Fecal Sacs Help Keep Nests Clean

photograph of yellow, white and brown bird flying with white fecal sack in beak

Biological Strategy

Birds

Eleanor Banwell

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Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Expel Solids

Living systems must often discharge solids–such as seeds, eggs, pollen, and mineral salts–for reproduction, protection, or other reasons. Solids can be easily moved when a living system applies force to them. But creating that force requires energy, so living systems must either have efficient strategies worth the energy investment or use an outside force (such as gravity). For example, a tropical mistletoe positions its pollen so that the weight of a bird landing nearby triggers the pollen’s release onto the bird’s forehead. The bird then fertilizes other mistletoes that it visits.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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Fecal sacs produced by baby birds enable parents to maintain a clean nest by easing clearance of waste.

Baby birds can stay in nests for many weeks before fledging (learning to fly). The Eastern bluebird takes 3 weeks to fledge, while larger birds like crows take 5 weeks. During that time, the nestlings are confined to a very small space that must be kept clean to prevent infection.

Birds do not urinate and defecate separately, instead they excrete all their waste in one action. Nestlings of some species excrete over the side of the nest, however for others this can attract predators and put the offspring at risk. In order to enable the parents to remove the waste and dispose of it elsewhere, the nestlings of many bird species defecate into fecal sacs. These membranous sacs collect the waste in a tidy parcel that the adult can easily collect and carry away.

The nestlings of different species of birds use different methods to draw attention to the sacs so that the parents notice them. Some have specialized shivering or presenting behaviors, while others deposit the sacs on the rim of the nest.

Birds Build Responsively

Birds

Birds evaluate external threats, their previous success, and the success of others in determining where and how to build their nests.

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Last Updated April 19, 2018

References

“The removal of feces of nesting birds provides sanitation and lessens the attractiveness of the nest to predators (Welty and Baptista 1988), but the presence of a mucosal sac around the feces allows parents the option of carrying them away” (McGowan KJ. 1995:50)

Journal article

A Test of Whether Economy or Nutrition Determines Fecal Sac Ingestion in Nesting Corvids

The Condor, 97(1): 50-56 | 01/02/1995 | McGowan KJ

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Reference

“The manner of defecation of the thrasher and wren was strikingly different. In both cases, the parents remove the feces when present, after they have fed the young. Thus, it is essential that the parents be aware of the presence of fecal sacs in the nest…The thrasher’s cup-shaped nest is deep, and when defecating, the nestling raises its posterior as high as possible, making the fecal sac readily visible from above. This action was invariably displayed on the flat tabletop in the absence of a nest rim. When the nestling is large enough, it can defecate onto the nest rim, making the fecal sac conspicuous without the parent’s having to observe the defecation. Towards the end of its “nestling” period, the hand-raised thrasher frequently defecated between feedings. Body-shaking did not accompany defecation in the thrasher as it did in the wren.” (Ricklefs 1966:49)

Journal article

Behavior of Young Cactus Wrens and Curve-Billed Thrashers

The Wilson Bulletin, 78(1): 47-56 | 01/03/1966 | Ricklefs RE

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