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The Great Egg Debate - Science, Not Emotion, Shows that Caged Laying Hens Have Poor Welfare
By Sara Shields, PhD on HamAndEggonomics.Blogspot.com
There is a long history of scientific work demonstrating that animals have behavioral needs and that when these needs are not met in invariant, deprived, captive environments, there are real consequences for the animals.
Previous blog posts on Ham and Eggonomics have addressed the often-touted claim that concerns about the welfare of animals in intensive production facilities are based on emotion rather than science. However, the basis for opposition to the confinement of hens in battery cages is deeply rooted in objective scientific inquiry, and research on the topic is almost as old as the use of the battery cage itself.1
From the beginning of the debate, ethology (the study of animal behavior) has advanced understanding of the effects of cage confinement on the well-being of laying hens and, without a doubt, shown that there are very serious welfare consequences. Studies have demonstrated that there are two basic reasons for this: 1) the animals are deprived of the opportunity to express important natural behavior; and 2) the constraints of the cage prevent exercise, which has profound physical consequences for the health of the birds.
There seems to be a general lack of appreciation for the importance of behavioral expression as a component of animal well-being. Historically, it has been easier to comprehend the role of health, for example, in ensuring good welfare, while it’s sometimes been more challenging to see how behavioral restriction can reduce welfare. The science, however, tells a very compelling story.
One of the most important behavior patterns that hens are prevented from performing in a conventional cage is nesting. Observational studies of feral hens and wild Jungle Fowl (the progenitor of today’s domesticated chickens) have shown that hens will seek out a secretive, sheltered nesting site when they are about to lay an egg.2,3,4 Ethologists have investigated this behavior further in laboratory studies. They have shown that when hens do not have a nest box—as is the case when confined inside a typical battery cage—they express frustration with stereotyped, repetitive pacing movements just prior to oviposition (egg-laying),5 and make “gakel-calls,” the same types of behavior expressed in experiments with hungry hens who are able to see an expected food reward but are prevented from access by a clear Plexiglas-like cover.6,7
Using a methodology borrowed from psychology, ethologists have also investigated the “motivation” or “drive” that compels a hen to seek a suitable nesting site. By requiring an animal to “work” for access to a resource (in this case, a nest box), motivational level can be quantified. A common work task now used in animal welfare research is to require animals to push through a weighted door. Weight can be added to the door to determine how hard an animal will push to gain access to something they want, feed, a social companion or more space, for example, thereby giving an objective measure of motivational strength. It has been found that hens will push at a rate greater than 3000 Newtons/second to access a nest box 20 minutes prior to oviposition, harder than they will work to gain access to feed after several hours of food deprivation.8,9
The behavioral evidence is complemented by physiological studies. The internal drive to display nesting behavior is under hormonal control. Progesterone and estrogen released from the postovulatory follicle initiate pre-laying behavior 24 hours later, when the egg is nearly ready to be laid.10 In other words, the hen seeks a nest because her hormones tell her to do so when she is about to lay an egg. These hormonal signals are present no matter what the environment, whether the hen is in a backyard flock, a barn, or a cage.
While any one of these experiments alone would be highly suggestive, together they support a solid, scientifically-based argument that a hen is frustrated when she does not have access to an appropriate nest site. I could write much more (and, in fact, have done so here, with my co-author Dr. Ian Duncan) on the strong scientific evidence suggesting that hens also need to forage, perch, and explore, and that they enjoy dustbathing. Given the plethora of scientific research in the field of ethology and complementary scientific disciplines, it is disconcerting when the behavioral needs of hens are brushed aside in favor of arguing that concerns about the well-being of caged hens are based on emotion rather than science. There is a long history of scientific work demonstrating that animals have behavioral needs and that when these needs are not met in invariant, deprived, captive environments, there are real consequences for the animals.
If the aforementioned psychological impacts weren’t enough, there are also physical consequences when movement is severely restricted. For all hens, osteoporosis is a concern because calcium needed for shell formation is diverted from bone.11 Studies have demonstrated that restriction of movement, especially the thwarting of normal behavior such as stepping and wing-flapping, are a primary cause of bone fragility for laying hens12,13 and that exercise improves bone strength.14 Along with high-energy diets, restriction of movement and lack of exercise are also factors that predispose hens to fatty liver hemorrhagic syndrome,15,16,17,18 a disease in which excessive fat is deposited in the hen’s liver and abdomen.19,20
Cage layer fatigue was first identified when laying hen flocks were moved into cages during the advent of intensive farming in the 1950s and continues to be a “major issue.”21 The disease is “virtually unheard of” in birds who are not raised in cages.22 The skeletal system of hens suffering from cage layer fatigue can become so weak that hens become paralyzed.23 However, if they are removed from their cages and allowed to walk normally on the floor (that is, if they are allowed to exercise) and are given food and water, some may recover spontaneously.24,25,26 Unattended birds die from dehydration and starvation in their cages.27,28
Proponents of cage confinement will point to infectious disease concerns in cage-free housing, especially those that are transmitted by contact with manure. Yet cage-free barn eggs can be produced using slatted or perforated plastic mesh flooring, which just as effectively separate the hens from their manure. These types of solutions have always been used in breeding flocks (the parent birds of hens used in egg production), who are not confined in cages in commercial production enterprises. Disease concerns have been minimized in breeding flocks to levels these producers find acceptable. Many free-range egg producers also practice pasture rotation and often reduce stocking density, both of which are effective management techniques. These examples demonstrate that there are ways in which to appropriately address any potential disease concerns in cage-free systems. In contrast, severe restriction of movement is inherent to cages and thus will always be a problem for hens in intensive confinement.
