retains heat longer, and also loses heat faster
These two points are contradictory. Something either holds heat longer or loses it faster.
I read your second link and it seems that color matters way more than composite vs real wood. Though in any case they were measuring the upward-facing surface temperature of the decking material, not the inside temperature of a structure made from the material.
I'm no bird building engineer, but here is what I'd consider if I was worried about bird house temperatures:
- ventilation: helps bring the temperature down/up to the ambient air temperature
- solar absorption: lighter colors tend to absorb less warmth from sunlight
- insulation: more insulation means less heat/cold will transfer from the outside surface in, and will make the temperature inside more stable throughout the day/night
And addressing each point in terms of composite vs real wood:
- ventilation: same for both composite and real wood
- solar absorption: unpainted light-colored wood appears to be fairly cool, but if it's painted/stained then it doesn't matter
- insulation: I can't find a good source, but it seems like real wood is a better insulator than composite. You can use thicker boards to increase insulation.
So, if you make a bird house with unstained unpainted untreated wood and the exact same bird house design with composite wood, I think it's reasonable to assume that the composite one will get a little warmer on a hot day. If the bird house has some ventilation, I don't think there will be much of a difference.