How Are Nests Sanitized?
A parent either swallows or transports the fecal sac. Only nestlings, young birds that are yet to learn flying and share parent-built nests, can produce the portable sac for their poop. The sac is mostly white or transparent mucous membrane surrounding the nestling’s feces. Largely, passerines create the sac. Is swallowing the droppings not unsafe? All sacs are not swallowed. The sacs of very young nestlings are swallowed because poop is “nutritional.” This group of baby birds are unable to digest entire food eaten, so the crap still has nutrients and energy. Contrarily, as the digestion improves with growth of the baby, the sacs of old nestlings may lack nutrition. Thus, these are transported. Another reason for swallowing is keeping the nests away from the reach of predators because fecal odor may double as a compass. Swallowing or transporting the sacs to faraway places dismantles the compass. Ornithologists observed feeding and defecation happen in tandem in presence of parent. Feeding may stimulate excretion of earlier feces. So, nests always remain clean. But, research on this subject is limited. The sacs may be collected directly from rear of nestling. Some ornithologists compare them with diapers, “bird diapers.” The sac prevents spread of diseases from the feces.