I have come across several posts recently asking questions about cross beak and its origin. Numerous scientific studies have been completed on the subject. I reviewed several of the scientific studies that are available on cross beak and summarized the results below.
W. Landauer reported that the frequency of eye abnormalities with cross beak present is greatly influenced by incubation temperatures (Landauer, 1937). Landauer carried out several test matings of affected stock and found it impossible to produce a true-breeding cross billed fowl, despite considerable inbreeding (Landauer, 1938). In 1941, Landauer identified a lethal mutation in fowl that produces a shortening of the upper beak and of the long bones of the extremities. He reported the mutation to be recessive, autosomal and semi-lethal. In the experiments he completed, most homozygous embryos died near the end of the incubation period, but about 13% of all homozygotes hatched. The degree of shortening of the upper beak and of the long bones was variable. In roughly 50% of “short upper beak” chicks the beak became normal during growth. In the remaining 50%, a cross beak developed. (Landauer, 1941).
Hutt (1949) and Pflugfelder (1961) reported that there are non-hereditary deformed beaks associated with unilateral microphthalmia or anophthalmia in chick embryos. Hutt noted in his research findings that most cross beak cases that occur because of these conditions are never seen because the chicks die during the later stages of incubation. Hutt reported this abnormality most likely results from an accident in development, sometimes induced by an unfavorable environment. (Hutt, 1949).
Pflugfelder set up matings of 12 hens and 5 cocks affected with unilateral microphthalmia or anophthalmia to determine if the deformed beak that occurs with these conditions is hereditary. All the offspring obtained from his experiment were normal and he concluded that this type of cross beak was not genetic. (Pflugfelder, 1961).
Moriyuki Watanabe completed a small-scale study to explore non-hereditary causes of cross beak. He found that variations in incubator temperature due to interruptions in electrical current were causal of the condition. (Watanabe, 1966).
A breeding trial performed on Appenzeller Barthuhn chickens, a breed believed to have a genetic predisposition to cross beak, showed significantly higher prevalence of offspring with deformed beaks from the mating of parent stock with cross beak present compared to the mating of non-affected parents. The mating of parent stock with cross beak resulted in 67 (80.7%) offspring with normal beaks and 13 (15.7%) with cross beak. The mating of parent stock with normal beaks resulted in 95 (93.1%) offspring having normal beaks and 3 (2.9%) having cross beak. The findings elude to a possible hereditary cause of cross beak, but variations in the phenotype and inconclusive molecular genetic results indicates the need for additional research. (Joller et al., 2018).
A scientific study completed in 2019 on Huiyang bearded chickens found that expression levels of bone morphogenetic protein 4 (MBP4) in the craniofacial bones was associated with the occurrence of cross beak. The expression of MBP4 was highest in chicks that had severe cross beak followed in succession by those with moderate and minimal cross beak. This study provided insight into the potential role of BMP4 in the development of this congenital abnormality. (Hong et al., 2019).
A scientific study was completed on congenital anomalies by A. Azizpour in 2019. A total of 1,796,863 broiler chicks were examined during this study. 1,740,832 chicks were labeled as normal at hatch and 56,031 chicks were identified as having a congenital anomaly. Cross beak was identified in 27.43% of the chicks that had a congenital anomaly present. Azizpour reported that genetics and management factors were causal of cross beak. (Azizpour, 2019).
In summary, a few of the factors that have been found to be associated with and/or causal of cross beak are incubation temperatures and fluctuations, hereditary factors, accidents in development, bone morphogenetic protein 4 and management factors.
Azizpour, Aidin. (2019). A Study on Congenital Anomalies in Hatched Broiler Chickens at the End of the Incubation Period. 10.22092/vj.2018.123598.1505.
Hong, Y.; Pang, Y.; Zhao, H.; Chen, S.; Tan, S.; Xiang, H.; Yu, H.; Li, H. The Morphology of Cross-Beaks and BMP4 Gene Expression in Huiyang Bearded Chickens. Animals 2019, 9, 1143.
Hutt, F.B. 1949. Genetics of the Fowl. 1st ed. pp. 43-44. McGraw-Hill Book Comp. INC.
Joller, Sara & Bertschinger, Flurina & Kump, Erwin & Spiri, Astrid & Rotz, Alois & Schweizer-Gorgas, Daniela & Drögemüller, Cord & Flury, Christine. (2018). Crossed Beaks in a Local Swiss Chicken Breed. BMC Veterinary Research. 14. 10.1186/s12917-018-1398-z.
Landauer W. A Semi-lethal Mutation in Fowl Affecting Length of the Upper Beak and of the Long Bones. Genetics. 1941;26(4):426-439.
Landauer, W. Notes on Cross-beak in Fowl. Journ. of Genetics 37, 51–68 (1938). https://doi.org/10.1007/BF02982143
PFLUGFELDER, 0. 1961. Nichterblich Kreuzschnabelbildungen beim Haushuhn. Roux' Arch. Entw. 152: 655-661.
Watanabe, Moriyuki. 1966. Non-hereditary Crooked Beaks in Poultry. Department of Animal Husbandry, Faculty of Fisheries and Animal Husbandry, Hiroshima University. 6: 339-34.