While reduction of disease and predation are indeed important, the way that we choose to address those concerns should not be at the expense of other important welfare components. The price hens have paid as a result of caging them is far too high. They have lost all opportunity to display their rich, species-typical behavioral repertoire, and they are so intensively confined that they suffer severe physical consequences. The opportunity for a hen to have a good quality of life is completely denied to her in a battery cage.
Many animal protection organizations advocate for a housing system in which not just one or two of the welfare needs of the hen are met, but one in which hens are healthy and safe and in which they can express natural behavior that is important to them. That can be achieved in a well-managed cage-free environment, but, in a cage it is impossible to provide enough space for hens to express the behavior they want and need to express for their physical and psychological health.
At this summer’s Poultry Science conference, attendees learned that cage-free systems are being extensively adopted. While I attended the Keynote Symposium, Tomorrow’s Poultry: Genomics, Physiology, and Well-being, professors and breeders repeatedly asserted that we can incorporate welfare-friendly traits into selection indexes by, for example, breeding hens who are not predisposed to engaging in injurious pecking behavior. This conference only reiterated the successful studies that are already in the scientific literature.29,30,31,32,33
The scientific evidence is clear that battery cages reduce welfare and that cage-free egg production is a viable alternative. Although the reaction of large U.S. egg producers to cage-free systems has thus far been tepid, I trust that we all share the common goal of wanting to provide the best possible welfare for hens. The way to do that is to move forward, using innovation to address the needs of the hen. Working together, producers with more experience and know-how in cage-free production could lead the way in a transition to more welfare-friendly egg production systems, leaning on the scientific community to facilitate this much-needed shift. All the tools are available, leaving only the need for collective will within the industry.
Sara Shields earned her B.S. in Zoology from Colorado State University and her Ph.D. in Animal Behavior from the University of California, Davis, where she studied the welfare of chickens, and subsequently served in a post-doctoral capacity in the Animal Science department of the University of Nebraska. There, she was engaged in scientific research focused on laying hens and teaching courses on companion animals as well as animal welfare. Presently, Dr. Shields serves on the animal welfare advisory committee for Safeway stores and as a consultant for the Humane Society of the United States, among other organizations.
1 Brambell FWR. 1965. Report of the Technical Committee to Enquire into the Welfare of Animals Kept under Intensive Livestock Husbandry Systems. London: HMSO Cmnd. 2836.
2 McBride G, Parer IP, Foenander F. 1969. The social organization and behavior of the feral domestic fowl. Animal Behaviour Monographs 2:125-81.
3 Duncan IJH, Savory CJ, and Wood-Gush DGM. 1978. Observations on the reproductive behaviour of domestic fowl in the wild. Applied Animal Ethology 4:29-42.
4 Collian NE and Collias EC. 1967. A field study of the red jungle fowl in north-central India. The Condor 69:360-86.
5 Yue S and Duncan IJH. 2003. Frustrated nesting behaviour: relation to extra-cuticular shell calcium and bone strength in White leghorns. British Poultry Science 44(2):175-81.
6 Duncan IJH and Wood-Gush DGM. 1972. Thwarting of feeding behaviour in the domestic fowl. Animal Behaviour 20:444-51.
7 Zimmerman PH, Koene P, and Hooff JARAM. 2000. Thwarting of behaviour in different contexts and the gakel-call in the laying hen. Applied Animal Behaviour Science 69:255-64.
8 Follensbee ME, Duncan IJH, and Widowski TM. 1992. Quantifying nesting motivation of domestic hens. Journal of Animal Science 70(Suppl.1):164.
9 Cooper JJ and Appleby MC. 2003. The value of environmental resources to domestic hens: a comparison of the work-rate for food and for nests as a function of time. Animal Welfare 12(1):39-52.
10 Wood-Gush DGM and Gilbert AB. 1973. Some hormones involved in the nesting behaviour of hens. Animal Behaviour 21:98-103.
