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I've just read in a book that birds' guts can digest almost all the consumed seeds with the exception of mistletoe and loranthus (which stays stuck on the branches). On the other hand, I know that the co-evolution with birds caused the enlarging of the fruits - for the fruit trees as an advantage in fast spreading.

But how do exactly birds spread the seeds they consume if they're digested in their guts?

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Birds may indeed digest seeds under conditions of rest. It has been postulated that almost all current knowledge on mechanisms of internal seed dispersal has been obtained from experiments with resting animals.

A study with the mallard Anas platyrhynchos (common wild duck), claimed to be quantitatively one of the most important seed dispersing animals in aquatic habitats in the Northern Hemisphere, has shown that physical activity affects gut passage survival and retention time of ingested plant seeds. The authors fed seeds of nine common wetland plants to mallards trained to subsequently swim for six hours. They compared gut passage survival of seeds against a control treatment with resting mallards.

Intact gut passage of seeds increased significantly with mallard activity (up to 80% in the fastest swimming treatment compared to the control), and hence revealing reduced digestive efficiency due to increased metabolic rates. This enhances the dispersal potential of ingested seeds.

An extensive work-out after a meal generally slows down metabolism, as blood is redistributed to the musculature and away from the digestive tract.

These findings imply that seed dispersal potential by mallards calculated from other experiments with resting birds may be underestimated.

Reference
- Kleyheeg et al., Oikos (2015); 124(7): 899–907

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    $\begingroup$ Reduced digestive efficiency due to increased metabolic rates? That seems a bit counter-intuitive, no? You would think that higher demands on the system would translate to a higher need for efficient energy and nutrient extraction! $\endgroup$ – Mason Wheeler May 17 '16 at 15:05
  • $\begingroup$ @MasonWheeler - An extensive work-out after a meal slows down metabolism, as blood is funneled to the muscles away from the digestive tract.. Good point, and added to answer. $\endgroup$ – AliceD May 17 '16 at 15:13
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    $\begingroup$ So, are they saying that motility increases with exercise, leaving the seeds less time to be digested, and "pushed out" more quickly, or that the energy demands of the muscles reduce the amount of energy devoted to digestion, and motility remains constant? $\endgroup$ – MattDMo May 17 '16 at 18:48
  • $\begingroup$ @noncom I'm asking what the study concluded, if anything. $\endgroup$ – MattDMo May 18 '16 at 15:16
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The fact that birds are able to digest seeds doesn't mean all seeds get digested. That may depend on activity as in the article cited by @Christiaan. There will probably be other factors like size and shape of the seed and protective coats around the seed. Humans, whose digestive system is longer than that of birds can digest seeds as well. However, seeds of stratiotes aloides can pass through a human and germinate afterwards.

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