Why laying eggs kills chickens (yes, even when they are cared for and treated with love)
Or: what does the science say about egg-laying?
Analysis of Genetic Relationships Between Various Populations of Domestic and Jungle Fowl Using Microsatellite Markers
During 8,000 yr of domestication, the chicken has been considerably changed and much differentiated by natural and artificial selections.The presumed ancestor of the domestic fowl, the red jungle fowl, lays 10to 15 eggs per year in the wild, whereas commercial laying hens are capable of producing more than 300 eggs a year. Current breeding strategies for commercial poultry concentrate on specialized production lines derived by intense selection from a few breeds and very large populations with a great genetic uniformity of traits under selection.
CHICKENS (HENS) RAISED FOR EGGS by Food Empowerment Project (mostly focuses on large scale animal farming and other human rights/animal rights issues related to food)
Rather than the 10 to 15 eggs laid naturally in a year by red jungle fowl, the presumed ancestor of today’s domestic chicken, industrial farmed hens have been manipulated to lay more than 300 eggs per year. The calcium devoted to this volume of egg laying leaves their bones extremely brittle and prone to breaking.
Ovarian Tumors by PoultryVDM:
Ovarian tumors occurs frequently in domestic laying hens, especially those bred for excessive egg production, such as ex-battery hens from factory farms. More than 80% of commercial laying hens develop ovarian tumors by the time they are 2 years of age. [...] Ovarian tumors usually don't become apparent until their growth is well advanced. The bird may have a history of egg binding or oviductual prolapse. They may also have problems associated with egg laying, such as increased infections, soft-shelled, shell-less and other abnormal eggs, andgeneralized signs of lethargy/depression.
More on Egg Yolk Peritonitis. In general http://www.poultrydvm.com/ is a good resource.
Lots of info on Chicken Health Resources by Chicken Run Rescue, another site worth checking. Also from them and regarding implants: No Such Thing as a Harmless Egg: Battling Reproductive Disease in Domestic Hens.
From this paper by an avian vet:
Reproductive disease: The most common system affected by disease seen in hens. This is believed to be due to their prolonged reproductive period with commercial hybrids laying over 300 eggs a year. It is the cause of up to 90% of mortality in barn egg-laying flocks. However it is less common in less productive breeds.
Many hens are laying over 300 eggs a year compared with the wild red jungle fowl which would rarely lay more than 20 eggs per year.
Calcium Homeostasis: 2-3 times more dietary calcium is required by a laying hen compared to the requirements of a growing chick, so diet is critical. They need to be eating predominately layers pellets. These birds are on a nutritional knife edge as regards their calcium/phosphorus balance. The original jungle fowl would have laid at most 2 clutches a year, rarely more than 20 eggs a year compared with a bird laying approx 300 eggs a year. Home-made diets will result in calcium deficiency, however breeds that are not very productive may scrape by on homemade diets.
Laying birds are susceptible to problems associated with deficient diets, as when in lay they are constantly utilising medullary bone as a temporary store of calcium.
Regarding chronic laying: It's recognized as a serious health problem in virtually all birds. Laying 300 eggs when you are phisiologically prepared to lay 20 a year definitely accounts for overlaying:
Chronic Egg Laying (The Parrot University at Harman Aviary)
Chronic Egg Laying (Beauty of Birds)
Chronic Egg-laying: What you can do (Harrison's Bird Foods)
Excessive or Chronic Egg-laying in Birds (PetCoach)
Also addressed in Backyard Chickens: The Key Issues by faunalytics.
In conclusion: chicken birth control saves their lives, chicken laying enough for us to eat eggs regularly kills them.