Classification/Diagnostic Characteristics
Pigeons are a part of the kingdom animalia, meaning they are multicellular eukaryotic organisms. Within that kingdom they fall into the phylum chordata which implies that at some point in their life they have a notochord as well as a post-anal tail. Because Pigeons have backbones, they are part of the sub-phylum vertabrata (vertabrates). They are further classified as Aves, meaning they are winged, are feathered, are warm-blooded, move on their two rear legs, and lay eggs. The evolution of feathers was a major force of diversification, as it allowed birds to fly.

Relationship to Humans
Humans and Pigeons are both vertabrates, however Pigeons are amniotes; they are NOT mammals like humans are. This means that Humans and Pigeons are in the same Phylum, but not Subphylum. They do however share a common ancestor with a back bone.

Another piece of evidence which suggests the link between pigeons and humans is the presence of homologous structures. Both the wing of a bird and the arm of a human contain a humerus, ulna, radius, carpals, metacarpals, and phalanges which suggests divergence from a common ancestor. (10) (SR)

Habitat and Niche
Pigeons can be found around the world and are able to live in a variety of environments, due to their ability to self-regulate their temperature, water, and salt systems. They are able to fly and travel great distances, allowing them to spread locally. They are very adaptable to their environment.

They are capable of living in almost any environment. There are the greatest variety in pigeon species in tropical rainforests. Pigeons are found at altitudes ranging from sea level to 3 miles above sea level. They have also been able to colonize islands due to their ability to fly. Pigeons are often found in urban, suburban, and agricultural environments. The original habitat of the most common pigeon, the rock pigeon, was the sides of cliffs. When they live in cities, they scavenge discarded human food like bread and popcorn to eat. In the countryside, they find seeds and grain to eat. (JM 12, 13)

Predator Avoidance
Pigeons use flight to avoid predators as well as practice group living and alarm signaling. The more Pigeons in a group, the sooner one will spot a predator and signal the danger to those surrounding them.
Hawks, falcons, owls, and cats are some of the more common predators to pigeons. Since most of these predators attack from above, overhead shelter provided by buildings helps keep pigeons safe. In addition, cats and other predators who can't fly are unable to reach them atop buildings or telephone poles (19). (SP)

Nutrient Acquisition
Pigeons are both primary and secondary consumers. They aqcuire nutrients by digesting their food in digestive enzymes and absorbing the nutrients within their digestive system. Pigeons eat seeds (Primary Consumption) as well as insects and worms (Secondary Consumption).
Pigeons are scavengers meaning that they'll eat almost anything except anything that requires a lot of effort. They're also granivorous birds which mean they eat seeds and cereal grains. Often in cities, pigeons will eat leftovers and get their water from puddles on the ground or even birdbaths. Pigeons tend to have different diets depending on whether or not they're in the reproduction period or raising squab, baby pigeons. (14) (15) (LK)
(16) (LK)

Reproduction and Life Cycle
Pigeons are monogamous and may have the same mate for consecutive years. Breeding season is prompted by food availability and day length and can be either seasonal or year-round. The clutch size, or number of eggs the female pigeon will lay at one time, is usually limited to three eggs; some fruit-eating pigeons may have a clutch size of only one due to the lack of protein in their diet. Both the mother and the father participate in the the incubation period of the eggs, which can span from under two weeks to a full month, and the hatching of the eggs is done either synchronously or asynchronously, at the same time or in time intervals, which depends on whether incubation begins after all eggs are laid or during the span of the process. After being hatched, chicks are altricial, born with their eyes shut and with little to no down, and are dependent on their parents for nourishment as they are incapable of departing from the nest. During their first few days of development, chicks are fed crop milk, which is mostly composed of water and contains 11 to 13 percent protein, 5 to 6 percent fat, and less than 2 percent minerals and amino acids and is accountable for rapid growth, and then are weaned onto fruits and nuts. Although chicks may continue to depend on others' for food for a full month and a half after birth, the feathers and wing muscles necessary for flight may develop at 10 to 36 days. The short breeding cycles allow for more frequent broods, and a pair of pigeons may have up to five broods per year; this compensates for the pigeon's relatively small clutch sizes and relatively high predation rate. (AC) (4, 5)

Pigeons are amneotic and produce sexually. The eggs of Pigeons contain a mass off yolk, which the blastulas develop on top on as a disc. The embryo matures within the egg, which surrounds the embryo in a hard shell. The egg then hatches into a young that is genetically similar to the adult Pigeon, but will mature and grow.

