The Warbler builds a much smaller nest, literally creating the sides of the nest around its own body. The warbler gathers grass and stems, clumping them in to the fork of a tree. Then it sits in the middle of this debris and rotates itself, pushing with chest and feet to develop a cup shape. Once this base is established it will twine grass together to strengthen the nest's structure. The warbler will line the nest with hair from animals or people (it doesn't care which) and finish the edging with spider's silk. The warbler collects the spider threads and, after arriving back at the nest, brushes them off its beak onto the nest's twigs. As the silk adheres to the nest, the warbler draws the threads out and weaves them into the rim of the nest, making it smooth and strong.
Don't have branches, grass, silk or someone else's hair to build with? Then how about your own spit? The Edible-nest Swiftlet makes its nest out of saliva. The male of the species can produce condensed saliva from a special salivary gland in its throat. The extruded spit resembles strands of clear spaghetti. The swiftlet forms the strands into a pseudo-cuplike shape, attaching it to a vertical surface such as a cliff face. The strands harden as they are exposed to the air. They most be glorious to see, glistening on the cliff sides in their hundreds. Unfortunately, these nests are primary ingredients in the Asian delicacy, Bird's Nest Soup. Poachers are no longer waiting for chicks to fledge before they take the nests, thereby driving this species towards extinction.
I'll be turning my attention away from birds tomorrow. I've had the pleasure of seeing our next structure and it's very impressive. Want to try and guess at the next architect? Here's a hint. If humans were to build something similar, we would produce a skyscraper . . . 6 miles high!
Thank you Paul Bolstad from the University of Minnesota for bald eagle nest picture.