Weasel told you right, they can be called occassionally during the daytime. But you might get pretty frustrated if you only go during the daytime. They are much easier called at Dusk and Dawn, than during the mid-day hours. Unless naturally, you are in their bedroom with them. The first hour or so after civil twilight in the morning and the last half-hour of daylight are the witching hours, get ready. Have fun! Love dem' grays! They have the heart of a lion! Over-aggressive, I hate to call them dumb like kendog did, even if they do resemble that remark.
As for calls, if everyone around your parts are using the distress rabbit calls, shift to a high pitched shrill bird sound, it really works well also. A Johnny Stewart PC-3 and get out on the end of the reed and bite down and let er rip. Works for me. If your into electronics the FoxPro adult bluejay, quail, starling, shorebird, game chick, turkey in distress all work very well. Haydel's Mouse squeaker is a tube, mouth blown call that has more volume than most squeakers, especially the bulb types. Get in close in heavy cover where you think they are at and use that one.