Bird nests are round. Construction plans are part of the animal’s genetic code. Sometime long ago, a bird or birds learned that there is an advantage to having a round, tubular, or globular nest. A circle is the most economical way to define space. For investment of material and effort, circles, tubes, and globes hold more at least expense  circumference to volume ratio). Smart birds.

Songbird

Hummingbird

Cliff Swallow

Grebe