Throughout the globe, there are approximately 11,000 species of bird, across 30 orders. Every species of bird eats a certain diet. And every species is adapted to that very diet. So what are their diets – or more specifically – what do birds eat?
Sure, you may have seen birds come to your bird feeder. But birds are so much more than mere seed-eaters.
There are specialist snake hunters, capable of delivering kicks up to six times their own body weight.
There are fishing experts that can lift like-to-like their own body weight.
And, of course, there are birds that eat nothing but fruit.
The world of bird dieting is fascinating and we’re about to explore some of the most common diets, and the adaptations needed, for these birds to thrive.
An Introduction to Bird Diet
Biologist or not, most of us have heard the terms carnivore, herbivore and omnivore. For me, it was drilled into my head in my early school days.
A carnivore eats meat, a herbivore eats plants and an omnivore eats a bit of both.
It’s easy for our minds to go straight to the big cats as carnivores or cows and sheep as innocent herbivores.
But did you know, there are carnivorous, herbivorous and omnivorous birds?
Then things get a bit more complicated…
Categories of Bird Diet
As a group, birds have incredibly varied diets. Many species fall under the omnivorous category, eating a wide range of food.
However, there are some species with specialized diets.
So specialized, they have their own terms.
Below, we’ll discuss a variety of different diets most commonly observed in bird feeding behaviors.
Some species are incredibly strict with their diet and will not deviate away from a certain food source. These are known as obligate feeders. The hummingbird, for example, is an obligate nectivore. These birds will eat mainly nectar.
Other species, whilst specializing on a certain type of food, are more adaptable. These are called facultative feeders. Toucans are facultative frugivores, supplementing their fruit-heavy diet with many other food sources.
The bird diet categories are:
- Piscivorous
- Insectivorous
- Avivorous
- Molluscivorous
- Frugivorous
- Granivorous
- Nectivorous
- Mucivorous
- Ophiophagous
- Kleptoparasitism
What Do Piscivorous Birds Eat?
Piscivores are birds that predominantly consume fish and other aquatic animals – both marine and freshwater.
Body Adaptations
Some piscivores have sharp talons that pierce the body of their prey. Specialized textured skin on the base of their feet, called ‘spicules’, prevent the fish from slipping away. An example of this is the osprey.
Other piscivores have webbed feet and paddle-like flippers. These adaptations help birds propel themselves through the water in pursuit of prey. Penguins use their powerful flippers when diving depths of up to 500m in search of fish.
Then, there are some oddities.
Whilst puffins have webbed feet, it is their unique jaw that sets them apart from other piscivores.
Puffins use their textured tongue to lock captured fish against their upper jaw, where a series of backwards-facing spikes holds them in place. Whilst diving, puffins can catch and hold, on average, 10 fish.
Incredibly, researchers have recorded upwards of 120 fish in the beak of a single puffin.
Some piscivores, such as terns and petrels, barely come into contact with the water when hunting for fish.
One particular example is the skimmer. This species, which can be found throughout tropical oceans, flies along the water surface with the lower mandible in the water, shutting as soon as the bill touches prey in the water.
What Do Insectivorous Birds Eat?
Birds that lead an insectivorous lifestyle typically hunt a range of arthropods – insects and spiders.
An insectivore diet is one of the most common bird feeding strategies, with an estimated 80% of all bird species relying on insects, at least temporarily, as part of their diet.
Insects, such as ants, grasshoppers, caterpillars, moths, and flies, are readily available and are an important protein source in nestlings.
Nearly all passerine bird species will feed their chicks a diet of insects, before becoming specialized feeders.
Body Adaptations
Insect feeders are diverse.
Some species, such as the woodpecker, have a long tongue covered in sticky saliva. Once a woodpecker has found a suitable drilling site, they’ll bore into trees with their chisel-like beak. Then, the long and sticky tongue is put to use extracting insects from within the bark.
Other species, such as the unique tawny frogmouth, have incredibly large beaks. They are capable of swallowing insects whole. To avoid damage caused by biting or stinging insects, frogmouths have stiff bristles around their beak.
However, most insect feeders have smallish, curved or pointed bills. The bee eater uses their bill like a pair of forceps, snatching flying insects on the wing. Larger insects that cannot be eaten whilst flying are taken to a perch to be dismembered. Delightful.
