We've looked before at the skeletal make-up of birds and concluded that
compared to its counterparts, there is relatively little diversity. Even
though the skeletal features of these birds are almost the same, they
do have quite unique bills evolved for diverse ways of feeding. In this
section we'll examine some of the many ways that birds obtain their food.
Getting food in the aquatic environment:
If you were to analyze the behavior of birds in the ocean, you'd notice they have
very diverse ways of obtaining food. In a 1971 classic paper on Seabird
Biology, several techniques were noted, including:
- Aerial piracy
- Aerial pursuit
- Pattering (petrels) : flapping wings and kicking feet to stay
above the water.
- Hydroplaning (Petrels) : open their mouth, flap their wings and
move through water.
- Surface feeding.
- Surface seizing
- Pursuit plunging: See the prey and pursue it.
- Deep plunging: Boobies and Gannets.
- Surface plunging: See the bird and just try to make one shot.
- Pursuit Diving feet:
- Pursuit diving wings.
Owls have unique modifications of their ears to detect prey.
They also have asymmetric ears and the feathered face acts as a reflective
dish to hear sound.
Besides their amazing hearing adaptations, they have large raptorian feet.
They have claws to grip the prey. They can hold the prey tight to suffocate
them and use their claws to pierce them.
While most avian predators have large feet, these birds of prey
don’t. They get prey by knocking it
out as they collide with it in the air at speeds up to 190 mph. They actually
hit their prey with their feet. In fact, they are so good that they actually
aim for and hit their prey in the head. They have a tomial 'tooth' used
to disarticulate the cervical vertebrae.
These raptors, are birds that do not feed on the same type of
prey as most of the birds in this group. Instead they have modified beaks
to get snails out of their shells.
Vulture head shapes:
In Africa, one can actually see four different species of vulture
that feed on the same carrion. The question is, how do all these species
co-exist in the same environment? Fish (1944) noted that there are different
sizes and proportions of the beaks and skulls of these different catharid
vultures. In fact there seems to be a consistent degree of difference
between the different size classes. The most likely answer is that these
vultures partition their food. On a kill, for example, only the largest,
rarest vultures are actually able to break open the skin of the large
mammals. Once they arrive on the scene, the other vultures can
Beak of a filter feeder (Duck / Flamingos etc).
The lamellae of the duck on the side of the horny covering (culmen) of
the upper mandible is specialized for filtering out food from the water.
They get a mouthful of water and the tongue moves the water out. Flamingos
are interesting because they are upside-down filter-feeders whose mouths
have adapted to this upside-down lifestyle.
The palate of a caprimulgiform bird known as a potoo has enlarged palatines
that protect the roof of the mouth against the impact of large insects
caught in the air.
Ways of soil probing/gapping
Many birds attempt to get insects and worms from the soil. There are two
methods to this: probing and gapping. Probing is where
no hole exists and the prober sticks its bill into the ground and can
open the bill underground just enough to obtain its prey. Gapping (common
in Mynas) occurs when the birds stick their bills in the ground and open
their bills in the ground to make a hole. This is exposing a prey by moving
the soil away with its bill.
Woodpeckers are bizarre in that their hyoid apparatus curls around and
connects to the skull. Its an amazing adaptation for sticking out the
tongue (which is attached to the hyoid apparatus). Their tongues, which
have a spiny tip for stabbing prey, aren't much longer than any other birds,
but this long hyoid apparatus, which is attached to the tongue, in essence
gives them a really long reach.
This is a process of flexing back the tip of the upper bill.
The bill of a snipe, for example, can be opened near the tip. This occurs
through a rotation of the quadrate whereby it moves the jugal bar forward.
The basal two-thirds of the bill is rigid, hence the mandible is bent
Seed eating birds have modifications of the skull including the trabeculae,
and nasofrontal hinge which allow them to exert lots of pressure on seeds
but have a flexible hinge that protects the jaw joint. Some birds are
very powerful. The hawfinch, for instance, can crush olive pits. The
amazing strength in the upper and lower jaws of these organisms allows
the birds to deal with hard pits and seeds by shearing forces.
Sunbirds are basically the Asian equivalent of the hummingbird. They
have beaks that are optimal for collecting food from flowers. The length
of the bill is important because of the way these birds take in nectar.
They stick out their tongue a bit from the bill and the nectar is extracted
through capillary action.
Hummingbirds have simple stomachs because of the simplicity of the food they eat. (Turns out that sugar water is not good for birds. Birds can’t
assimilate sucrose because it inhibits the assimilation of other dietary requirements.)
Crops are part of the esophagus and are used as storage areas for food.
A number of birds have them. Birds consume food, store it
in the crop and then regurgitate it to their offspring. Digestive juices in the
saliva break down food in the crop. Fish eating birds don’t have
much of a crop because they are more commonly used for the digestion of nuts and seeds.
There are many different types of stomachs
in birds including those of fish eaters, nectar feeders, and insect eaters.
Birds will have some sort of stomach specialization depending on their diet. Seed-eating
birds have very powerful stomachs. It is said that turkeys can bend pennies
in their stomachs! The avian stomach has a glandular portion known as the Proventriculus which is an avian novelty. It is the place where food is chemically
digested. The gizzard or the muscular portion is located behind the proventriculus. Another
part of the stomach is the CECUM. Its the place of bacterial
digestion. Birds with the best cecums eat leafs or seeds (ie.
material that their own digestive systems can’t handle).
Central Place Foraging:
Birds are great for studying the theory of central place foraging because they have a central
nest and then go out and forage for food. In this theory, the question
is: should parents forage differently close to the nest compared to
far from the nest? The answer is YES! Far from the nest parents return
with more prey. In essence, this is an aspect of Optimal Foraging Theory.
By Rob Nelson
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