Buzz
Ken Saladin:
Birds lack a urinary bladder altogether. This is a key adaptation for flight, as a bladder full of urine would weigh them down. Instead, they excrete a white, paste-like substance, similar to toothpaste, mixed with feces through a single opening called a vent, not an anus—though some sources, including one I'll reference next, occasionally stretch the terminology.
In contrast to a previous explanation on this topic, one ornithology website describes birds' “outer anal opening, closed by a strong sphincter muscle,” which is actually the cloaca. Based on my observations, I strongly agree with this description.
Blue-footed booby producing guano in the Galápagos Islands. | pilesasmiles/iStock via Getty ImagesAmong other observations, I’ve often observed the blue-footed boobies that nest on the ground in the Galápagos Islands. When they need to relieve themselves while sitting on their nests, they rise, angle their tail away from the sun, and release a white, pasty mixture of feces and urine. As the sun moves across the sky, they reposition themselves throughout the day like feathered sundials, creating a spoke-like pattern of guano around the nest center. I believe this would be impossible without control over their cloacal sphincter.
I also know that many birds, including bats, tend to defecate just before flying in order to lighten their bodies. In birds that nest in trees, such as those with cup-shaped nests in branches, hawks and eagles in stick nests, or owls and woodpeckers in tree holes, they usually stand and extend their tail beyond the nest’s edge or outside the tree hole to defecate. When seagulls and terns dive-bomb a person or predator, they hover above and drop large amounts of waste on their target. These actions also suggest they have voluntary control over their sphincters.
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