A recent posting has given me an idea to conduct an experiment on what perch types are preferred by my chickens. This will be an ongoing learning experience and results will be added as found. This is not intended to be medically based and will not take into account extreme cold.
My flock is 6 weeks old and mixed standard size chickens. When I built the coop I installed 2x4 roosts narrow edge up. They have been using these for 4 weeks since they were moved into the coop so this will be my control.
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They always roost on the top and vary on which side. The lower will stay in place for access. I will be dividing the top into two movable sections. One side will be 2x4 as they are accustomed to, the other will be the experiment material. Each material will be tested for 2 weeks. One week on left and one on right so it's not a location preference. Planned tests are 2x4 flat(wide side up), natural branches, wood closet rod(1 1/4" dia), and PVC pipe (1"dia). I am willing to take suggestions for additional materials in the comments. Considering safety and availability I will do what I can to try them. The experiment will run Sunday to Sunday beginning on 19, Aug 2018 and run until testing is complete.
Link to discussion thread:
https://www.backyardchickens.com/threads/the-perch-experiment.1267000/

8/18/2018
The roost has been modified to divide it in the center. The original height has been maintained as well as the original orientation. I am trying to change as little as possible in order to more accurately determine if preference is due to material rather than another factor.
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Since the beginning of this idea I have more closely monitored their roosting behavior. It appears they have a preference for the center and left of the perch.
I understand that the results will be from a limited number of data points. I also understand that some material tested is not considered ideal. This is mainly being done out of curiosity and for fun. Any results collected are not a representation of every flock or set of conditions. If you have anything you'd like to discuss or observations you have from your own experiences. Please join me on the linked discussion thread, I'd love to hear from you.

8/19/2018
The perch modifications are complete and today will be the first test. The bottom of my coop is sand, I will be judging where they roosted by the concentration of waste. The floor is cleaned daily so this should give a fairly accurate representation of where they slept.
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From current observation this appears to be their typical location. Centered slightly favoring the left. Variation from this will be counted as a result.

Test 1)
2x4 with wide side up. Test perch on left, control on right.
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Results will be posted 8/26/2018 when next test begins.

8/262018
Test 1 results:
After the first week of testing the 2x4 wide side up, left side of the coop, here is the raw data. Sun, Mon, Tue, Thu, and Sat they all slept on the left side on the 2x4 flat. On Wed and Fri six of eight slept on 2x4 flat and two of eight slept on control on right. Since they have shown a preference for the left I can not call this conclusive as to preference for the wider perch as yet. So far it has shown that either A) the left side is more preferable regardless of perch type. Or B) the flat perch was not different enough from the control to be a factor.

Test 2:
2x4 wide side up on right, control on left
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This week will be the reverse test of test 1. If there is a preference to the wider perch, hopefully results from this test will confirm. Results will be posted on 9/02/2018.

9/02/2018
Test 2 results:
Results from week two are almost identical to week one. Left side (control) was used by all Sun,Tue, Fri, and Sat. On Mon, We'd, and Thu there was a 7/1 split with the control being favored. It is beginning to appear that the location is preferred. I cannot state this conclusively since ,1) it is still early in testing, and 2) there may not be enough difference in the orientation of the 2x4 to cause a behavior change.

Test 3:
2x4 control vs 1 1/4" dia. Round
This week will be the first square vs round test. The material chosen is pine closet rod with 1 1/4" diameter. (Unfinished, no further surface preparation) This will be placed on the left side first same as start of previous test material. Control is the same, 2x4 on edge, placed on right.
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While there is a height difference. The effect was negligible in the previous batch of testing, no compensation will be made. Results will be posted on 9/09/2018. As always discussion is welcome in the thread linked at the beginning of the article.

9/09/2018
Test 3 results:
This week's results have varied from the previous testing. I will add their daily roost as a fraction (left round / right control) to make easier reading. Results are as follows:
Sun 8/0, Mon 4/4, Tue 3/5, Wed 3/5, Thu 0/8, Fri 1/7, Sat 0/8. Monday was the only day all used the round perch. After Monday they began a preference for the control 2x4, Mon being an even split. With Thu and Sat being the most notably different. There is definitely a preference shown of square over round when it comes to my flock. At least with the tested material. It is unclear wether this preference is learned or instinct. Since my flock was raised on the square perches. Possible causes for the preference, the manufactured round stock may be too smooth for a good grip. The diameter may not be affording proper foot position. There could be a balance difference between round and flat surfaces that is preferred.

