The Strange Anatomy of Hummingbirds

Weight and Size

The weight of the smallest hummingbird, the Bee Hummingbird (endemic to Cuba) ranges from 0.056 to 0.071 ounces (1.59 to 2.01 grams).  It measures 2 - 2.4 inches (5.1 to 6.1 cm) in length.

The weight of the largest hummingbird, the Giant Hummingbird ranges from 0.63 to 0.85 ounces (17.8 to 24.1 grams), nearly double that of the nearest recorded species.  It measures 9.1 inches (23 cm) with a wingspan of 8.5 inches (21.5 cm).  This birds is found along the Eastern and Western slopes of the Andes from Ecuador to Chile.

The Brain

The weight of the hummingbird brain is 4.2% of its total body weight, much larger than other birds.

Researchers at the University of British Columbia have found that hummingbirds process motion differently in that they are able to detect movement equally from all directions.  Other birds as well as amphibians, reptiles and mammals better detect motion in their peripheral vision, which better enables them to flee from predators.  Hummingbirds, however, who move at high speeds often in cluttered environments, need to be aware of subtle movements from all directions.  

Researchers in Alberta have determined that the hippocampal formation is two to five times larger compared to other birds, and it is this area that is responsible for the hummingbird's "episodic-like" spacial-temporal memories.  A hummer will not only remember where every flower in its territory is located, but also how long it will take for each flower to replenish its nectar.

The Beak or Bill

The hummingbird beak is longer in proportion to their body than other birds because it is adapted to reach deep into flowers to obtain the nectar they produce.  There are also adaptations to allow the bird to catch insects, which is how they get their protein.  The upper beak (maxilla) is slightly flexible and the lower beak (mandible) can widen at the base, bending downward as much as 20 degrees. 

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The Tongue

The tongue of a hummingbird is able to lap nectar at a rate of up to 18 times per second.  The bird can extend its tongue as far as the bill is long.  The tip is forked and the tongue is rapidly extended when it reaches liquid.  This rapid movement pulls the nectar into grooves or troughs on the surface.  The tongue is then retracted and the bird actively presses downward with its upper bill and the nectar is forced down its throat.  

Where does the bird put all that tongue?  It is coiled up inside its head, around its skull and eyes!

The Bones

Hummingbird bones must be lightweight to ensure their maneuverability.  The bones of their legs and wings are hollow and more of their other bones are porous.  They have a deep keel bone, necessary for their relatively large pectoral muscles.  They also have eight rather than six sets of ribs like other birds.  Their elbow and wrist bones are fused together, but their shoulders can rotate 180 degrees.

The Cloaca

The cloaca serves a dual purpose.  It is the only opening a bird has for intestinal, urinary and reproductive purposes.  Liquid and solid waste mix here and are expelled, actually "shot" from the body.  They do not have a bladder, as carrying around extra weight would make them heavier for flight.  

Reproduction occurs as a result of a "cloacal kiss" which transfers sperm from the male to the female.

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The Crop

The crop is the storage area between the mouth and the stomach.  Generally hummingbirds only fill their crop 1/10 to 1/3 full when they drink.  Birds that don't own territories and must drink when they are not being chased generally fill their crops more when they have the chance.  The crops are then filled before sunset for the bird to have a supplementary energy source at night in torpor.

The Duodenum

This is the portion of the digestive system where bile from the liver and enzymes from the pancreas are added to continue digestion.  The nectar largely bypasses most of this area and is deposited very close to the beginning of the small intestine where the sugar is absorbed.  The duodenum is mostly for digestion of the insects.

The Ears (Hearing)

Hummingbirds have excellent hearing.  Their ears do not have earlobes, but are holes with a light covering of feathers.  They can hear small changes in tone better than humans.

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Erythocytes

Erythrocytes are red blood cells and they carry oxygen to all parts of the body.  Hummingbirds have very small erythrocytes and the greatest concentration of erythrocytes in the blood than any other animal in the animal kingdom.

Esophagus

The esophagus is the part of the digestive system that connects the throat of the bird with the crop.

Eyelids

Hummingbirds have two outer eyelids (upper and lower) to block the light.  They also have an additional pair of eyelids called the nictitating membrane.  These are clear and allow the hummingbird to see while blocking the wind when they fly.  Think of them like a pair of aviator goggles, but under their outer eyelids.  Hummingbirds also have eyelashes.

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Eyes (Vision)

Hummingbirds have excellent vision and they are able to see ultraviolet light, which makes their world even more colorful than ours.  

