I think any advice regarding light should be considered good rule-of-thumb, but I'd caution not to take any rules as unbreakable. In general, most will say to shoot on sunny days when possible, shoot birds in open areas with direct light, keep the light behind you and on the subject, avoid noon and high-in-the-sky sun shots which produce high contrast and flat, poor directional light, and so on. All good tips - but not hard fast rules one must adhere to. As others mentioned, the gear can make a difference - better sensors, better focus systems, better lenses, faster apertures can all help to counter poorer lighting conditions. Good processing skill can drastically recover noisy, dull, or off-color images. And sometimes, even straight-up 'breaking the rules' can result in an interesting photo because it's different from all the others - shooting into the sun can create interesting backlight and halo effects, shooting in heavy shadow or shade can produce interesting light and shadowplay on the bird, highlighting a certain feature or hiding details on a low-key style.
When I was shooting with P&S digital cameras with small sensors back when I started, I tried to keep my ISO at 400 or less - anything else produced such a mess of noise that even good processing skill couldn't recover much. So I was more cognizant of getting very good light. As I moved into larger sensors and interchangeable lens cameras, I was willing to stray up to ISO 1,600, even 3,200. As those sensors got better and better, I was willing to stretch to regular ISO 6,400, even ISO 12,800 range. All things improved - sensor performance, noise reduction software, post-processing software...which allows me more and more to venture out in poor light - at dusk, or in very dark, dense forested areas where light barely penetrates.
And while I love to get a great, crisp, perfectly sunlit ISO 100 shot of a bird on a branch with nothing in front of it, and a clean, blurred background, with count-the-feathers detail as much as the next guy, I found I was also enjoying the more challenging shots - and finding some of them more interesting - look at 1,000 perfectly lit photos of the same bird on a branch in the late afternoon with the sun at the photographer's back, then suddenly come upon an ISO 6,400 shot of the same bird painted by a thin shaft of light dropping through a dark forest with droplets of water on its head and glimmers of light stars popping off the blown out highlights on the droplets, and that shot stands out as something so different. Often, that's the shot that the 'professional' who refuses to touch his camera at noontime or shoot when the sky goes overcast, will miss if he's too stringent about following the 'rules'.
