Question: What's going on when large birds pose like satellite dishes?
Answer: Most birds have their own group of ectoparasites (feather mites for one), and they have different behaviors for dislodging those parasites.
Because some birds sunbathe in temperatures which are quite hot, sunbathing appears to leave them over-heated. They can be seen panting to cool down, as are the Yellow-crowned Night Heron and the Great Blue Heron at the right. So warming up is not a likely reason for that behavior.
That has led scientists to theorise that the sun's warmth is important in helping dislodge feather parasites. Spreading the undersides of the wings so that they are exposed to the sun would be an effective way to heat up and move those parasites.