Chickens wear backpacks!

Whenever I tell people that I research chicken behavior and welfare, I usually receive two types of responses: 1) Raised eyebrows coupled with a disinterested “Huh… As long as it makes you happy, I guess” or 2) “So what kind of eggs should I be buying?” I prefer the latter, but I have to be mindful that not everyone wants to listen to a 20-minute answer. Many friends and family express confusion over the numerous claims made on egg cartons, particularly cage-free vs free-range vs the conventional cage eggs. People ask me, “What is actually best for the bird?” It’s not a short or simple answer, but it’s a really important one because while there isn’t a unanimous ‘best’ or perfect type of environment, the industry is rapidly changing. Currently only 16% of hens are housed in cage-free environments. But consumers – people like you and me who buy eggs – have recently begun to shift demands to favor cage-free eggs because it’s perceived to give the birds a better-quality life. Your purchases and demands really DO matter, and they have resulted in recent state legislation and pledges from numerous food processors, restaurant chains, and supermarkets (e.g. McDonalds, Wal-Mart, Costco, even Walt Disney Parks and Resorts!) that will shift the industry towards 75% of all hens housed cage-free by 20261. Eggs are an example of how our purchasing can be political, and how we can affect change through what we buy and who we buy it from.

Image: An overwhelming number of choices of eggs at the grocery store. [Source.]
Let’s get real – going from 16% to 75% of birds living in a certain housing environment is a MASSIVE shift for millions of animals! What does this mean for the birds’ welfare? Animal welfare can be scientifically evaluated in three broad overlapping categories2.

  1. Behavior: What is the range of normal, highly-motivated behaviors that the animal is able to express? Or, to put it simply, is a chicken able to be a chicken? Can she do the things she wants to do – like take dust baths or lay her eggs in a nest? Highly-motivated behaviors are those that animals are willing to overcome obstacles in order to perform. Some high motivation examples include chickens pushing a weighted door to be able to access a nest3, or still ruffling her feathers to go through the motions of dust bathing even if she doesn’t have any “dust” to work through her feathers to clean them4.
  2. Affective state: Does the animal experience negative states, such as pain, fear, hunger, and distress? Are opportunities for positive affective state available, such as comfort and pleasure? When we check on the hens, are we walking through the barn quietly and calmly to avoid scaring them? Are predators prevented from getting into the barn? Are we keeping the temperature in a comfortable range?
  3. Health: Is the animal healthy? What measures have we taken to promote good health and prevent illness or injury?

Finding a balance across the three categories ensures optimal welfare. But, before we assess welfare of chickens, let’s first understand the different housing systems. In cage-free systems, birds live indoors and roam freely through horizontal and vertical space. They have access to perches, nest boxes, and scratch areas that contain substrate for dustbathing and foraging5 (which adds up to more amenities than my current apartment, #jealous). In cage systems, birds live indoors and are housed in cages, but it is important to note that there are two types of caged systems that are very different – conventional and furnished. Conventional cages are what usually comes to mind for cage systems, where birds have access to feed and water but no other enrichment (e.g. perches, nest boxes, or scratch areas). Furnished cages on the other hand contain perches, nest boxes, and a scratch pad within the cage. If a hen is housed in a cage, the type of cage is not distinguished on egg cartons, but conventional cages are the most common type of cage housing in the United States.




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Now to assess chicken welfare. When comparing the welfare between a conventional cage and cage-free, hens in cage-free do have more space per bird and greater expression of those normal, highly-motivated behaviors because of the enrichment. Sounds like a check mark for the behavior category! However, cage-free hens are also more likely to express abnormal behavior, such as aggression that manifests in harmful pecking on other hens6. So, maybe that check in the behavior category is actually a question mark? Additionally, cage-free hens have higher rates of death and parasites than conventional caged hens (a bad X-mark for the health category), as well as keel bone fractures that may cause discomfort for the bird4 (I’ll explain keel bones more later, but discomfort is bad for the affective state category). While these are some serious drawbacks to cage-free environments, many of these issues can be mitigated through management strategies, which is where the Makagon Lab and I come in!

A chicken’s keel bone represented in blue. [Source.]
One of the key welfare issues we are interested in addressing through management strategies is keel bone damage. Keel bones are essentially a bird’s sternum bone, and the damage refers to deviations (bends, twists) and fractures. The fractures in particular have been associated with reduced mobility of the hen, discomfort, and lower egg production7,8. There is still a lot to learn about keel bone damage, including if deviations alone cause discomfort, the degree of discomfort for the bird for both fractures and deviations, and how long that discomfort lasts.



