By whom, I'm not sure. My guess is a solitary bee, such as Osmia, which I often see at the back of the house in spring and early summer. There seem to be four egg-cells affixed to the bulb, oriented left to right in the above image. I have two mason bee houses that get a fair amount of use every year. They're on the wall, not far from the light. Perhaps one bee decided to think outside the box, had a lightbulb over its head.
Update: Sally, from Foothills Fancies, suggested these to be mud dauber pots, perhaps of genera Sceliphron or Chalybion (See comments). I haven't seen any here, but that may be due to my haphazard monitoring of the back yard. Mud dauber would make more sense, as mason bees tend to use pre-existing tubes or tunnels.
I don't really need that light to be working over the next several months--the dark, rainy season when the back porch is used more for storage than anything else. I carefully returned the dead bulb to its socket.
How many zoologists does it take to screw in a lightbulb?
Only one, but don't be surprised if the light still doesn't work.