Growth and Development
As chordates, Pigeons develop with a notochord at the embryonic level, this notochord will become their vertabrae as they develop. Pigeons also have a post-anal tail that they will continue to have throughout their development.

Baby pigeons are called squabs, and usually take about 24 hrs to hatch from the egg. For the first two days after hatching, pigeons need rest and high temperatures, or they will lose too much energy and will die. After the two day period, pigeons are ready to receive a mixture of regurgitated food and water from their parents. At about 4 weeks, the young are ready to leave their nest and start living independently, while the parents are preparing for a new litter. (S.S.-9)

Like all birds, Pigeons have feathers.These feathers contribute to the Pigeon's ability to fly as well as serve as an insulation system, allowing them to live in a variety of climates. Flight is an extremely metabolically taxing process that results in the release of heat energy; Pigeons also are able to elevate their feathers to cool themselves during this process. Under their feathers, Pigeons have dry skin.

Pigeons are bipedal, meaning they walk on their two rear legs. Pigeons also use their wings to fly.

Pigeon's wings are useful for flying in two ways: achieving thrust through flapping and maintaining flight through gliding. Pigeons flap their wings to lift off of the ground or to accelerate while already in flight. While flying, the design and lightness of their wings allows them to create lift and remain airborne. Pigeons can fly upwards of 50-60 miles per hour, and though they don't typically fly long distances, they have the endurance to do so. (6) (YR).

Sensing the Environment
Pigeons sense their environment through pheromones, visual signals, and acoustic signals. Pheromones are odorants used for communication, they generally act as alarm signals, territory marking, and sexual signals. Using their eyes, Pigeons are also able to interpret visual signals, which offers rapid delivery of information. With fairly acute hearing Pigeons can also sense their environment based on surrounding sounds.

Gas Exchange
Pigeons have lungs, but these lungs are not exactly like human lungs. Like all birds, pigeons have air sacs that supply a continuous unidirectional flow of fresh air and oxygen. Even when they are exhaling pigeons are taking in oxygen.

Pigeons have an avian respiratory system like most other birds. This system delivers oxygen from the air, through the body and to different tissues, while also removing carbon dioxide. This gas exchange is also very important with thermoregulation, maintaining normal body temperature. This respiratory system is different than other vertebrate respiratory systems because birds have small lungs and nine air sacs, that although are not directly involved with the exchange of gas, play an important role in respiration. The air sacs permit the unidirectional flow of air through the lungs meaning that their is more new oxygen available (11). (AG)

Like all bird, pigeons have rigid lungs and air sac because they require more air than most other organisms. The unidirectional airflow of air avoids mixing of oxygen rich and oxygen poor air in the lungs that hinders the efficiency of gas exchange. There is a two-cycle pump involved in the intake of gas: first the air is inhaled into the posterior sacs and exhaled to the lungs, then a second inhalation brings the air into anterior sacs and a second exhalation brings the air out of the body while more air is simultaneously inhaled by the first posterior sac to restart the cycle. These steps maximize the exposure of oxygen rich air with the lungs of the bird so that more oxygen rich air is taken in. Once air is taken in, it travels down the trachea, or windpipe, which divides into two bronchi that in turn divide into smaller tubes until the air reaches capillaries that exchange air with blood capillaries. (17) (CM)

Two-cycle pump in birds that maximizes oxygen intake (CM) (18)

Waste Removal
Pigeons are Uricotelic, meaning they excrete uric acid. Uric acid is not very soluble in water so urine is excreted as a semi-solid (a whiteish paste in Pigeons) Pigeons lose very little water in excreting their wastes because uric acid allows water from the urine to be reabsorbed.

