Crossed beak, also known as cross beak and scissor beak, is a

congenital deformity A birth defect is an abnormal condition that is present at birth, regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can range from mild to severe. Birth de ...

in birds, particularly

poultry Poultry () are domesticated birds kept by humans for the purpose of harvesting animal products such as meat, Eggs as food, eggs or feathers. The practice of animal husbandry, raising poultry is known as poultry farming. These birds are most typ ...

, where the upper and lower parts of the

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and ...

do not align properly. This condition results in the beak crossing or growing in a way that prevents the bird from closing its beak properly. As a result, affected birds may have difficulty eating and performing other essential activities.

Identification

Crossed beak is a deformity characterized by a misalignment of the upper and lower beak, with one or both beaks exhibiting lateral deviation from the head's longitudinal axis. The upper beak frequently exhibits horizontal bending at its base, alongside the affected

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

, and the skull, particularly the nasals and orbits, shows asymmetry.

Causes

Several factors, including incubation temperatures and variations,

hereditary Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic inform ...

elements, developmental mishaps, bone morphogenetic protein 4, and management practices, have been identified as potential associations with or causes of crossed beak. In a 1966 study focused on non-hereditary factors associated with crossed beak, it was identified that fluctuations in incubator temperature, stemming from electrical disruptions, emerged as a primary cause. Recent studies have identified potential candidate genes for beak deformity in domestic chickens, with several belonging to the

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

family. In a breeding trial involving Appenzeller Barthuhn chickens, believed to have a high genetic predisposition for crossed beak, mating affected parent stock led to a notably higher proportion of offspring with deformed beaks compared to mating unaffected parents. Specifically, mating affected parents resulted in 13 (15.7%) offspring with crossed beak and 67 (80.7%) with normal beaks. Conversely, mating unaffected parents yielded 3 (2.9%) offspring with crossed beak and 95 (93.1%) with normal beaks. These findings suggest a potential hereditary link to beak deformations although the genetic basis of this condition remains unclear. A 2019 study on Huiyang bearded chickens revealed a correlation between the levels of bone morphogenetic protein 4 (BMP4) expression in craniofacial bones and the presence of crossed beak deformities. BMP4 expression was highest in severe cases of crossed beak.

Association with unilateral microphthalmia and anophthalmia

In 1949, Frederick Bruce Hutt documented non-hereditary beak deformities linked to unilateral

microphthalmia Microphthalmia (Greek: , ), also referred as microphthalmos, is a developmental disorder of the eye in which one (unilateral microphthalmia) or both (bilateral microphthalmia) eyes are abnormally small and have anatomic malformations. Microphthalm ...

anophthalmia Anophthalmia (Greek: ἀνόφθαλμος, "without eye") is the medical term for the absence of one or both eyes. Both the globe and the ocular tissue are missing from the orbit. The absence of the eye will cause a small bony orbit, a constricte ...

in chick embryos. He observed that most crossed beak cases stemming from these conditions often go unnoticed, as affected chicks typically don't survive the later stages of incubation. Hutt suggested that the abnormality likely arises due to developmental accidents, possibly influenced by unfavorable environmental factors. A 1961 study involving 12 hens and 5 cocks with unilateral microphthalmia or anophthalmia was conducted to investigate the heredity of the associated cross beak deformity. All the offspring produced in the experiment exhibited normal beaks, leading to the conclusion that this specific type of cross beak is not inherited.

Prevalence

Crossed beak has been reported in at least 12 chicken strains worldwide, including commercial strains like leghorn (Landauer, 1938) and

Rhode Island Red The Rhode Island Red is an American breed of domestic chicken. It is the state bird of Rhode Island. It was developed there and in Massachusetts in the late nineteenth century, by cross-breeding birds of Oriental origin such as the Malay wit ...

(Landauer, 1956), as well as native breeds like the Appenzeller Barthuhn, Schweizer (Joller et al., 2018), Chinese silk chicken (Bai et al., 2014, 2016), Beijing-You (Bai et al., 2018a), and Huiyang bearded chicken (Hong et al., 2019), with frequencies ranged from 0.2 to 7.4%. The deformity is present in around 30% of 114 Chinese native chicken strains. In a 2019 study conducted by Aidin Azizpour on congenital anomalies, 1,796,863

broiler Breed broiler is any chicken (''Gallus gallus domesticus'') that is bred and raised specifically for meat production. Most commercial broilers reach slaughter weight between four and six weeks of age, although slower growing breeds reach slaug ...

chicks were evaluated. Of these, 56,031 chicks exhibited congenital anomalies, with crossed beak detected in approximately 27.43% of those affected. The occurrence of crossed beak appears to show no distinction between male and female offspring.

Impacts

Crossed beak in birds lead to decreased feed consumption, hindered growth, subpar production performance, and reduced survival rates. The abnormality poses both economic and animal welfare challenges within the poultry sector.

References

{{Reflist Congenital disorders Musculoskeletal disorders