Birds do not chew their food.
Indeed, only a few birds (mainly raptors) can even tear their food into smaller chunks before swallowing it. Certainly, a few birds will pick at fruit on a tree and gulls will pick at dead fish on a beach. However, most birds just swallow whole whatever they eat. This places constraints on the size of things most birds can eat. A kingfisher does not even bother looking for a fish that a heron would happily swallow.
I reflected on this when last week I was watching two birds feeding: a Snow Bunting that was picking off grass seeds and swallowing them; a Hooded Merganser that had caught and swallowed a small fish. These are the final two pictures. The first five pictures are older and are chosen to illustrate the issue.
A raptor can grip its prey and tear it apart with its hooked bill. This allows it to eat prey much larger than it could swallow whole. This is a Merlin feasting on a Mallard (2017 April 13).
Most birds swallow their prey whole and this constrains what they can eat. A robin can easily swallow a worm or moth (2016 May 19).
Dippers mainly swallow insects and fish eggs. A fry is perhaps the largest fish it can swallow whole (2012 Dec. 10).
This is a large fish for a Common Merganser to swallow whole (2016 August 30).
However, the merganser would not have been able to handle the rather large Kokanee being swallowed by this Great Blue Heron (2015 August 25).
A Snow Bunting picked off grass seeds and then swallowed each whole.
A Hooded Mergansers dived and then surfaced with a small fish. The event was interesting not just because the fish was swallowed whole. Unexpectedly, the catch is a member of the sunfish family, a black crappie or a pumpkinseed, both of which are native to eastern North America. They seem to have turned up in Kootenay Lake only within the last decade and neither is to be welcomed. Unfortunately, sunfish prey on small indigenous fish and have the potential to alter the local aquatic food chain. The only thing we have in our favour is that the coolness of the Lake may prevent them from becoming a nuisance.