What Do Avivorous Birds Eat?
Specialized carnivores, avivores are birds that eat other birds.
Birds of prey, such as peregrine falcons and some hawks, actively hunt other bird species.
Body Adaptations
Like piscivores, avivores have physiology adapted to snatch fast-moving prey.
Avivorous raptors, such as the peregrine falcon, have adapted to live in urban environments, where they hunt other common city birds, such as pigeons. Often, they will use their strong, curved and sharp talons to capture and kill prey.
However, peregrines also use speed to their advantage. Being the fastest known animal, they stun avian prey by colliding with them at speeds of over 200 miles per hour – that’s three times faster than the fastest land animal; the cheetah.
What Do Molluscivorous Birds Eat?
A molluscivore, you guessed it, has a diet consisting of mostly molluscs.
Molluscs are invertebrates, categorized by their soft, unsegmented bodies. They can range from terrestrial species, such as snails and slugs, to marine species such as oysters, clams and periwinkles.
Many species have an outer, calcareous shell, which provides vital protection for their vulnerable bodies.
However, these shells are no match for molluscivores.
Body Adaptations
Molluscivores have a range of feeding techniques and adaptations that enable this.
Some, such as oystercatchers (which incidentally I’m watching a flock of as I write this), use their durable beak to hammer at the shell, revealing the adductor muscle of the bivalve. A quick snip of this and it’s an easy meal for the oystercatcher.
Other species, such as song thrushes, have gone one step further and use their intelligence to work out how to break open the shell by dropping it from great heights.
There is at least one known species of raptor that feeds exclusively on mollusks – the snail kite.
A unique foraging behavior in the raptor family, the snail kite hunts apple snails close to water sources in areas such as the Everglades in Florida. Then, using their uniquely curved bill, the snail kite is able to take the snail straight from the shell.
What Do Frugivorous Birds Eat?
Frugivores are specialized herbivores. They consume a fruit-heavy diet, including the seeds and pulp of a range of fruits.
Body Adaptations
Many frugivores have adapted a strong stomach – capable of withstanding compounds that would otherwise be toxic to us humans. This is the case for the gray catbird, which can tolerate the toxic urushiol oil found in poison ivy berries.
The bohemian waxwing has evolved to thrive on nitrogen-poor sugary fruits. The intake of high sugar and low protein requirements enables the waxwing to thrive on a fruit diet.
The beak shape of frugivorous birds is also advantageous to them. The toucan, for example, have large, thick curved beaks in which they use to pluck fruit from trees.
What Do Granivorous Birds Eat?
Granivores are birds whose diet consists primarily of feeding on seeds and grain.
Whilst many passerine birds are granivorous during adulthood, most will supplement their developing chicks with insects.
However, American goldfinches are one of the few obligate granivores. They even feed their young seeds – a unique behavior in birds, as seeds typically have less calories than insect protein for developing young
Body Adaptations
Seed and grain eaters are often characterized by their beaks – larger birds have thicker bills which they use to exert force enabling them to crack hard shell exteriors. Smaller birds, such as finches and sparrows, have smaller and thinner bills in which they use to pry seeds from pine cones and flowers.
The common crossbill, a member of the finch family, has an unusual beak – slightly curved and crossing over at the tip. But this beak is perfectly evolved to allow the crossbill to extract seeds of pine cones.
What Do Nectivorous Birds Eat?
Another easily deducible name, nectarivores consume a nectar-heavy diet.
Body Adaptations
The most recognisable nectivorous birds are the hummingbirds.
Found throughout the Americas – from Canada to Argentina – hummingbirds have evolved long and thin beaks to access tubular flowers.
The beaks of hummingbirds are so unique, some flowers have evolved to be pollinated by just one particular species of hummingbird.
In their home range, competition is fierce. In Ecuador, over 130 different hummingbird species can be found, so each hummingbird has to evolve some advantage over another.
And the Sword-billed Hummingbird has done just that. Their bill can measure up to 11 cm – the largest with respect to body length for any bird. Oh, and the sword-billed hummingbird happens to be one of those birds that one flower – the passiflora – depends on.
However, hummingbirds aren’t the only nectar feeders.
Native to Hawaii, the liwi is a species of honeycreeper. Like the hummingbird, they too have a long, proboscis-like beak in which they use to lap up the sweet nectar. Their beak is down-curved, which allows them to access the elongated flowers of Hawaiian lobelioids.