11 Riddell C. 1992. Non-infectious skeletal disorders of poultry: an overview. In: Whitehead CC (ed.), Bone Biology and Skeletal Disorders in Poultry. Poultry Science Symposium Number Twenty-three (Oxfordshire, U.K.: Carfax Publishing Company, pp. 137-8).
12 Knowles TG and Broom DM. 1990. Limb bone strength and movement in laying hens from different housing systems. Veterinary Record 126(15):354-6.
13 Nightingale TE, Littlefield LH, Merkley JW, and Richardi JC. 1974. Immobilization-induced bone alterations in chickens. Canadian Journal of Physiology and Pharmacology 52(5):916-9.
14 Meyer WA and Sunde ML. 1974. Bone breakage as affected by type housing or an exercise machine for layers. Poultry Science 53(3):878-85.
15 Mississippi State University Cooperative Extension Service. Miscellaneous management related diseases. www.msstate.edu/dept/poultry/dismisc.htm. Accessed March 25, 2008.
16 European Food Safety Authority, Animal Health and Animal Welfare. 2005. Scientific report on the welfare aspects of various systems for keeping laying hens. EFSA-Q-2003-92, p. 28. Annex to The EFSA Journal 197, 1-23. www.efsa.europa.eu/EFSA/Scientific_Opinion/lh_scirep_final1.pdf. Accessed March 25, 2008.
17 Crespo R and Shivaprasad HL. 2003. Developmental, metabolic, and other noninfectious disorders. In: Saif YM, Barnes HJ, Glisson JR, Fadly AM, McDougald LR, and Swayne DE (eds.), Diseases of Poultry, 11th Edition (Ames, IA: Iowa State Press, pp. 1082-3).
18 Squires EJ and Leeson S. 1988. Aetiology of fatty liver syndrome in laying hens. British Veterinary Journal 144(6):602-9.
19 Merck Veterinary Manual. 2003. Fatty liver syndrome: introduction, Merck Veterinary Manual Online, 8th Edition. www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/202400.htm. Accessed March 25, 2008.
20 McMullin P. 2004. A Pocket Guide to Poultry Health and Disease (Sheffield, U.K.: 5M Enterprises Ltd., p. 123).
21 Leeson S. 2007. Metabolic challenges: past, present, and future. Journal of Applied Poultry Research 16:121-5.
22 Leeson S. 2007. Metabolic challenges: past, present, and future. Journal of Applied Poultry Research 16:121-5.
23 Riddell C, Helmboldt CF, Singsen EP, and Matterson LD. 1968. Bone pathology of birds affected with cage layer fatigue. Avian Diseases 12(2):285-97.
24 Mississippi State University Cooperative Extension Service. Miscellaneous management related diseases. www.msstate.edu/dept/poultry/dismisc.htm. Accessed March 25, 2008.
25 Webster AB. 2004. Welfare implications of avian osteoporosis. Poultry Science 83:184-92.
26 Riddell C. 1992. Non-infectious skeletal disorders of poultry: an overview. In: Whitehead CC (ed.), Bone Biology and Skeletal Disorders in Poultry, Poultry Science Symposium Number Twenty-three (Oxfordshire, U.K.: Carfax Publishing Company).
27 Riddell C, Helmboldt CF, Singsen EP, and Matterson LD. 1968. Bone pathology of birds affected with cage layer fatigue. Avian Diseases 12(2):285-97.
28 Riddell C. 1992. Non-infectious skeletal disorders of poultry: an overview. In: Whitehead CC (ed.), Bone Biology and Skeletal Disorders in Poultry. Poultry Science Symposium Number Twenty-three (Oxfordshire, U.K.: Carfax Publishing Company, pp. 137-8).
29 Appleby MC and Hughes BO. 1991. Welfare of laying hens in cages and alternative systems: environmental, physical and behavioural aspects. World’s Poultry Science Journal 47(2):109-28.
30 Appleby MC, Hughes BO, and Hogarth GS. 1989. Behaviour of laying hens in a deep litter house. British Poultry Science 30(3):545-53.
31 European Food Safety Authority, Animal Health and Animal Welfare. 2005. Scientific report on the welfare aspects of various systems for keeping laying hens. EFSA-Q-2003-92, p. 80. Annex to The EFSA Journal 197, 1-23. www.efsa.europa.eu/EFSA/Scientific_Opinion/lh_scirep_final1.pdf. Accessed March 25, 2008.
32 Flock DK, Laughlin KF, and Bentley J. 2005. Minimizing losses in poultry breeding and production: how breeding companies contribute to poultry welfare. World’s Poultry Science Journal 61(2):227-37.
33 Ellen ED, Visscher J, van Arendonk JA, and Bijma P. 2008. Survival of laying hens: genetic parameters for direct and associative effects in three purebred layer lines. Poultry Science 87(2):233-9.
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