The bicycle provided mankind with a mode of transport that was more efficient than walking or running, almost right from the get-go. The first wooden velocipede was barely there, but it was there. Once past the velocipede stage, a man with a bicycle was already more efficient and capable than man and horse. What makes this efficiency possible is the wheel. In order for an avian species to realize a similar gain, or ANY gain, they would have to invent something that functioned, for flight, in the manner that a wheel does on the ground - by making moving through the air more energy-efficient. The proposition of machines or tools to enhance an avian's flight is mostly nonsensical, given the physics of our known world. However, there might be some exceptions. I'll return to the wheel later.
ALL human flying machines are LESS efficient than any avian's nature-given ability*. All flight involves 3 stages: ascending, traveling, and descending. Avian physiology is dictated by these requirements. Ascent requires the most significant energy expenditure. Gliding is traveling and descending. For flighted birds, the energy requirement for gliding is near zero. Adding a gliding device to the ascent would add weight to the ascent, and would require greater energy expenditure to attain altitude. Thus, the net gain would be negative for a single glide-slope.
Some avians use multiple glide-slopes by utilizing updrafts to ascend. You could offer improved glide characteristics for avians who are poor gliders, but such avians would likely not have the innate flight power required for ascent with the added weight (think turkeys). But gliders, or paragliders, are out, due to insufficient gains and too high an energy cost. **
Most avians have sufficient muscle power to provide lift. Musculature for other uses is reduced to optimize weight to lift ratios. Most avians would have to use their wings to power an avian-powered machine. Adding any device adds weight, with no consequent increase in power (or reduction in lift requirements or enhancement of the power to lift ratio). This would negate any gain. The only way an avian-powered machine could improve on the avian's flight capacity would be if the avian was flightless, like an ostrich, or an emu. A flight-capable avian species could not have enough muscle power in other muscle groups, due to weight restrictions required for lift and flight, to power some driving force, such as a propeller. So, you'd have to have a flightless avian species.
Now, when it comes to powered aircraft, it is conceivable that an avian species could invent such. Just like for earth-bound humans, the power source would have to be essentially free or low cost, but such a development arc is conceivable. After all, the development of mechanical engines came about because of ground-based needs. And, an avian species could have an ecology with similar needs, where, at some point in time; a mechanical engine provided greater efficiency and productivity than attainable without the device.
But an avian-powered flying machine would have to be more efficient than the avian or there's no point. The machine would have to have some way to act as a multiplier to the avian's innate efficiency. I could imagine that a sapient flying squirrel could invent something like a glider - to make them more efficient at gliding, and to able to use thermals to gain altitude. But a flying squirrel is not an avian. Flying is not its nature.
Returning to the wheel. Man's natural means of transportation, walking and running, are on the ground. These natural means of transportation were made more efficient by the invention of the wheel. The combination of wheels, in a relatively light and steerable human-powered device, rendered a huge increase in energy-efficiency. In order for an avian species to realize a similar gain, or ANY gain, they would have to invent something that functioned, for flight, in the manner that a wheel does on the ground - by making moving through the air more energy-efficient. The only thing I can think of that might fit would be some sort of small anti-gravity device (currently unknown technology). Balloons don't work to enhance flight due to wind resistance. Design a world where your avian species has anti-gravity devices, and those devices could improve flying efficiency.
I have, in comments, posited some possible exceptions to what I've argued here. Mankind has, to date, not invented anything (non-motorized) that would logically enhance a flighted bird's natural abilities to fly. You can't make an albatross out of a turkey, due to the physical limitations of each body. The turkey doesn't have the musculature to power the albatross's flight gear. The albatross can't walk on ground as well as the turkey. However, we are dealing with science fiction. If you posited materials development that was lighter, more flexible, more workable, and stronger than anything yet known to man, you might be able to design something that would work. Or, if you had anti-gravity capacity that was as small as the avian or smaller (small size required due to wind resistance problems). A bicycle works because a man's major muscle group is almost completely re-tasked to do something that was a relatively minor function. Legs no longer had to hold us upright. All of a sudden they were only devoted to moving us forward, and they could use all that "standing-up" energy to move forward.
*This is not entirely true, as an aircraft like the Gossamer Albatross is more energy efficient than some flighted creatures. But for the sake of simplicity, and not writing a book, that is the exception, not the rule.
**Because of the extended discussions involved in posting comments on this question, and editing this answer, I have realized that there might be one way, given known technology (and physics), for an intelligent avian species to enhance flight capacity, although in a limited way, and only for avians with limited flight capacity, or non-flighted. Lighter-than-air craft would not work to enhance flight, due to wind resistance. However, let us propose that our alien avian is like a turkey, with limited flight capacity, but intelligent. A lighter-than-air balloon could be used for ascent, carrying our avian and a gliding device. Thus, the balloon delivers the primary energy requirement for a flight. Once at altitude, the gliding device could then be deployed, and our avian has a net gain. This isn't precisely avian-powered, but it could be an enhancement.