I think once the lightning properties hit the birds they get diffused. Gaara's sand/magnet style is weak to lightning and sand is Earth + Wind fusion. It can't be earth + lightning like some people believe (because you know electricity, magnetic forces etc) because Shukaku uses wind, not lightning, and Rasa knows every element except lightning jutsu so he couldn't have been using lightning to manipulate his magnet style. The wind in magent style should counteract lightning jutsu from breaking through it but it doesn't. maybe depending on the power or chakra in the move some might have priority over others.