It is literally a birds nest made from edible fibers. Usually a swallow's nest :)
the size of a soup bowl
Spain. they nest every summer mostly in the south
No, it's not cruel. However, it depends on how you define cruelty. If you think honey is cruel, then, yes, you might consider birds' nest soup cruel. Most harvesters would not bother those nests with egg on them. So, consider birds' nest as an expensive honey that only can be afforded by those few.
Authentic bird's nest soup is made using the nests of the swiftlet, a tiny bird in southeast Asia. The swiftlet lives in dark caves. Instead of twigs and straw, the swiftlet makes its nest from strands of its own gummy saliva, which hardens when exposed to air.
It is quite possible as there IS a delicious soup that is called "Bird's Nest Soup". It is considered to be quite the delicacy & IS NOT made from a bird's nest.
Sometimes they can make out where a nest came from or if it is man-made or not. They will probably live there if they don't have a choice, but some birds would rather build their own nest.
it looks like a birds nest
The nest was built by birds.
Because the design is based on a birds nest. If you look at the contruction of an actual birds nest it is very similar to the bird cage. Especially if viewed from the air.
The nests of cave swifts are used in bird's nest soup.
Te specific study of the birds nest is calle "ornithology".