Researchers at the University of British Columbia have found that hummingbirds process motion differently in that they are able to detect movement equally from all directions.  Other birds as well as amphibians, reptiles and mammals better detect motion in their peripheral vision, which better enables them to flee from predators.  Hummingbirds, however, who move at high speeds often in cluttered environments, need to be aware of subtle movements from all directions.

Feathers (Tail and Wing)

Hummingbirds have ten tail feathers (called retrices), five on each side.  These are numbered from inside out as R1 to R5.  There are  smaller feathers above and below these retrices that are called coverts.  

The wings are divided into primary and secondary feathers (remiges).  The primary feathers are the ten long ones on each side.  These are also numbered from inside out as R1 to R10.  The secondary feathers are the six shorter ones from the primaries to the shoulder.  They are numbered from the shoulder to the R1 primary.  These feathers as well as the tertial feathers (which protect the folded remiges) are also covered with coverts, just like the tail.  

Hummingbirds have no down feathers to trap heat like other birds.  They must regulate their temperature by altering their metabolism.

Feet

Hummingbirds cannot walk or use their feet to push themselves upward like humans jump.  They use only the power of their wings to lift them off of the branch.  The feet are used only for perching and scratching.  Hummingbirds spend most of their lives perching.  They have three front facing and one back facing toe.  The toe that faces back works much like the human thumb, allowing the bird to hang onto the branch.

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Female Reproductive System

Female hummingbirds are born with two ovaries, but the right one shrinks and disappears after birth in order to lighten the bird's weight.  The left one produces several eggs which mature and produce yolks.  These enter the oviduct and if fertilized they travel to the albumen gland where the egg-white is added.  They continue together to the shell gland where the calcium shell membrane is added.  Two eggs are usually laid and the unfertilized eggs will be reabsorbed by the body.  The fertilized eggs leave by the cloaca.

Male Reproductive System

The male hummingbird has a testis that shrinks after breeding season to conserve weight.  When breeding the testis will produce sperm which flows through the vas deferens to the cloaca.  The cloaca swells and protrudes in order to transfer sperm to the female during mating. 

Gizzard

The hummingbird gizzard is a pocket before the stomach that holds the solid food (insects).  Mucous is secreted to begin digestion.  The nectar bypasses this pocket.

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Gorget

The gorget is the bright colored patch on the throat of the male hummingbird.  The term comes from olden times, when knights wore metal collars called gorgets to protect their throats in battle.  

The feathers are irridescent and the color is only visible when illuminated by light, much like the colors of an oil slick floating on water are visible.  The irridescence is produced by microscopic grooves and bubbles within the feather itself that scatter and reflect light.  When the bird looks away from the light source the feathers appear to be dark.  The gorget feathers are used to attract a female for breeding purposes. 

Heart

A hummingbird's heart is large compared with other birds.  It beats 500-600 beats/minute at rest. The highest recorded rate of 1260 b/m was that of a Blue-Throated Hummingbird measured in SE Arizona.  Rivoli's Hummingbirds also measured recorded up to 1200 b/m.  This heart rate is required to deliver the oxygen they require for their lifestyle.  

During torpor the hummingbird will slow its heart rate to conserve energy.  In the same study that measured the adult Blue-Throated and Rivoli's heart rates, it was shown that the Blue-Throated slowed to 36 b/m and the Rivoli's to 55 b/m.

Kidneys

Hummingbird kidneys are extreme compared to human kidneys.  90% of the tissue cortical, meaning the the primary function of their kidneys is to excrete excess water.  Their kidneys are not effective at concentrating urine. 

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Legs

A hummingbird's legs are small and weak.  They are not muscular for hopping or walking and the bones are hollow. The legs are used only for perching.

Liver

In order to maintain their incredibly high metabolic rate, hummingbirds may possess the most biosynthetically active livers in nature.  Their bodies burn glucose and fructose equally (unlike humans) and they are able to power themselves without turning sugar into fat.  Fat stores in the liver are usually about 15%.  During the pre-migration phase, however, they accumulate fat which is stored in the liver at rates as high as 10% of body mass per day, and the fat content of the liver can rise to 45%.  Hummingbirds do not have a gall bladder.

Lungs

Hummingbirds have two lungs and nine air sacs.  Air enters the lungs by way of the nostrils and oxygen is transferred into the bloodstream to feed their extraordinarily high oxygen consumption.  Contraction and expansion of their muscles fills the air sacs which serve to ventilate their body for cooling purposes, as their flight muscles generate a great deal of heat.

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Lymphatic System

The lymphatic system consists of the thymus gland, the lymphatic circulatory system and the Bursa of Fabricius which is located near the cloaca. 

