.
Now, what was it you really wanted to ask?
Jon
