This section is intended to be an exact copy of what the survivor Helena Walker, the author of the dossiers, has written. There may be some discrepancies between this text and the in-game creature.
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Species
Hesperornis avenatantes
Time
Late Cretaceous
Diet
Piscivore
Temperament
Passive
Wild
Possessing the appearance of a half-duck/half-dinosaur, Hesperornis is a medium-sized, fish-eating bird, common in the rivers and lakes of the island. It would be about two-thirds the height of a human if it stood tall, but it rarely does. Hesperornis spends most of its time gliding along the surface of the water, where it is much more maneuverable.
Hesperornis is barely a threat to any land-dwelling creature, as its legs are too short for it to move around effectively, but it is surprisingly fast in the water. It can easily hunt down fish and other small water-dwelling creatures.
Domesticated
Not particularly useful for hunting, and not being affectionate, Hesperornis is primarily kept for the eggs it produces after consuming much fish. When rendered correctly, the eggs separate into two useful substances.
One is a protein substance that is high in calories, and the other is an oily liquid that is effectively the same as the Oil found in the ocean!
Hesperornis is quite content to ignore animals and survivors, unless if they get too close for comfort. They will bite any player that touches them before fleeing. They devour fish such as Coelacanths and are in turn hunted by predators and survivors who seek their organic polymer.
Appearance
The Hesperornis is a strange bird resembling a dinosaur mixed with a loon. Like a loon, it has a long neck, waterproof feathers, and strong webbed feet that propel it through the water at the cost of being rather helpless on dry land. Unlike a loon, it has vestigial wings that prevent it from flying, as well as small needle-like teeth to snatch fish - its favourite meal.
Color Scheme and Regions
This section displays the Hesperornis's natural colors and regions. For demonstration, the regions below are colored red over an albino Hesperornis. The colored squares shown underneath each region's description are the colors that the Hesperornis will randomly spawn with to provide an overall range of its natural color scheme. Hover your cursor over a color to display its name and ID.
This information can be used to alter the Hesperornis's regions by entering cheat SetTargetDinoColor <ColorRegion> <ColorID> in the cheat console. For instance, cheat SetTargetDinoColor 0 6 would color the Hesperornis's "main body" magenta.
Type in values of a wild creature to see on which stats it's emphasized. Green values on a high-level creature are very good for breeding. If you have already tamed your creature you can try to recover the breeding stats with an external tool.[1]
Hesperornis
The stat-calculator does not work in the mobile-view, see here for alternatives: Apps
Note that after the creature is tamed it gets bonuses on some stats depending on the taming effectiveness. This makes it hard to retrieve the levels on a tamed creature, so this tool is only for wild ones, but gives a first impression, how well the stats are distributed.
This bird uses an odd version of passive taming. You must kill fish and then bring it to the Hesperornis to tame it. It appears type of fish and, most importantly, size affects how quickly it's tamed. A small fish might give 2-3% taming progress, while a large one can give 9-10% or more. There is a 30-second cooldown between feedings.
A good strategy to use is to put a fish in a foundation with a ramp and three railings, with one open side towards the birds. Then, they'll walk into it without any effort. Another method is to use a Grappling Hook to drag them to your pen. Alternatively, it can also be picked up using the jaws of the Kaprosuchus, completely immobilizing it throughout the taming process.
Sabertooth Salmon seems to be more effective in taming than Piranha, which again seems to be more effective than Coelacanth. Taming it with fish obtained by an Ichthyornis gives more taming percentage.
The Hesperornis deals barely any damage with its attack, unless in water, so getting it to attack you on land which prevents it from running away is a good idea. Luring it into a pen by walking near and gaining its attention is the best way to prevent you from losing it while finding more fish.
Using an Ichthyornis to kill fish will drastically improve taming effectiveness (roughly 5 to 10 times greater effectiveness per fish) due to the "prime buff" given by fish it kills.
Although the Hesperornis is tamed by the player dragging a dead fish up to the bird. Once it is tamed Hesperornis only eats raw fish meat just like other aquatic carnivores.
Combat
This section describes how to fight against the Hesperornis.
General
It is quite easy to kill a Hesperornis due to their low amount of health. It is best to keep it out of the water, as it does more damage while swimming.
Strategy
The Hesperornis is very slow and poses very little threat, so using a pike should easily do the trick.
Weaponry
Only low-end weapons are needed, such as a spear, bow or pike.
Dangers
The Hesperornis poses essentially no dangers to even a low-level survivor.
Care should be taken while hunting or taming them however, as their preferred hunting areas are rivers, often teeming with other dangerous creatures. Most notably, the piranha, which will occasionally try to grab them from the surface of the water.
Weakness
They are slow runners, meaning you can easily out run it, or hit it with a weapon and run before you get hit yourself, they also have low health.