The thymus produces T-cells and a hormone which stimulates their maturation.  The Bursa of Fabricius produces the B-cells responsible for producing antibodies.  Humans do not have this bursa, as our B-cells are produced in our bone marrow.  The lymphatic system circulates the lymph plasma which carries these cells throughout the body and is important for fluid homeostasis.  The lymphatic system carries nutrients to the cells and collects their waste products. 

Neck Vertebrae

The neck vertebrae are the bones that hold the hummingbird's head.  Humans have seven.  Many hummingbirds have 14 or 15, so their neck is very flexible. 

Nostrils

A hummingbird's nostrils are located at the base of the beak and is the place where air enters the lungs.

Very little research has been done on the hummingbird olfactory apparatus, but what has been done suggests that hummingbirds do have a functioning sense of smell.  Plants that they feed from generally do not have much of a scent, so without further research their sense of smell is considered to be unimportant in nectar gathering.

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Pectoralis and Supracoracoideus Muscles

These are the muscles responsible for flight.  The pectoralis is responsible for the downstroke and the supracoracoideus for the upstroke.  In most birds the supracoracoideus is a fifth of the size of the pectoralis, but in the hummingbird it is about half the size.  The enlarged supracoracoideus together with the hummingbird's ability to invert their wings, they are able to aerodynamically support their weight through amazing maneuvers.

Small Intestine

The small intestine is the portion of the GI tract where 90% of the absorption of nutrients from food occurs.  The protein from insects and the sugar that a hummingbird drinks is delivered here and is absorbed into the system to produce the energy that the bird needs to survive.

Sternum

The sternum (or keel bone) in a hummingbird is large and this bone is where the large pectoral muscles (flight muscles) are attached. 

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Body Temperature

Average body temperature for a hummingbird is 40ºC or 105ºF.  At night their temperature drops to ambient temperature, but never below 18-24ºC or 65-68ºF.  It is life-threatening for humans to have a core temperature of 28ºC or 82ºF.  This classifies them as "heterothermic," since they have the ability to turn "cold-blooded" at night.

Hummingbirds, as well as all birds, have air sacs connected to the lungs which extend into the hollow bones.  These act as ventilation shafts for cooling the body.  Unlike humans, they do not dissipate heat by perspiration.

Researchers at George Fox University with the University of Montana determined that hummingbirds actively alter where heat is dissipated in flight.  While flying heat is dissipated primarily in its eyes, shoulders and feet.  


 

Ureters

The ureters are thin tubes that connect the kidneys to the cloaca.  They transport the water that the kidneys eliminate from the system.

Uropygial Gland

This is a gland located at on the lower back at the top of the tail.  Oil is secreted to protect the hummingbird's feathers.  The bird will take tiny droplets of this fresh oil and work it into its feathers during the preening process.

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Wings

The wings of a hummingbird are unique in the bird world.  Other birds fly primarily by generating lift on the downstroke of the wing.  Hummingbirds don't flap but rotate their wings 180 degrees.  This enables them to generate lift on both sides of the wing, and this wing pattern is what allows them to fly backwards and forwards and even briefly upside down.  It also allows them to hover in one place.  The wings rotate in an oval pattern when flying and in a figure of eight pattern when hovering.  

The wings beat from 15 to 80 beats/second.  The Giant Hummingbird's wings beat at the lowest range, because their wings are bigger and they can support their weight with a slower beat.  The tiny Amethyst Woodstar Hummingbird holds the fastest recorded rate of 80 beats/second. 

Metabolism

Hummingbirds have one of the highest metabolism to mass ratios among vertebrates.  Their metabolism is actually 100 times greater than an elephant.  Hummingbirds can eat 1.5 to 3 times their weight in insects and nectar per day to support this metabolism.  

Welch and Groom at the University of Toronto were able to measure oxygen consumption rates and concluded that they burning and using the sugar they have consumed in the last 30 to 60 minutes.  They move sugar through their system so fast that a human would have to drink the amount of sugar in a can of Coca-Cola every minute to match it.  They are able to utilize sugar directly to support their metabolism.  Humans use sugar for a small portion of exercise, but must turn to fat stores to keep going.  Hummers do not need to do that.  

Larger hummingbirds have even a higher efficiency.  They convert an even greater portion of their sugar energy into power than the smaller hummingbirds.

Hummingbirds have to slow down their metabolism through torpor or they would never survive the night.  They cut their metabolic rate down to 60 - 90% of their daytime rate by slowing their temperature, heart and breathing rates.

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