I was told a long time ago that having the weather radar turned on when at low altitudes helps to scare birds out of your way because the can sense/feel the radar. Casey also mentioned this in another answer of his.
Is there any evidence that weather radar actually scares off birds, or is this another "urban aviation legend"?
My guess is it is an urban legend. Ground based weather radar have been used track and analyse behaviour of birds. Part of this research was done with the radar in fixed beam mode to record individual wing beat frequencies for species identification, keeping the bird in the radar beam. If the birds would be scared away by the beam this research would be almost impossible (and invalid as well). Of course, that's no hard evidence that the birds aren't scared. Also the distance between the bird and radar plays a role of course.
The radar used for that research was operating in the C-band, which is also the frequency band that most aircraft weather radars operate in. Newer aircraft weather radars operate in the X-band, which is a higher frequency band. My gut feeling is that the higher frequency is even less likely to bother birds, but then again, I am not a bird so what do I know?
jwenting suggested in a comment that this myth might have been started because of incidents where animals and humans have been harmed and killed by high power radar transmissions.
The FAA has published an Advisory Circular on recommended practices and precautions for operating weather radar on the ground. While not concerned with birds, it provides an example calculations of the output power and safe distance of a weather radar. It the example, the safe distance is less than 4.5 meter. That is too close for a bird to avoid a mid air collision.
There are studies indicating that radars might affect avian behavior. A 2015 study found evidence of radar exposure increasing bird alertness.
(FID is Flight Initiation distance)
High-power military radars can outright kill birds crossing the beam, and have effects, some of which are noticeable by the target. Radio waves have been experimented with as a less-lethal weapon through thermal effects. You don't have to be well-done to notice being cooked.
Per studies, it appears that birds change behavior when exposed to radars at low power. The perception is through either thermoreception or acoustic effects, like a primitive radio formed by conductive body tissues. This effect leads them to momentarily interrupt foraging behaviors and look around for threats. It doesn't scare them per se, just alerts them a little sooner. In human terms, the effect is probably most comparable to tinnitus.
However, the range of this effect is <200 ft, so a weather radar can't be relied on to prevent bird strikes. It's also not well-studied and not verified across a wide range of species. The beam might also completely miss the birds due to scanning.
So it's not a reliable bird deterrent. Still, it might be better to have at least some birds pay attention and possibly evade 150 ft away rather than 75 ft away.
This effect appears to be much more pronounced with solid-state radars, used in modern transport aircraft, than with the more basic magnetron units used in older or smaller planes, and appears to be proportionate to mean (rather than peak) radiated power.
...at taxiing aircraft speeds (approximately 3–10 m/s) birds responding to an aircraft with the solid state radar would escape 2–6 s earlier. However, these effects may be minimized at higher speeds. For instance, approach speeds of large aircraft using solid state radars (e.g., Airbus A330) range from 62 to 73 m/s, leaving birds with 0.3 s more to escape in response to the radar.
