VERY interesting study! It basically says that although many intelligent species (dogs, primates, etc) show that they prefer an equal distribution of resources given an equal amount of effort, parrots do not seem to be bothered by unfairness - AND they think that this is because they are monogamous, mated for life animals and, as finding a new mate would have a cost too high (time, effort, lack of reproduction, etc), they accept the inequality.
https://www.nature.com/articles/s41598-019-52780-8