As I mentioned above, we know we see more keel bone damage in cage-free systems than conventional caged systems, with furnished cages being in the middle. Why is that? In cage-free housing, birds have the benefit of moving through all that horizontal and vertical space we mentioned above (a check for behavior!), but collisions with objects in the environment, such as perches or platforms, can cause keel bone damage. Collisions happen in both cage-free and furnished cage systems. (I told you that this wasn’t a simple answer to “which kinds of eggs should I buy?”) The fact that birds are struggling to jump and fly between platforms and perches suggests that the birds have impaired spatial abilities. Shouldn’t a chicken be able to do those things because she’s a chicken? We are collaborating with the Blatchford Lab at UC Davis and the Bennett Lab at Cal Poly San Luis Obispo to figure out how spatial abilities develop in chickens and correlate that with keel bone damage.

Researching development is important to consider because chickens are raised in a different facility for the first 16-18 weeks of their lives, called a pullet house (pullet is the term for an immature chicken that hasn’t laid an egg yet). During this time as pullets, birds are most sensitive to learning and brain development, so the way that we raise them could have long-term effects on behavior9. However, the design of the pullet environment can be highly variable because it’s not affected by those state laws and restaurant pledges I mentioned earlier. Pullets may be raised in conventional cages or cage-free, with large differences in group size and enrichment. Additionally, little research has been done on pullet environment design and management, but the research that is out there suggests that birds reared in a cage-free environment will hang out in vertical space more, are less fearful, and have stronger bones and muscles than birds reared in a cage environment once they go into their adult housing10. So, we started to wonder if raising birds in cage-free pullet housing would help minimize those collisions with objects in their environment. Maybe they would navigate their adult environment better if they got practice doing it earlier in life. If they can navigate better and have less collisions, would this reduce keel bone damage? This is what we set to find out!

Image: The sun rising behind the hills at the Cal Poly San Luis Obispo poultry unit. [Source: Allison Pullin]
Personal sidenote – I have to say that I felt very lucky to have my first field site of my PhD be at the poultry facility at Cal Poly San Luis Obispo (SLO). The 4.5 hour drive between Davis and SLO is stunning, especially along the 101. My favorite parts were driving through Gilroy, the “Garlic Capital of the World” (where the garlic scent hangs heavy and garlic ice cream is a real flavor!), and the Salinas valley area. Around Salinas, there were miles and miles of berry farms and orchards, and I would often drive past trucks carrying lettuce, onions, and tomatoes. I find it so interesting to see where food comes from, which is how I got into agriculture in the first place! Additionally, each field trip also meant indulging in all of my favorite podcasts – 2 Dope Queens, Reply All, This American Life, Radiolab, Nancy, More Perfect, and Casefiles. Those drives landed me in SLO, a charming coastal city enveloped by dramatic hills that I couldn’t stop staring at.

Image: The sunset at Pismo Beach during a trip for chicken backpacks, only 20 minutes from the farm. [Source: Allison Pullin]
Okay, back to the chickens. For this study, chickens were housed in furnished cages. To evaluate the chickens’ spatial abilities, I looked at the collisions that hens had in their environment by utilizing customized chicken backpacks containing an accelerometer with an impact sensor on the keel bone. The chickens wear these backpacks (they look like 80s workout gear – hello Chicken Simmons) and the sensor records any date and time that the bird hits her keel bone on something. You might be wondering what the chickens think of these backpacks? Well, their first reaction is like your pet when you put Halloween costumes on them – the birds first do a “drunk chicken walk” but then adjust and are walking and perching normally after a few minutes. On backpack days, I start the day at 3 AM with our collaborators when the barn lights are off and we can quietly catch sleepy hens from their roosts to clothe them. By coming in dark and early, we can keep all of the birds calm – sometimes at our own expense. Dark barns are creepy, especially if the wind blows the door open at 3 AM while you’re in there with only a headlamp and one other person. Factor in a 4.5-hour drive of murder podcasts and you can’t help but wonder if your last moments on Earth will be putting clothes on a chicken. (Side note: A graduate student in the Bennett Lab, Mieko Temple, met me at the chicken barn at 3 AM for every backpack day. Always professional, well-organized, knowledgeable, and smiling, she even cooked me a delicious dinner once – I worked with amazing collaborators and could not have done it without her!

