Structural engineer in action

Whether you're a bird person or not, this is stunning!!! Not to detract from the sheer magic of it, but in practical terms, how M A NY trips would a bird have to make with that tiny little quantity of mud/clay it could carry? (and how far from the nest is the source?) If you take the construction of a circular bowl in your stride as instinctive  how the heck does the bird come up with the windbreak/entrance design that shields the eggs/chicks from the elements? And at what point in fashioning the bowl do they start to construct it? Structural engineer in action. Even if you think you could build this?  Try it without using your hands! Cliff Swallows are even more impressive - they do this against gravity - building under eaves of buildings.