Terror birds were massive flightless birds from the Phorusrhacidae family that lived during the Cenozoic era. Perhaps the largest of these was Kelenken guillermoi, a species that stood almost 10 feet tall and weighed upwards of 300 pounds. While these birds certainly live up to their colloquial name, it’s interesting to consider how they would fare against modern animals. That’s why we’re going to explore a hypothetical battle between a grizzly bear vs terror bird with an emphasis on data from Kelenken guillermoi.

We’ll show you which of these large animals would win a fight against one another using data about each animal as well as a bit of imagination.  

Comparing a Grizzly Bear and a Terror Bird

Grizzly Bear Terror Bird
Size Weight: From 600 to 700 pounds on average, but upwards of 1,300 pounds
Height: 3 to 4.5 feet when measured at the shoulder
Length: Between 6 and 10 feet
Weight: 150 to 300 pounds, but possibly higher
Height: 3 to 10 feet tall
Length: 6 to 10 feet in length
Speed and Movement Type Capable of running 35 mph when chasing prey Could reach speeds of 30 mph while running
Defenses – Possesses dense fur and thick skin along with a layer of fat that can be several inches thick in vital areas
– Their bodies can take a lot of punishment, even gunshots in some cases, and allow them to fight
– They can travel quickly on land and in the water
– Defensive threat display makes many creatures flee outright
– Their height would make it hard to attack their head
– May have been agile creatures capable of making swift turns while running
– Good vision allowed them to spot predators in their area  
Offensive Capabilities – Uses claws, paws, and teeth to kill prey
– Has claws that measure between 3 and 5 inches in length
– Fangs measure between 2 and 3 inches in length
– Bite power reaches 975 PSI, capable of causing deadly puncture wounds
– A smack from its paws could render animals unconscious or disorient them in a fight
– Used their speed to chase down prey
– Utilized their long, hooked and sharp beaks to peck prey or grab and tear into it
– Could have used powerful legs to subdue prey, but their talons weren’t especially long or sharp
– Their lack of a sense of smell suggests that they were more predators than scavengers
Predatory Behavior – Primarily scavengers and gatherers
– Opportunistic predators that find prey in their range and chase it down
– May have been scavengers as well as cursorial predators
– Could have hunted small or large prey by pecking them to death  

What Are 3 Key Differences Between a Grizzly Bear and a Terror Bird?

The most significant differences between a grizzly bear and a terror bird can be found in their morphology, size, and predatory behaviors. While grizzly bears are opportunistic predators and large mammals that weigh between 600 and 1,300 pounds while standing up to 4.5 feet tall, terror birds were large flightless predator birds that stood between 3 and 10 feet tall and weighed up to 300 pounds or more.

An important distinction to make is that while grizzly bears are between 3 and 4.5 feet tall while walking on all fours, they can stand on two feet to gain a height advantage during a threat display, allowing them to stand much taller in the thick of a fight.

These differences are important, and they will impact the fight. However, we must examine more information before declaring one the winner of this battle!

Gastornis
Terror birds were large flightless predator birds that stood between 3 and 10 feet tall and weighed up to 300 pounds.

iStock.com/Aunt_Spray

What Are the Key Factors in a Fight Between a Grizzly Bear and a Terror Bird?

The key factors in a fight between a grizzly bear and a terror bird are their size, speed, defenses, and attack methods. We’re going to compare and contrast these factors as they apply to each animal, ultimately determining which of them has the greater advantage over the other.

When we’ve finished going over the relevant data, we’ll combine all of the information and figure out how the fight would go and which animal survives!

Grizzly Bear vs Terror Bird: Size

Although terror birds are taller than grizzly bears, they are smaller in terms of length and weight. The Kelenken guillermoi, the largest terror bird, is believed to have stood about 9.8 feet in height. Also, it grew about 6 to 10 feet long and weighed between 150 and 300 pounds.

Meanwhile, a big grizzly bear can weigh between 600 and 700 pounds, but it’s not uncommon for them to reach over 1,000 pounds! The largest grizzly bear weighed over 1,300 pounds! Meanwhile, they stand about 3 to 4.5 feet on all fours and also measure between 6 and 10 feet long. These animals are larger, heavier, and have thicker bodies than terror birds.

Grizzly bears have a size advantage over terror birds.

Large grizzly bear
A big grizzly bear can weigh between 600 and 700 pounds, but it’s not uncommon for them to reach over 1,000 pounds!

Perpis/Shutterstock.com

Grizzly Bear vs Terror Bird: Speed

Terror birds are believed to have been swift animals. They could run at speeds of up to 30 mph. Although that may seem too fast for a heavy grizzly bear, they surpass those speeds. Grizzly bears that are closing in on their prey can run at speeds of 35 mph or a little more.

