What is a columbiform?
Pigeons and doves belong to order Columbiformes and family Columbidae. Columbids range in size from the 50-gram diamond dove (Geopelia cuneata) to the 1300 gram crowned pigeon (genus Goura) (Fig 1)(Hooimeijer and Dorrestein 1997). The rock dove (Columba livia) is the common ancestor of all domestic pigeons. Life expectancy varies greatly among columbid species, ranging from 3 to 15+ years. When natural predation is not a concern, domestic pigeons can live 20 to 30 years (Pigeon Control Resource Centre 2009). Despite the diversity of family Columbidae, there are some features of anatomy and physiology which many species share and LafeberVet has summarized 15 facts below:
1. Large, bilobed crop
Many aspects of the columbid gastrointestinal tract are unique, however one of the most well-known features is the crop or ingluvies. Members of order Columbiformes possess a large, well-developed, bilobed crop (Fig 2). In fact, a common problem observed in injured wildlife is rupture of the distended crop when a juvenile bird falls, or is pushed, from the nest.
2. Crop milk or pigeon milk
In reproductively active pigeons and doves, rising levels of prolactin stimulate broody (or reproductive) behavior and crop milk secretion. In both male and female birds, the mucosa becomes heavily congested. Crop milk consists of desquamated, fluid-filled cells lining the crop wall. The serosal surface of the crop appears as if it is covered with cooked rice and should not be confused with a pathologic condition (De Herdt and Pasmans 2009).
Crop milk production begins 2 weeks (to several days) before squabs hatch until the juveniles are 10-16 days old (Hooimeijer and Dorrestein 1997). While most birds meet their young’s protein needs with insects, pigeons and doves feed their newly hatched chicks crop milk. Crop milk is rich in protein, immunoglobulin A, and fat, but low in carbohydrates. In fact, hand-reared squabs can suffer fatal gastrointestinal impactions when fed complex carbohydrates that they are unable to digest. Crop milk is all that squabs receive from parent birds during the first week of life (Video 1). After 5 to 10 days, the parents regurgitate seeds, grains or fruit to the chick.
Video 1. All squabs receive crop milk from parent birds during the first week of life.
3. Vestigial ceca
4. Specialized ventriculus in fruit-eating pigeons
In some fruit-eating pigeons (Ducula spp.), the protective cuticle layer or koilin lining the ventriculus or gizzard possesses rows of hard, conical projections that are used to crush hard fruits. Modifications of the ventriculus are most advanced in the Nicobar pigeon (Caloenas nicobarica) in which a ventricular stone or “nicobar” allows these birds to eat hard-shelled nuts (Fig 3). In addition to being hunted for food and trapped for the local pet market, the Nicobar pigeon is also hunted for the ventricular stone, which is used in jewelry.
While most birds drink by dipping their bill in water and throwing their head back to swallow, many pigeons and doves can immerse their beaks and drink by using suction using their beaks like straws (Fig 4) (Pigeon Control Resource Centre 2009).
Like many members of class Aves, most pigeons do not have a gall bladder although bile is still produced in abundance (De Herdt and Pasmans 2015). Therefore it is normal for the anorectic bird to produce green droppings that contain bile but little fecal material.
Although most pigeons lack a gallbladder, all fruit pigeons or fruit doves studied (genera Ptilinopus, Lopholaimus, Carpophaga, Ducula, Gymnophaps) possess this structure. Fruit pigeons are generally described as a distinct group from other columbids, and the presence of a gallbladder in fruit doves may be an important characteristic for taxonomic classification (Hagey et al 1994, Garrod 1874).
7. Liver enzyme activity
Much of the research performed on liver enzymes in avian patients comes from work in the pigeon (Lumeij et al 1988). Although not a standard component of the avian biochemistry panel, glutamate dehydrogenase (GLDH) was found to be the most specific indicator of severe liver necrosis in racing pigeons. Increased activities of GLDH in the plasma are likely seen only in the acute stage of severe liver cell damage. Lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) seem to be the most sensitive indicators of liver cell damage, however LDH was a poor indicator of liver damage despite its relatively high tissue concentrations, probably due to its rapid disappearance rate from plasma (Lumeij 1988 et al, Lumeij 1988 et al).
