Disclaimer: the branch is wood, but I had to put the nest on something... and I preferred the contrast of a natural material over a piece of wire. I could have put them on a big electrical transformer, like some birds do for reasons I can't understand, but I didn't have a transformer handy.
However, I do own loads of surplus resistors in different colors, collected over the years, which I use to create mosaic effects in my art. If you want a similar aesthetic, don't buy new resistors -- get some old surplus components and/or unsorted resistors for cheap. You can always measure them (and combine them in series or parallel) if you want to use them in a functional circuit.
The shape of the mama bird is made entirely out of resistors and she's hollow inside, which is where I carefully, painstakingly installed her freeform singing circuit. The baby bird is nestled in a nest of wires --not altogether different from the real thing-- and the light-colored object in the nest is an 8 ohm speaker wrapped in heat-shrink tubing (to mechanically dampen the sound). Both birds have a photoresistor that enables you to interactively affect their chirping. Here's a video showing the pair making their avian sounds: