Nisqually Wildlife Refuge, December 2023. Part 1 of 5
I saw five bald eagles, four adults and one subadult, in my last visit to Nisqually Wildlife Refuge.

With Nisqually’s ponds, canals, and marshes topped up by the recent rains, its dense concentration of waterfowl creates a rich hunting ground for eagles. So, the cackling geese and ducks are VERY wary and burst into flight at the slightest sign of an eagle soaring overhead.

But the geese and ducks need to land soon to continue feeding.

The Canada geese are wary too.

But they appear to be too big of a meal for an eagle, especially with so many bite-size morsels around. The Canada geese tend to hold their ground when an eagle passes by.
Many mammal species consume foliage = folivores. But the energy contained in plant foliage is very hard to access. To digest cellulose efficiently, mammals have evolved elaborate structures that serve as homes for symbiotic bacteria that produce celluloses. These structures include a rumen (= multi-chambered stomach of a cow, deer, or sheep) or an enlarged cecum (blind sack at the start of the large intestine). Repeated chewing by grinding molars increases the surface area of the ingested plant material and speeds the rate of digestion. The mammals consume the easy-to-digest bacteria or the by-products of bacterial fermentation of foliage. Still, it takes time to digest foliage efficiently.
But flying requires a light body mass and that constrains the abilities of most birds to evolve these elaborate digestive structures or to fly around large masses of slowly-digesting plant material. Large flightless birds, such as ostriches and rheas, are less constrained and consume more plant material. But geese, swans, and some ducks are major herbivores and excellent fliers.

How do these waterfowl do it? First, like many birds, they consume gravel and sand (aka, grit or gastroliths) which combines with the plant fibers in the muscular gizzard.

Contraction of these muscles power pebbles and sand rip open the cells, releasing their aqueous contents, much like mortal and pestle. [Rounded rocks, called gastroliths, have been found in the digestive tracks of toothless duck-billed dinosaurs, supposedly to aid mechanical digestion of plant material.]. The aqueous contents of a cell are easy to digest, but without symbiotic bacteria, the cellulose (i.e., fiber) is indigestible. So, geese and other folivorous waterfowl (e.g., American wigeons)

use a high through-put strategy - skim off the easiest-to-digest components and defecate the rest = the fiber. This explains the voluminous poo piles left behind by a flock of feeding geese.
Steve