Test 4:
Test 4 will be the reverse of 3. Control will be placed on the left and the round stock on the right. If current theory is true they should quickly return to the left preference previously shown.
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Results will be posted on 9/16/2018.

9/16/2018
Test 4 results:
This week's results are consistent with the theory that the smooth round perch was not preferred. Daily results are as follows (2x4 control / round ) Sun 4/4, Mon 8/0, Tue 8/0, We'd 7/1, Thu 6/2, Fri 8/0, Sat 7/1. There was an almost immediate switch back to the left side where the control perch was located. On days where the round was still used they were located as far left as they could get towards the control. My opinion, they would have used the control but were prevented access by the location of other chickens.
My personal conclusion on the use of smooth round perch materials. Though the 1 1/4" wood rod was used. The frequency of use of the control far outweighs the experiment material. Even though they have a tendency to use the left side of the roost, they moved away to use the control material. When control was returned to the left the return of the birds was almost immediate. Either by being raised to use a certain material, or other reasons of their own, the small diameter smooth round perch was dramatically inferior to my flock.

Test 5:
Natural branch vs 2x4 control. This week I will be testing a natural maple branch against the standard control. The branch I have cut is larger in diameter (2 1/2"-3") than the previous test. I will also be leaving the bark on to see if the rough texture has any effect. I will be continuing the same test pattern with the branch starting on the left and following test on right. Results will be posted on 9/23/2018.
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9/23/2018
Test 5 results:
The results from this test were pretty balanced when viewed in comparison to the last test. The raw data, (branch/control) Sun 5/3, Mon 5/3, Tue 5/3, Wed 4/4, Thu 3/5, Fri 4/4, Sat 3/5. The raw data suggests that there is still some preference to the left side. The larger diameter natural branch was not disliked enough to force a change to the right side for all the birds. Two of the birds that used the control remained the same (both GLW) throughout the test, the other control use varied. Only one was observed using the branch exclusively.(Austra White) At this time no significant preference to the natural branch can be determined. Results of the next test will help determine if the material is preferred since it will be on the non-prererred side.

Test 6
Control on left, branch on right. This is the reverse of the previous test. All parameters remain the same with the exception of the side of the coop. Results will be posted on 9/30/2018
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9/30/2018
Test 6 results
The raw data for test 6 (control/branch)
Sun 3/5, Mon 4/4, Tue 4/4, Wed 5/3, Thu 4/4, Fri 5/3, Sat 4/4. The total use this week is rather well balanced. The larger diameter branch seems to be equal to the control with my flock. Coupling with the previous test, the use is almost identical. There was one bird in hat preferred the left side both weeks (EE) other than that, use was mixed. Based on the information, I can make the assumption that natural branches are acceptable to my flock as nightly roosts. Which should come as no surprise since left to their own, they would most likely be using trees at night.

Test 7
1" PVC pipe vs control
This week's test is probably going to be one of the more controversial tests. As I noted in the beginning, some material will be tested for potential elimination. This is one. Before any comments or concerns are voiced about this choice. Let me give some facts. PVC is a poor conductor of heat. (Thermal conductivity of 0.19, steel 43, oak 0.17, pine 0.15)
So it is no more likely to cause cold concerns than wood. In addition, the forecasted lows for my area are above 65°F for the duration of the test. The smooth surface has been roughed with abrasive cloth and cleaned of debris prior to install. The test will progress in the same manner as previous tests. Results will be posted 10/7/2018.
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10/07/2018

Test 7 results
This week's test of the PVC had significant results. Only Sun night had one bird use the pvc, all other days it was avoided. To the extent that 2 started roosting on the lower level, which has never been observed before. My conclusion is that pvc is definitely a material that,is avoided by my flock.
This has led me to abandon test 8. The results from this week to me are conclusive enough to negate the need for further Data.

Now for some unfortunate bad news. The perch experiment is going to be suspended for some time. My job is requiring me to travel a greater distance from home to another work site with extended hours. For the next 8 weeks my family will be providing the main care of the flock and I would t be available for daily observations. Once the projected completion date is reached we will be into winter here. This combination has prompted me to suspend further testing until further notice. I'm hoping with a mild winter and early spring I can resume in the march, April time frame. Thank you for all of the support and understanding you have all given during this experiment. With luck, spring will bring new tests and more results.