I looked at this a little while ago.
Near infrared emitters, which Blink use, have a wavelength of 850nm. This is beyond what most people can see easily, though in tests, some people can even just about perceive wavelengths as long as 900nm. These LEDs do not produce a perfectly pure wavelength at 850nm, instead they produce a spectrum, having their peak at 850nm. Their range is probably as much as 200nm, with lowering intensity as you move away from this central wavelength, in either direction. As a result, at least some of the illumination is closer to 750nm, and visible to all, hence why it can be seen as a red LED. Even so, we are not very sensitive at this part of the spectrum, so the LED seems relatively dim, when we know in fact that they must be really very bright indeed, because of how they illuminate the scene. Of course, it will be dimmer at 750nm, where we see it, so that’s also part of why it doesn’t seem extremely bright to us.
To me it is a very obvious, bright red, LED, but I can’t notice any illumination of the scene. Some people may, and most animals will be able to at least see the LED itself, even if they cannot see the illumination of the scene. This may explain their interest in the camera itself, when operating, with the click perhaps drawing their attention to it. We can see it, why wouldn’t they? I hear the click everytime it catches me in the dark, and I always look up to see that red light.
Medium and far infrared, which we would not see at all, requires more expensive emitters, and much more expensive camera sensors, but it is done, just not in consumer grade equipment.
Birds pecking at the lens, I’m with ceedee on that one, probably arguing with their reflection, lol.