Air rifles are effective but are not legal for hunting crows. Might be legal for killing in the act of depredation but I dont know. Just a friendly heads up.
http://wdfw.wa.gov/publications/01184/wdfw01184.pdf Page 70
Hunt game birds or game animals with
anything other than a firearm, a bow and
arrow, or by falconry. Bullfrogs may only
be taken by angling, hand dip netting,
gigging, and bow and arrow.
Are crows considered game birds?