Behavioural priorities are those behaviour patterns that are important enough to chickens that will work hard to perform them. There is common consensus in the poultry ethology community that there are four main behavioural priorities beyond biological imperatives like eating and drinking: nesting, foraging, dustbathing and perching (or roosting). Close runners-up also include maintenance behaviours like preening.
How do we know if a behaviour is a priority?
There are a few ways to determine what a chicken thinks about their environment. They fall broadly into two categories: preference and motivation tests. Preference tests tell you which option a chicken prefers out of a set number of choices. Motivation tests tell you how hard a chicken will work to access that choice. The harder a chicken will work, the higher they prioritize their choice.
As an example, chickens have demonstrated that they prefer enclosed nests to open nests. They will choose enclosed nests over open nests, even if they are occupied by other hens. They will even shift their preference if the location of the enclosed nest changes. Or they will choose the most enclosed area to create nest, even one isn't provided (e.g. the corner of a pen).
Another way to tell if a behaviour is a priority is to observe what the chicken does when the goal (of the behaviour) is already present. If they still perform the behaviour, then this suggests it is actually a behavioural priority.
The best example of this is foraging behaviour. Birds will spend an inordinate amount of their time foraging (working) for food, even in environments where food is freely available. This concept is called contrafreeloading. Therefore, the act of foraging itself, and not just getting enough food to eat, is important to chickens.
Finally, a behaviour can be considered a priority when birds will attempt to perform it even when not provided with the proper resources. For example, chickens will dustbathe, although in an incomplete form, even when there is no dusty substrate available. This is called sham dustbathing and it is typically performed in cages close to the feed trough. Another demonstration of this is when chickens use any elevated structure to roost at night, even if it is not intended as a perch (see below). Even though they no longer need to fear predators in an enclosed barn, chickens are still highly motivated to roost as high as they can which illustrates their a self-protective instinct.
Preening and other feather maintenance behaviours like feather ruffling are important but may not be priorities in the same way. Preening takes place when other behavioural priorities have been met, and may be temporarily abandoned if a bird is facing hardship. Therefore, preening may actually be considered an indicator that all is "well with the world" (Nicol, p. 90).
Aside: These behaviour patterns are sometimes referred to as "natural behaviours" in some animal welfare assurance schemes (i.e. "freedom to express natural behaviours"). However, the concept of natural behaviour patterns can be deceiving. I am always cautious of this wording because not all behaviours that occur in nature are desirable. For instance, vigilance behaviour in response to predators as well as escape behaviour are both perfectly "natural," but I do not feel the need to provide opportunities for chickens to perform these behaviours. Although it is mostly a semantic difference, it is more accurate to refer to these behaviour patterns as "motivated," behavioural "needs" or "priorities" rather than natural behaviours.
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