Comments in the YouTube chat show some misconceptions about loon anatomy, particularly about anatomical structures lacking in loons. Here are some features found in a wide range of birds but not found in loons:
The Crop – A crop is an enlargement of the esophagus that allows storage of food prior to continuing to the stomach (more properly known as gizzard or ventriculus). It allows birds to gorge on food where it is plentiful and then digest the food when they’ve moved to a safer location. It also does some predigestion preparation and may be useful in combating harmful bacteria. Omnivorous and herbivorous birds are most likely to have a crop, and to have a well developed, complex, lobed crop. Loons are nearly exclusive fish eaters (piscivores) and that may have something to do with why they don’t have crops. Penguins are another piscivorous bird without a crop. Cormorants have a rudimentary crop, which is nothing more than an extra wide section in the esophagus.
Brood Patch – A brood patch is a featherless area on the underside of an incubating bird. It usually has a number of blood vessels near the skin and it aids in heat transfer to the egg. Brood patches typically are only present during breeding season. Loons seem to manage well without a brood patch. Exactly why loons don’t have a brood patch is unknown but it might have something to do with waterproofing. A loon’s feathers form a watertight envelope and they can control buoyancy by expanding and contracting their feathers. Perhaps a brood patch could compromise the watertight seal.
Egg Tooth – An egg tooth is a protuberance used by the offspring of egg-laying animals to break through the eggshell. On birds, it is normally found on the top of the upper mandible and falls off within a few days after hatching. Loon chicks can do well without an egg tooth. Their bill is well formed at hatch time, and although it isn’t as sharp and dagger-like as an adults, it’s still more than adequate for punching a hole in the eggshell. Loon chicks are precocial, meaning that they are fully developed and ready to jump in the water and swim with their parents the same day that they hatch. They are well past the stage where they would need an egg tooth.