Can a claim of ineffective counsel be established by assuming that there are a number of potentially available witnesses who could be able to provide mitigating testimony?

On direct appeal, a claim of ineffective counsel cannot be established by mere speculation regarding the "likely" testimony of potentially available witnesses. (See, e.g., People v. Jackson (1980) 28 Cal.3d 264, 288-289, 293, 168 Cal.Rptr. 603, 618 P.2d 149.) We cannot assume from a silent record that particular witnesses were ready, willing and able to give mitigating testimony, nor can we speculate concerning the probable content or substance of such testimony.