Utility
Roles
Egg farm: Female Hesperornis can lay Golden Hesperornis Egg and Hesperornis Egg when brought out of water after killing and eating 5 fish. This process can be somewhat sped up by killing fish as the player and then commanding Hesperornis to attack fish corpses in the water. Note that there is about 10 seconds delay between Hesperornis reaching the shore and laying an egg. The Golden eggs give a temporary 500% experience boost to smaller creatures such as Dilophosaur, Troodon and Compies or a 10% boost to larger creatures when consumed. They are also an ingredient in Extraordinary Kibble which is the only kibble that's effective at taming Yutyrannus, Thylacoleo, Megalania, Griffin, and Rock Elemental (Technically, extraordinary kibble can effectively tame any creature that normally has a preferred kibble.) The Golden Egg is also one of the possible cravings that can be requested when trying to acquire a Rhyniognatha. The normal Hesperornis Egg can also be placed in a cooking pot to produce Oil. Extraordinary Kibble is better obtained from eggs of very low-level Yutyrannus or Deinonychus, and Golden Eggs are currently too much of time-wasters to invest in. However, if you are looking to rush a Thylacoleo, and have the proper saddle for it, this could very well be the method you obtain one with, in addition to a trap.
Polymer: Hunting Hesperornis is a great way to obtain Organic Polymer for people who do not have Fur Armor or who are far from the snow biome where the more numerous Kairuku are found. The Wooden Club is the most efficient tool to harvest the polymer.
Polymer Farm: Although it is not the most popular option, the Hesperornis are easier to spot than Kairuku on early game, so even if the Hesperornis takes so much time to tame, it will be a good inversion to tame a pair in order to start the breeding project to eventually make a good Organic Polymer farm.
Fisher: The Hesperornis will insta-kill a small fish (e.g. coelacanth, sabertooth salmon, etc...). This makes it an excellent fisher, which is good because it only eats fish.
Collectibles
Notes/Trivia
For information pertaining specifically to the real-world Hesperornis, see the relevant Wikipedia article.
The ability of Hesperornis to lay golden eggs is likely in reference to the fairy tale trope of a magical goose that lays golden eggs, appearing in stories such as the Aesop fable The Goose that Laid the Golden Eggs and the English tale Jack and the Beanstalk. Hesperornis is not closely related to geese, but does superficially resemble a goose.
Hesperornis in real life was rather different from its Ark counterpart: it could not walk upright and instead probably hauled itself on the ground clumsily, a bit like a seal, due to its legs being on the sides, unlike in Ark where they are located underneath. Its legs and feet were a lot larger and the webbing on them was more like that of a moorhen or coot rather than a duck or seagull.
The real Hesperornis was native to the shores of shallow shelf seas like the Western Interior Seaway and acted as an easy prey for ocean predators like Xiphactinus, Tylosaurus, and Mosasaurus.
Like Archaeopteryx, Hesperornis was a proto-bird, meaning it still retained some Dinosaurian features, such as teeth and visible fingers, which most modern birds lack.
Real Hesperornis were a social animal that lived in flocks, Ark's Hesperornis is mostly seen alone but occasionally can be found together.
The diving attack Hesperornis uses against prey fish deals 100,000,000 damage to them. (Tested with floating damage numbers.)
Hesperornis does not have an Oxygen stat unlike other air-breathing swimmers such as Otters.
On PC and PS4 it is almost impossible to feed hesperornis with fish that are too big, since the player character will always be inside the collider of the fish causing the "[E]:Release Body" prompt to take priority over the "[E] : Feed Fish" for taming the hesperornis. Your best bet is to strafe along side it while aiming directly at the head. If the fish is spinning, drop it and quickly pick it up before the hesperornis eats it. (Have not confirmed on XB1 yet)
Yes
No
Region 0:
Region 2:
Region 4:
Region 5:
Health
Stamina
Oxygen
Food
Weight
Melee Damage
Movement Speed
Torpidity
Hesperornis Egg or less frequently 
Golden Hesperornis Egg.
Grappling Hook to drag them to your pen. Alternatively, it can also be picked up using the jaws of the 
Kaprosuchus, completely immobilizing it throughout the taming process.
Sabertooth Salmon seems to be more effective in taming than 
Piranha, which again seems to be more effective than 
Coelacanth. Taming it with fish obtained by an Ichthyornis gives more taming percentage.
Ichthyornis to kill fish will drastically improve taming effectiveness (roughly 5 to 10 times greater effectiveness per fish) due to the "prime buff" given by fish it kills.
Dilophosaur, 
Troodon and 
Compies or a 10% boost to larger creatures when consumed. They are also an ingredient in Extraordinary Kibble which is the only kibble that's effective at taming 
Yutyrannus, Thylacoleo, Megalania, Griffin, and Rock Elemental (Technically, extraordinary kibble can effectively tame any creature that normally has a preferred kibble.) The Golden Egg is also one of the possible cravings that can be requested when trying to acquire a Rhyniognatha. The normal 
Oil. Extraordinary Kibble is better obtained from eggs of very low-level Yutyrannus or Deinonychus, and Golden Eggs are currently too much of time-wasters to invest in. However, if you are looking to rush a Thylacoleo, and have the proper saddle for it, this could very well be the method you obtain one with, in addition to a trap.
Organic Polymer for people who do not have Fur Armor or who are far from the snow biome where the more numerous 
Kairuku are found. The 
Wooden Club is the most efficient tool to harvest the polymer.
Oxygen stat unlike other air-breathing swimmers such as 
Otters.
ARK: Survival Evolved Changes
Hesperornis is added to the game
