I have not tried using beneficial nematodes to get rid of grubs but they have been successfully used by others so you may want to give them a try if you know you have a lot of grubs. The crows could be harvesting earthworms or even moss. You may want to consider giving this problem some time. Crows can cause problems during just a few weeks when no other protein source of food is available but when spring arrives they may lose interest in your lawn. They could also be after crane fly larvae that will mature into flying insects soon and no longer be an attractive food source.  Keep growing, Marianne Binetti