8. Vascular plexus
Pigeons possess a complex network of subcutaneous vessels in the skin overlying the neck called the plexus venous intracutaneous collaris. This vascular plexus extends from the head to the crop in both male and female birds. Dorsally the plexus is divided into left and right portions separated by a 1 mm gap on the median plane. Dilation of this plexus is used for sexual and territorial display. The plexus also helps regulate circulation and body temperature (Hooimeijer and Dorrestein 1997).
The interested reader is referred to Figure 44.1 of Chapter 44. Columbiformes on AvianMedicine.net to view a vascular cast of the plexus.
Subcutaneous injections in racing pigeons are normally given dorsally in the distal one-third or base of the neck by pigeon fanciers. However the presence of this extensive vascular plexus means that injections given during hot weather or behavioral displays can lead to fatal hemorrhage (Hooimeijer and Dorrestein 1997).
9. Powder down feathers
There are several unique features of columbiform plumage, Down feathers are absent or sparse in columbids, however powder down or feather powder plays an important role in the maintenance of feather condition. Powder down production is quite high in some pigeons (Hooimeijer and Dorrestein 1997). Frequent exposure to the proteins found in these feathers causes hypersensitivity in susceptible humans. This hypersensitivity reaction occurs in up to 10% of pigeon fanciers and appears first as a persistent cold that develops into allergic pneumonitis also known as “bird fancier’s lung” or “pigeon breeder’s disease” (Tudor 1991).
10. Fright molt
When attacked or frightened, any bird species is capable of dropping feathers in a “fright molt”, however doves are particularly adept at this anti-predator adaptation. Adult mourning doves (Zenaida macroura) have the ability to suddenly shed feathers leaving a predator with a mouthful of feathers while they escape OR a veterinarian with a wild bird whose release might need to be delayed until new feathers growth occurs. Therefore exercise caution when handling free-ranging doves, restraining these birds as little as possible.
Pigeons replace all feathers at least once a year. Molt is normally completed within 3 to 6 months (Tudor 1991).
11. No uropygial gland
The uropygial or preen gland is absent in most pigeons and doves. Among class Aves there tends to be a relationship between the absence of a preen gland and the production of powder down, and in columbids there is a correlation between high powder production and a decrease in preen gland size.
12. Long keel
Pigeons are strong fliers with the necessary long keel or carina for flight muscle insertion (Fig 5). These flight muscles can make up to 44% of the bird’s body weight and allow columbids to display excellent flying and maneuvering capabilities (Camfield 2004).
This musculoskeletal feature also affects the physical examination. The keel of the columbid is so long that it is particularly difficult to palpate the coelom because the space is so very small (Fig 6).
13. The basics of pigeon reproduction
All columbiformes are monogamous, mating for life. Pigeons can breed all year long with the peak breeding season occurring during the spring and summer months. Pairs typically produce two eggs that are incubated by both parents for 17-19 days. Chicks fledge at approximately 30 days of age. Birds reach sexual maturity or puberty at 6 months of age (Tudor 1991).
14. Sexual monomorphism
Columbidae generally lack sexual dimorphism, however experienced pigeon fanciers can sometimes distinguish male and female birds using subtle physical and behavioral cues. Males tend to have a larger beak, crop, beak wattle, and “eye cere” (Fig 7).
The cock’s head also tends to be larger and more globoid with a thicker neck. During courtship displays, the male pigeon often turns in complete circles while cooing loudly and insistently (Video 2).
Video 2. Pigeon courtship dance
15. Markers of renal disease
Pigeons have also been used as a laboratory model of renal disease (Wimsatt et al 2009, Marshall et al 2003, Lumeij 1987), and much of our basic understanding on the markers for avian renal disease is based on work in racing pigeons (Lumeij 1987). Thanks to this racing pigeon work, we know that uric acid, blood urea nitrogen (BUN), and creatinine are all insensitive markers of renal disease (Wimsatt et al 2009, Lumeij 1987) however pigeons deprived of water for 3 days showed a 6.5- to 15.3-fold increase in BUN, 1.2- to 1.5-fold increase in creatinine, and a 1.4- to 2-fold increase in uric acid when compared to values before water deprivation (Lumeij 1987).