Image: A chicken in a backpack! The black mark on her back is the shape of a square so that we can individually identify her in the video footage. The backpack is holding the sensor against her keel bone (not shown) to record any impacts to her bone. She can freely stretch her wings, lay eggs, walk, and perch with no problems. [Source: Sydney Baker]
After 10 days of chickens in backpacks, I took them off to download the data. During those 10 days, we also recorded video footage in order to pair their behavior with the data. Let’s say we have an impact at 10 AM on Tuesday from the data, I would go to the video footage to see what the hen was doing that caused the impact. A total of eight, trusty, undergraduate interns coded the video footage, and quickly identified their favorite chicken. I would hear them laughing and saying, “Wow, that’s totally something that ‘Spots’ would do.” Chickens are unique individuals too! Maybe in the future I’ll produce “Real Housewives of Henhouse #9.”

A chicken backpack with a “heart” individual identification shape. The chicken’s head will come through the top opening and her tail through the bottom opening. [Source: Sydney Baker]
While results are still being analyzed, preliminary results suggest that the pullet environment does have an effect on the types of collisions that chickens have in their adult environment. Birds raised in conventional cages collide with objects and other hens in their horizontal space (e.g. bumping into objects while walking or running through the cage), while birds raised cage-free collide with elevated perches in their vertical space. It’s possible that the birds raised cage-free are using the vertical space more and thus having more opportunities for collisions on the elevated perches, which is something we are also evaluating. These results will help us to understand how rearing environment affects collisions in the laying environment – a possible behavioral mechanism for keel bone damage. Ultimately the results will provide insight into how the rearing environment could be designed to reduce collisions later in life. The Makagon Lab will continue to investigate rearing environments for laying hens in an exciting new grant that you can read more about here, which will make up the later chapters of my PhD!

Accelerometers (the keel bone sensor) charging in their docking station. [Source: Sydney Baker]
So where does that leave us on the ‘best’ type of eggs to buy? Chicken backpacks and other research can give us a lot of insight, but the final decision is really up to you. If you are interested in purchasing cage-free eggs but want to verify that the birds are managed in a way that helps them be successful in their environment, there are third-party certification programs (e.g. Certified Humane) that inspect farms against a set of standards, such as how often farmers monitor animals, how much access to resources hens should have, how employees are trained related to behavior and welfare, and much more. In the end, being a responsible consumer and doing your own investigating will help you make the most informed decisions as you help shape the future of the egg industry. While food labels can be overwhelming and confusing, paying attention to what the labels actually mean and the scientific evidence behind them ensures that you are supporting practices that match your values- and who knows, maybe the eggs you buy came from a chicken contributing to science by dawning a backpack!

Allison Pullin is a 2nd year PhD in the Animal Behavior Graduate Group at UC Davis in Dr. Maja Makagon’s lab. She is broadly interested in correlating behavior with bone health in farmed chickens raised for egg production (i.e. laying hens), specifically using accelerometers to assess the role of early life environmental complexity in spatial navigation for cage-free laying hens.


  1. United Egg Producers, 2017. Facts and Stats.
  2. Fraser, D., Weary, D.M., Pajor, E. A., and Milligan, B.N. (1997). A scientific conception of animal welfare that reflects ethical concerns. Animal Welfare 6, 187-205.
  3. Weeks, C.A. and Nicol, C.J. (2006). Behavioural needs, priorities and preferences of laying hens. World’s Poultry Science Journal 62, 296-307.
  4. Lindberg, A.C. and Nicol, C.J. (1997). Dustbathing in modified battery cages: Is sham dustbathing an adequate substitute? Applied Animal Behaviour Science 55, 113-128.
  5. United Egg Producers Certified, 2017. Choices in Hen Housing.
  6. Lay, D.C., Fulton, R.M., Hester, P.Y., Karcher, D.M., Kjaer, J.B., Mench, J.A., Mullens, B.A., Newberry, R.C., Nicol, C.J., O’Sullivan, N.P., and Porter, R.E. 2011. Hen welfare in different housing systems. Poultry Science 90, 278-294.
  7. Nasr, M.A.F., Nicol, C.J., and Murrell, J.C. (2012a). Do Laying Hens with Keel Bone Fractures Experience Pain? PLoS ONE 7(8), e42420.
  8. Nasr, M.A.F., Murrell, J., Wilkins, L.J., and Nicol, C.J. (2012b). The effect of keel fractures on egg-production parameters, mobility and behaviour in individual laying hens. Animal Welfare 21, 127-135.
  9. Wichman, A., Heikkila, M., Valros, A., Forkman, B., and Keeling, L.J. (2007). Perching behaviour in chickens and its relation to spatial ability. Applied Animal Behaviour Science 105, 165-179.
  10. Janczak, A.M. and Riber, A.B. (2015). Review of rearing-related factors affecting the welfare of laying hens. Poultry Science 94,1454-1469.

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