Usually, I won't do give a birthday comment to someone I watch unless I'm a familiar face to them in some way, and I'm not just some anonymous watcher on their list.
In most cases, a lot of people whom I watch on DA are already people I have on Skype, usually from PMU. There are exceptions, of course. If that's the case, Skype is usually easier, but if not, I usually send them a DA comment. Sometimes I even send both if I have the extra time and feel like they are a closer friend or someone I have known over a long period of time.
As for drawings, wish I did more of those for people on their birthdays. I've been pretty good about it since I got my tablet, but usually I just whip up a nice little coloured doodle of one of their characters - nothing extraordinary enough to post on DA but a nice little thoughtful thing that still makes their day.
It oftentimes depends on how busy my schedule is and how close of a friend I consider the person. For the few people I consider close friends, I will take time out of a busy schedule to make sure they get something from me.
I also noticed recently a lot of my close friends have birthdays around the same 3 months, between April and June!