Pigeons excrete urine, which is mixed fecal material with a high concentration of uric acid. There is double the amount of nitrogen in uric acid than urea, which is the material most mammals excrete. Pigeons, like most birds, excrete convert ammonia into uric acid because they live in an arid area and must conserve water whenever able (3). Mammals and birds have a higher metabolism than other vertebrates, produce a large amount of nitrogenous wastes, and produce more urine (1). Uric acid is not water-soluble and can be excreted with no water with it. This is the white goo in bird droppings (2). The pigeon's kidneys are their primary organ for removing waste products from the blood and maintaining homeostatic balance of ions; however, it is less efficient than in mammals (3). Pigeons' kidneys are divided into 3 lobes: the cranial lobe, the middle lobe, and the caudal lobe. The ureters branch to each one of these lobes, passing over ventral side of the kidney. Each of the lobes has its own artery. Pigeons have cortical and medullary tissues in their kidneys, and these tissues are organized adjacently into lobules. Several cortical lobules surround each medullary lobule. A major portion of each nephron is located in the cortical tubule. Pigeons have small glomeruli, with only a few capillary loops. Pigeons, like humans, have developed the loop of Henle structure to save water, but most of the nephrons lack a large loop of Henle. The kidney produces uricotelic urine that is hypertonic to the blood. Some water is reabsorbed by the nephron tubules in the kidneys. As the water is reabsorbed, the urinary solutions in the filtrate form urate crystals, which are then coated with mucus that is secreted by cells in the duct lining. The mucus and crystals moves down by peristalsis down the ureters. The 2 ureters connect to the cloaca. Pigeons do not have urinary bladders, but instead the urine is diverted to the colon for water reabsorbtion. The uric acid is excreted along with the feces as a white mass and any fluid in the cloaca is also released (1). (SM)

Environmental Physiology
  • Temperature regulation - Pigeons are endotherms which they maintain a high and constant body temperature over a wide range of ambient temperatures. They can generate internal heat metabolically.
  • Water regulation - Pigeons are amniotes, meaning they do not need fresh water to reproduce. They use internal fertilization and lay their eggs with hard shells. They excrete their nitrogenous wastes as uric acid semisolids, which allows them to reabsorb water and minimize water loss in the process. Pigeons also have dry skin that minimizes evaporative water loss.
  • Salt regulation - Pigeons are osmoregulators, meaning they actively regulator actively regulate the osmolarity of their extracellular fluid.

Internal Circulation
Pigeons, like humans, have four-chambered hearts. Their pulmonary (lung) and systemic (bodily) circuits ae completely seperate. Oxygenated blood and deoxygenated blood do not mix. The systemic circuit always receives blood with the highest oxygen content and the pulmonary circuit always receives blood with the lowest oxygen content.

A bird’s heart consists of two ventricles and two atrium's. Blood from the body enters through the right atrium, passes thorough the right ventricle and gets pumped to the lungs where it becomes oxygenated. When the oxygenated blood returns, it comes through the left atrium and proceeds to the body through the left ventricle. This process prevents oxygenated blood and de-oxygenated blood from mixing, creating a highly efficient circulatory system. Avian hearts tend to be larger (in relation to body size) as well as pump more blood over all, which is attributed to keeping up the metabolic demands of flight.
external image Avian-Circulatory.jpg(SF)(4)(5)

Chemical Control (i.e. endocrine system)
Being a multi-cellular creature, Pigeons do have an endocrine system which is used internally to distribute chemical messages. They use their endocrine system in many ways, especially to regulate their internal functions. Pigeons actively regulate temperature, water levels, and salt levels; each of these processes is aided by the release of chemicals from the endocrine system.

Review Questions
1. Both humans and pigeons have notochords that evolve into vertebrae. What does this tell you about the evolution of the notochord?
2. Compare and contrast the pigeon's two-chambered heart and our four-chambered heart. What are the advantages for each of these systems and how do they contribute to each organism's respective niche? (SR)
3. How is the pigeon's respiratory system advantageous with regard to its flight? (NU)
4. Give three reasons for why pigeons are able to exist in abundance around the world. Support these reasons with evidence from their anatomy and adaptive mechanisms. (MT)

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