I don't know much about the engineering stuff behind this, but if fuel inefficiency is the only real reason they don't want to add grills, then why not have a m

I don't know much about the engineering stuff behind this, but if fuel inefficiency is the only real reason they don't want to add grills, then why not have a mechanical system that removes the grill after they pass a particular altitude (where flocks of birds don't exist) and then add the grill again for landing?