Now the Birds With More Unique Diets…
Ok, thus far the names are relatively self explanatory.
But not everything is that straightforward in biology. The next few diets are highly specialized – can you guess what they are without reading the description?
What Do Mucivorous Birds Eat?
No, not mucus.
Actually, kind of. Tree mucus.
The diet of mucivores consists mainly of plant sap.
Body Adaptations
Sap suckers, a type of North American woodpecker, are specialists when it comes to, well, sap sucking.
To do this, they have a sharp and pointy, chisel-like bill that allows them to drill holes into trees. The newly formed tree wound seeps sap that the sup sucker laps up with their equally adapted brush-tipped tongue.
What Do Ophiophagous Birds Eat?
If we’re talking specialism, this one takes first place.
Ophiophagus birds specialize in hunting and feeding on snakes. Bet you didn’t see that coming.
Body Adaptations
Surprisingly, there are a few species of birds that are seasoned snake hunters.
Also known as the crowned eagle, the chaco is a large snake-eating raptor native to South America.
Their diet consists of the highly venomous neotropical lanceheads. Members of this genus have venom so potent it can kill humans.
However, the chaco eagle has evolved morphological adaptations to help reduce risk of injury. Firstly, they have overlapping scales on their feet and toes, preventing snake fang penetration. Secondly, special proteins can be found in the blood of the chaco eagle, which is thought to help neutralize snake venom.
Another, perhaps more well-known, snake hunter is the secretary bird.
The dinosaur-like secretary bird is something to behold.
If you’re not dazzled by their eyelashes, you’ll be blown away by their hunting strategies.
Secretary birds have long legs and sharp claws in which they use to kick and stamp on the heads of venomous snakes. To avoid being bitten by deadly snakes, the secretary bird needs to strike quickly.
Incredibly, the secretary bird can deliver a powerful kick some 5 to 6 times their own bodyweight in a tenth of the time it takes to blink your eye. The forceful, yet incredibly accurate, blow requires high levels of cognitive coordination from the bird, enabling them to kill snakes efficiently.
What Do Kleptoparasitic Birds Eat?
Not a diet per se, but more of a lifestyle.
Those specializing in kleptoparasitism are typically generalist feeders, taking whatever they can, whenever they can.
Kleptoparasitism is a feeding strategy where an animal steals food from another animal.
Body Adaptations
Kleptoparasitism is most often seen in the bird kingdom.
It can either be stealthy or aggressive. For the latter, kleptoparasites chase down and harass another bird until it gives up its food.
One of the most infamous kleptoparasites is the group containing the frigatebirds.
Frigatebirds occupy and breed in warm temperate to tropical climates and typically target other tropical species, such as boobies (get your head out the gutter, this is a bird species).
Frigatebirds will chase down other species, grabbing their tail feathers and shaking them.
The sudden onslaught of arial harassment forces the victim to regurgitate their food midair. The frigatebird then swoops in to catch the falling meal – most often fish.
A Deep Dive into Convergent Evolution
Biology is confusing. Trust me, I know.
However, there are some logical explanations.
Take bird appearance, for example.
Why do many different unrelated birds look the same?
I’m talking about birds that aren’t even slightly related, found on the opposite sides of the world.
Toucans and hornbills. Ibises and curlews. The list goes on.
The answer – convergent evolution.
Convergent evolution occurs when unrelated species independently evolve similar features or behaviors in response to certain environmental pressures.
One of the biggest driving factors of convergent evolution is food.
Species evolve to adapt to a specific ecological niche. Some species evolve long bills to reach hard-to-access nectar whilst others evolve sharp talons to capture prey.
The key thing to remember about convergent evolution is the independent aspect.
Species from South America may evolve the same characteristics as species in India due to food availability.
It’s all very nifty stuff.
Final Thoughts
Birds eat a wide array of diets. They can be herbivores, carnivores, and omnivores.
Or, they can take dieting a step further and become incredibly specialized in what they eat.
There’s the snake specialists, the nectar gatherers, the fish hunters. The list goes on.
Now the resemblance to dinosaurs seems uncanny, right?
So, next time you’re watching that American Goldfinch on the feeder, just keep an eye out for what they’re eating.
