The elbow and 'finger' joints are fully extended, so are locked and don't require energy to support. The pectoral muscles are supporting the weight of the bird,

The elbow and 'finger' joints are fully extended, so are locked and don't require energy to support. The pectoral muscles are supporting the weight of the bird, but they are very much stronger proportionally than ours. But it's an interesting question. According to a couple of papers I found online [1][2], when soaring birds are soaring their energy use is not much higher than when sitting. Soaring is very efficient. 1. How Cheap Is Soaring Flight in Raptors? A Preliminary Investigation in Freely-Flying Vultures https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3893159/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3893159/ 2. Heart rate and estimated energy expenditure of flapping and gliding in black-browed albatrosses https://pubmed.ncbi.nlm.nih.gov/23661772/ https://pubmed.ncbi.nlm.nih.gov/23661772/