Once again the urban legend resurfaces.......
Birds are unable to detect and do not react to aircraft weather radar. I can assure you there is no scientific evidence to support the theory. I did a great deal of research with the avian biology experts on the issue for the Transport Canada book, Sharing the Skies. It is available online at the Transport Canada website if you wish to learn more facts about bird strikes and the urban legends.
Airborne weather radar is very low power and the signal strength decreases as the square of the distance to the birds. The legend appears to have been based on certain early military weapons radar systems, but it was only anectdotal and never tested with any sicentific methodology.
There is no value in turning it on, other than to make you feel good.