Grizzly bears are also known for being surprisingly agile, something that may or may not hold with terror birds depending on the the source of research and species.

In short, grizzly bears have a speed advantage over terror birds.

Grizzly Bear vs Terror Bird: Defenses

Terror birds did have some natural defenses that would have kept them safe in a fight. For example, their speed would probably be enough to get away from danger. Also, researchers believe they had good enough vision to spot prey and even potential predators so they could avoid trouble. Also, their height would make it harder for short animals to bite their vital areas without putting themselves in harm’s way.

Grizzly bears are defensive powerhouses. They have dense fur, thick skin, and layers of fat that protect their most vulnerable areas. Beneath all of that is a thick body, strong bones, and a lot of muscle. Reports of grizzly bears taking gunshots and still attacking exist, so it takes a great deal of effort to dispatch these creatures.

Also, grizzly bears are fast animals that can move swiftly in water as well as on land. Combined, they have a wide assortment of defensive measures that help keep them safe.

Grizzly bears have a defensive advantage over terror birds.

Terror birds used their speed, height and vision as defensive skills
While terror birds had defensive skills such as speed, height and keen vision it would not be enough to protect against a grizzly.

Michael Rosskothen/Shutterstock.com

Grizzly Bear vs Terror Bird: Offensive Capabilities

Terror birds had large skulls and long, curved beaks. However, we don’t exactly know how they used their beak in a fight. Some research claims that they hunted small prey while others insist that they had a biological toolkit that could hunt larger animals.

Their large, sharp beaks could have been used to peck large animals and cause severe damage before ripping them apart. So, they probably used their beak to deliver several blows before tearing apart animals that they couldn’t swallow whole. These sharp beaks would certainly be effective at harming other creatures.

Also, terror birds could have kicked their prey. Some species, like Kelenken guillermoi, lacked long, sharp talons on their feet to jump and slash prey. However, they could still use their body weight and feet to prevent animals from moving.  

Meanwhile, grizzly bears have long, somewhat straight claws to slash prey, large paws that they can use to smash into prey, and a reasonably powerful bite that can break vertebrae and puncture skulls. They use all of these elements in concert while climbing on top of their prey to maul them to death.

Grizzly bears have far more versatility when fighting. Yet, terror birds may have had a very effective, specialized method of killing their prey.

Grizzly bears have the offensive advantage over terror birds in a fight.

Grizzly bear claws for slashing
Grizzly bears have long, claws to slash prey giving them an offensive advantage.

TheWonderWays/Shutterstock.com

Grizzly Bear vs Terror Bird: Predatory Behavior

Terror birds were large cursorial predators, but they may have been scavengers to an extent. Also, scientists are divided on the size of the prey they hunted. Their bite power and skull morphology suggest they were better suited for taking down small prey.

However, terror birds were large, so they needed more energy than a few small mammals would provide. Also, their physiology suggests they could have run alongside large prey and delivered a powerful peck with their massive beaks, ultimately killing larger prey.

Grizzly bears are more likely to eat plants and scavenge food than they are to hunt down large prey. That doesn’t mean they can’t hunt down big animals. They are opportunists that can chase down and maul prey before consuming them.

Terror birds have the advantage in terms of predatory behaviors since they were probably true predators.

Terror Birds had massive beaks used to kill prey
Terror birds were considered to be true predators killing their prey with powerful pecks from their massive beaks.

Michael Rosskothen/Shutterstock.com

Who Would Win in a Fight Between a Grizzly Bear and a Terror Bird?

A grizzly bear would win a fight against a terror bird. The truth is that these animals would have avoided each other, but we’re going to break down this battle anyway. The decision to call the grizzly bear the winner may seem controversial or outright wrong, but let’s look at the smaller elements of the fight.  

Grizzly bears are shorter than terror birds. They’re also faster in short bursts, weigh a lot more, and love to use their claws, bites, and body weight to bring their prey to the ground. If this fight started on flat ground and the two animals knew they had to kill the other, that’s exactly what the bear would do.

The terror bird’s primary method of attack, pecking, may not have been all that helpful in this particular battle. Unless it struck and penetrated the grizzly’s head or broke its spine, it would be in serious trouble. We don’t know if the terror bird had the strength to do that. Moreover, the thick body of the grizzly bear would soak up a lot of damage before it died.

In the meantime, the grizzly would probably not give the terror bird time to do much in the way of attacking as it drags the large bird to the ground and mauls it to death by biting its neck and head. The grizzly is too swift, too powerful, and too strong for the bird to beat it.  

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