On a short timescale, feeding birds (like hummingbirds) may seem to be OK. However, hummingbirds are supposed to get their food from the nectar of flowers that are coevolved to favor each other. Feeding these birds will reduce the number of flowers that the birds will need to visit, reducing (subtly) the pollination rate of those flowers and causing less of them to produce seeds. On a shorter timescale, this likely will not have much impact, but over hundreds or thousands of years it will lead to the plants that hummingbirds formerly relied on reducing in abundance and range. Hummingbirds will then be dependent on their new food source. Natural ecosystems are complex systems and can be dramatically perturbed by very small inputs. An an example, you could look at how an invasive species of ant has effected lions in Kenya (https://www.science.org/content/article/how-ants-thwarted-lions-african-savanna). Here a seemingly minor change in the ecosystem, the arrival of a new species of ant, has dramatic knock on effects that eventually result in significant effects on the prey availability to lions. I'm not saying: Don't feed the birds. But, feeding the birds will have an effect, possibly one that cannot easily, or even possibly, be determined beforehand. Everything we do has effects, and some are larger than their input would suggest, since they act on complex and chaotic systems.